# ITA "blackboard" Colloquium

### Upcoming events

2019-04-15
11:15
tbd
Anastasia Filimonova
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2019-04-29
11:15
Lyman-alpha forest as a diagnostic for Warm Dark Matter
Andrew Magalich (Lorentz Institute, Leiden University)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
LambdaCDM paradigm is extremely successful at describing the large scale structure formation. However, there is a number of observations suggesting possible deviations from CDM on small scales, such as too-big-to-fail and core-cusp problems. Physically-motivated Warm Dark Matter candidates like Sterile Neutrino WDM could resolve this by introducing small-scale suppression in the Matter Power Spectrum. While the direct measurement of small-scale MPS is challenging, Lyman-alpha absorption spectra of distant quasars provide a proxy to the underlying matter distribution. Interestingly, Lyman-alpha forest Flux Power Spectrum exhibits a cut-off on the scales below ~30 km/s, that could be explained by peculiar velocities, Doppler broadening, physical size of absorbers as well as free-streaming of DM particles. We investigate the origin of the cut-off and implications for searches of DM particle.

2019-05-06
11:15
Nuclear fusion on Earth: A sustainable energy source?
Matthias Bartelmann
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Nuclear fusion in reactors on Earth has been advertised for decades now as a possible source for virtually unlimited amounts of clean energy. Triggered by a recent request, I will scrutinise this claim under several points of view: (1) fuel supply; (2) radioactive waste; (3) energy logistics; (4) cost efficiency; and (5) timeliness.

### Past events

2019-02-11
11:15
Thermal dark matter, who are the survivers?
Anastasiia Filimonova (ITP)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2019-02-04
11:15
A numerical study on the distribution of the satellite galaxies debris in the Milky Way environment
Matteo Mazzarini (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
The interaction of the Milky Way (MW) satellite galaxies with the Galactic environment produces tidal tails that are observed as denser concentrations in the sky (e.g. like in the case of the Magellanic Clouds tails or the Sagittarius Dwarf stream). I address via numerical simulations the distribution of Dark Matter (DM) and baryonic tidal streams in the Milky Way environment. Initial conditions (ICs) are modeled after Aquarius A2-to-F2 simulations and after previous literature, employing satellite models made both of baryons and DM. At the end of the simulations, most of the baryons are found still inside their progenitors, while most of DM is easily stripped and redistributed in the MW. Also, stripped baryonic debris ends mostly in the inner regions of the MW halo and it is flatter than the DM debris. A look at the angular distribution of the debris shows that this is oriented at specific angles with respect to the galactic disk. This is found also after rotating the satellites ICs of 90 degrees and running the simulations again. The next step into this investigation will be to extract ICs from latest cosmological simulations and to switch on gasdynamics and feedback to study the impact of baryonic physics on the distribution of satellite debris in the MW environment.

2019-01-28
11:15
Fermi Bubbles - An overview and more
Pooja Surajbali (MPIK)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
The Fermi bubbles are one of the largest structures known in the gamma-ray sky. From their discovery, they have been the subject of several studies, however, there are many aspects of it that are still a mystery. The bubbles have a non-trivial explanation for their origin, the understanding of which will provide an insight into the nature of processes occurring towards the center of the Milky Way. In this talk, I list the observed features of/ associated to the bubbles and align them with possible origin models and gamma-ray emission mechanisms. Furthermore, I use some of these features as a priori information in a toy model that I describe. Additionally, I comment on the cons of the gamma-ray emission mechanisms mentioned and conclude with future endeavors for the north Fermi bubble.

2019-01-21
11:15
How stellar feedback shapes nebular emission from star forming regions
Daniel Rahner (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Stars and the gas between them are intrinsically coupled. Massive stars form in clouds of molecular gas and illuminate them with their radiation creating regions of ionized hydrogen (HII regions) and photodissociation regions (PDRs). In all but the closest star-forming regions, observations of "star formation" are actually observations of young massive stars. These are detected either directly (e.g. in the far ultraviolet continuum) or more commonly indirectly, in the form of reprocessed radiation from dust or gas at certain wavelengths. The emission line spectrum of HII regions and PDRs is a widely used diagnostics of gas properties such as density, temperature, pressure or metallicity. With a newly developed scheme that couples a stellar feedback model (warpfield) to a plasma simulation code (cloudy) and a radiative transfer code (polaris) we can now produce synthetic observations both of individual sites of star formation and of star-forming galaxies. The method helps us derive basic properties of star-forming regions given their nebular emission and we show how these regions evolve in various diagnostic diagrams (such as BPT-diagrams).

2019-01-14
11:15
Astrometry and more
Gabriele Rodeghiero (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2018-12-10
11:15
The Fragmentation Behavior of Population III. Protostellar Disks under the Influence of Turbulence and Rotation
Katharina Wollenberg (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
In cosmological simulations, it is found that the degree of fragmentation of the Population III (Pop III) protostellar disk varies strongly from halo to halo. This is to be expected if turbulence within the cloud plays a role in determining when and where fragmentation actually occurs. It is also reasonable to expect that the amount of angular momentum present at small-scales will have a large impact on Population III protostellar disk formation and evolution. We conducted a numerical study with the Voronoi moving-mesh AREPO in which we examined how different levels of initial subsonic turbulence and rotation within the star-forming cloud affect the outcome of Pop III star formation in terms of the total number of objects formed, the overall mass spectrum and the individual accretion history. We used controlled initial conditions which examine the collapse of a gas cloud with pre-defined levels of turbulence or rotation within a small computational box. This allowed us to draw conclusions on the specific role, strength and importance of the particular physical parameters applied here. In this talk, I will present some of our results and discuss the relevance of our numerical approach in which we have performed several realizations per combination of initial conditions and have averaged over the realizations instead of just considering single, individual runs.

2018-11-19
11:15
A halo mass function from the non-linear density field: an analytic approach
Johannes Schwinn (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
The halo mass function is a very important probe for testing cosmological models and our theory of structure formation. However, the only analytical deduction of the halo mass function (apart from fitting formulas) is based on the linear evolution of the cosmic density field and a linear collapse threshold derived from spherical collapse. Since both of these assumptions are not ideal, I will present an analytical estimate of the HMF from the non-linearly evolved density field. Our approach is based on excursion set statistics with correlated steps, which we combine with a PDF model of the cosmic density field. The parameters of this PDF model are fixed by the Kinetic Field Theory (KFT) of cosmic structure formation. This yields a closed-form expression of the HMF that only depends on the overdensity threshold. Treating this threshold as a free parameter, we find very good agreement with measurements from numerical simulations over the mass range from 10^10 to 10^16 solar masses.

2018-11-12
11:15
Machine learning for cosmolgy explained by a human
Julian Merten (INAF-OAS Bologna & University of Oxford)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
On the use of machine learning to dissect observationally degenerate cosmologies -- A fully worked example of modified gravity and massive neutrinos.

2018-10-29
11:15
8000 galaxy clusters in the KiDS survey
Matteo Maturi (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
I will discuss methods to detect galaxy clusters in the X-rays, in the millimetric and in optical wide field surveys and I will present an application of AMICO (Adaptive Matched Identifier of Clustered Object) to the KiDS survey.

2018-10-22
11:15
EasyCritics: Arc-free detection of strong cluster lenses
Sebastian Stapelberg (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Strong gravitational lensing by galaxy clusters offers important insights into cosmology, dark matter and the high-redshift Universe. The blind search for giant arcs and multiple images in optical surveys has so far posed a considerable challenge due to the poor visibility of arcs and the presence of many false friends. This usually requires large visual inspection efforts to validate the results, which introduces a considerable bottleneck. In this talk I introduce the EasyCritics detection method, which identifies group- and cluster-scale lenses based on the optical properties of these lenses themselves instead of searching for arcs. Based on the well-tested assumption that optical light traces mass on cluster scales, EasyCritics models the mass and its lensing effect over the entire line of sight and throughout the search area to find the most likely strongly-lensing structures for a given, typical source redshift. In a first test application to the well-studied wide-field survey CFHTLenS, we find that EasyCritics recovers 70-80 % of the known lenses and identifies several new lens candidates missed due to poorly resolved arcs - while reducing the time spent on manual inspection by several orders of magnitude. Thus EasyCritics promises an important gain in efficiency and completeness for future lens searches.

2018-07-23
11:15
A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy M54
Mayte Alfaro (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Nuclear star clusters (NSCs) are the densest stellar systems in the universe with masses from 10^6-10^7 Msol and half-light radii of about 2-5 pc. They are found in a high fraction of galaxies across the Hubble sequence, including dwarf galaxies. NSCs are known to co-exist with black holes, being the Milky Way the best example. The origin of some of the most massive and metal complex Galactic globular clusters (GCs) is still unclear. One of the proposed explanations is they are former NSCs of dwarf galaxies accreted and disrupted by the Milky Way. In this context, the nucleus of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), M54 (1-2x10^6 Msol), at a distance of 28.4 Kpc offers a privileged view of a nucleus in a stage before the complete stripping of the host galaxy. This complex nucleus provides a link between high-mass metal complex globular clusters and nuclear star clusters. We performed an analysis of ~6600 member stars extracted from MUSE observations of this nucleus. This wealth of chemo-dynamical information allowed the study of a galactic nucleus at an unprecedented detail, unveiling the presence of at least three stellar sub-populations with clear differences in age and metallicity. I will illustrate how these populations can be explained by a combination of events in the nucleus of the Sgr dSph galaxy. Our data show evidences that the build-up of this nucleus could be the result of the interplay between infall and accretion of GCs by dynamical friction effects and of in-situ star formation from gas inflows.

2018-07-09
11:15
Simulating the progenitors of Milky Way like galaxies
Tobias Buck (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2018-06-25
11:15
An analytic description of structure formation in dark and baryonic matter
Robert Lilow (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Upcoming astronomical surveys like Euclid and LSST will probe the matter distribution down to scales of less than 1 Mpc/h with percent-level precision. Confronting these observations with theoretical predictions, though, will be challenging for at least two reasons: On the one hand, these scales lie far within the nonlinear regime of structure formation where conventional Eulerian and Lagrangian analytic approaches break down. On the other hand, baryonic effects like gas dynamics and astrophysical feedback become relevant on these small scales. In this talk I will describe an analytic framework that aims at overcoming both of these challenges. It is built on the Kinetic Field Theory approach developed by Bartelmann et al. and extends it to the treatment of a gravitationally coupled system of dark and baryonic matter. I will first present nonlinear results of the structure formation in pure dark matter and compare these to the results found in standard Eulerian perturbation theory. After that I will describe how baryonic gas dynamics can be taken into account and show linear results on the small-scale structure formation in coupled dark and baryonic matter. I will conclude with an outlook on the planned further developments of this framework.

2018-06-18
11:15
No LIGO MACHO: bounds on primordial black holes as dark matter from gravitational lensing of supernovae
Miguel Zumalacarregui (UC Berkeley & IPhT Saclay)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Black hole mergers detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) have revived dark matter models based on primordial black holes (PBH) or other massive compact halo objects (MACHO). These objects would be abundant in the mass range 1-100 M_Sun, where rather remarkably, previous bounds were the weakest. I will present new constraints on the PBH abundance and mass using the gravitational lensing magnification of type Ia supernovae (Sne) using current datasets (JLA, Union 2.1). Our results rule out the hypothesis of MACHO/PBH comprising the totality of the dark matter at high significance in the mass range M > 0.01 M_Sun. Eliminating the possibility of a LIGO-mass MACHO further strengthens the case for microscopic dark matter.

2018-06-11
11:15
Analytic description of baryonic effects on cosmic structure formation
Ivan Kostyuk (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2018-06-04
11:15
The expansion of ionisation fronts and what it means for star formation
Sam Geen (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Star formation efficiencies in molecular clouds are the result of accretion onto star-forming cores, ended by the expansion of HII regions that disperse this dense gas. I present a set of algebraic solutions for the expansion of photoionisation fronts and clump evaporation used to explain the results from recent simulations of star-forming clouds. I end by discussing prospects for a fully predictive model of star formation on molecular cloud scales.

2018-05-28
11:15
Modeling supermassive Black Holes in hydrodynamical simulations of galaxy formation
Sebastian Bustamante (HITS)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2018-05-14
11:15
Infalling clouds onto supermassive black hole binaries
Felipe Garrido Goicovic (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Massive black hole binaries (MBHBs) are expected to form during the assembly of galaxies through successive mergers, and understanding their formation and evolution is therefore extremely important to reconstruct the puzzle of the hierarchical growth of structures in the Universe. However, due to the complex processes at play during the interaction with their galactic backgrounds, the final fate of MBHBs is still unclear. In gas-rich environments, a plausible source of the gaseous fuel is cold clumps of gas formed due to turbulence and gravitational instabilities in the interstellar medium that later fall toward and interact with the binary. In this context, we present a suite of smoothed-particle-hydrodynamical models to study the evolution of turbulent gas clouds as they infall towards equal-mass, circular MBHBs. We focus on the dynamical response of the binary orbit to different levels of anisotropy of the incoherent accretion events. Our results suggest that sub-parsec MBHBs efficiently evolve towards coalescence during the interaction with a sequence of individual gas pockets, providing a viable solution to the "final parsec problem".

2018-05-07
11:15
The impact of a mixed model for intrinsic alignments on weak lensing surveys
Tim Tugendhat (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2018-04-30
11:15
Fuzzy Dark Matter in Kinetic Field Theory
Carsten Littek (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Recently, Fuzzy Dark Matter models such as Ultra-Light Axions have caught a lot of interest. Their dynamics is described by the classical equations of a condensate. This introduces a quantum potential in the Euler equation and is generally repulsive. We have developed an extension to KFT treating the effects of the quantum potential on the dynamics and on the initial density fluctuation power-spectrum. We find the effects to be largest on scales in the range of $3h/\mathrm{Mpc}\gtrsim k\gtrsim0.3h/\mathrm{Mpc}$, close to the onset of non-linear structures.

2018-04-16
11:15
Investigating scalar-tensor-gravity with statistics of the cosmic large-scale structure
Robert Reischke (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Future observations of the cosmic large-scale structure have the potential to investigate cosmological models with a high degree of complexity, including the properties of gravity on large scales, the presence of a time-evolving and clustering dark energy component, and the addition of neutrinos that change the fluctuation spectrum on small scales. I will briefly review Horndeski theories of gravity, the most general minimally coupled scalar-tensor theories of second order. While the cosmological background evolution can be described by an effective equation of state, the perturbations in this class of theories are characterised by four free functions of time. Here, we consider a specific parametrisation of these functions tracing the dark energy component. To forecast the expected constraining power of stage IV cosmological surveys I use combining temperature and polarisation fluctuations of the cosmic microwave background including their gravitational lensing signal, tomographic angular galaxy clustering and weak cosmic shear, together with all possible non-vanishing cross-correlations.

2018-02-12
11:15
Improving star cluster survival with centrally peaked star-formation efficiency profile
Bekdaulet Shukirgaliyev (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
We study the formation of the bound clusters after instantaneous gas expulsion, when the cluster forms with centrally peaked star-formation efficiency (SFE) profile. That means our embedded cluster has steeper density profile than that of initial and residual star forming gas at the time of gas expulsion. This is the consequence of star-formation proceeding with a constant SFE per free-fall time in centrally-concentrated spherically-symmetric molecular gas clump according to the local-density-driven cluster formation model by Parmentier and Pfalzner (2013). We assume our embedded cluster in virial equilibrium with the gravitational potential of whole clump (star+gas) at the time of gas expulsion. We perform a grid of direct N-body simulations, starting from instantaneous gas expulsion time with different global SFEs and cluster initial stellar masses. We found that the minimum global SFE needed to survive instantaneous gas expulsion is 13 percent for the isolated cluster and 15 percent for the cluster in the Solar Neighbourhood.

2018-01-29
11:15
Chemical properties of Galactic open clusters
Clio Bertelli Motta (ARI)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
TBD

2018-01-22
11:15
The two worlds of photometric redshift estimation: feature-based and fully automatic models
Disanto Antonio (H-ITS)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Redshift estimation is a fundamental task in astronomy. In particular, it serves as distance indicator in the cosmological field. As opposed to the expensive and time consuming redshift estimation via spectra, the use of photometry has emerged as a popular and practical alternative. In order to process the ever increasing number of sources, the use of machine learning methods has become mandatory. Manual feature extraction is the standard approach to train machine learning models. However, thanks to recent developments in GPU computing, it is now possible to perform massive feature selection, computing thousands of features combinations and selecting the best performing ones through a greedy forward selection scheme. On the other hand, the application of deep learning technologies and of convolutional neural networks allows estimating photometric redshifts directly from images, in a fully probabilistic way, precluding the need of pre-classification and feature extraction. In this framework, the process of feature extraction and selection is fully automatized. The success of several upcoming projects, such as the EUCLID mission, LOFAR, PANSTARRS and many more, requires the availability of highly affordable photometric redshifts. We believe that methods, along the ones presented here, will fulfill this requirement.

2018-01-15
11:15
Search for Dark Matter at the ATLAS detector
Daniel Narrias Villar (KIP)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Cosmological and astrophysical observations indicate the existence of Dark Matter. Many beyond Standard Model theories predict associated production of Dark Matter particle candidates (WIMPs) with a Higgs boson at energies that can be probed at collider experiments like ATLAS. Initially, a brief and broad introduction to ATLAS and to the topic in question is provided. The presented search for Dark Matter produced in association with a Higgs boson decaying to a pair of b-quarks is performed using proton-proton collisions at a centre-of-mass energy of 13~TeV with an integrated luminosity of 36.1 fb^-1 recorded with the ATLAS detector at the Large Hadron Collider. Experimental sensitivity improvements are presented. The observed data are found to be in agreement with the Standard Model predictions. Upper limits on the production cross-section of a beyond Standard Model scenario are presented.

2018-01-08
11:15
The Edge of Galaxy Formation: Simulating the smallest galaxies in our Universe
Jonas Frings (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The satellites of the Milky Way and Andromeda represent the smallest galaxies we can observe in our Universe. Because of the large range of mass and spatial scales it is quite challenging to resolve such objects properly in simulations. I will present a new hybrid method that we used to investigate satellites of Milky Way mass halos in hydrodynamical simulations and the effects of the satellite-host galaxy interaction that we can observe therein.

2017-12-18
11:15
Christmas thermodynamics: solve the riddle!
Mattia sormani (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Holidays are approaching and it is time to stop thinking about serious physics and discuss some recreational physics. I will discuss the following problem, which was assigned in the very first International Physics Olympiad in 1967, a competition for high school students: "Consider two identical homogeneous balls, A and B, with the same initial temperatures. One of them is at rest on a horizontal plane, while the second one hangs on a thread. The same quantities of heat have been supplied to both balls. Are the final temperatures of the balls the same or not? Justify your answer. (All kinds of heat losses are negligible.)" This problem is now considered a classic and can be found in several problem books and collections. The main reason for its popularity is that the solution usually presented, while being tricky to find, is very simple and elegant, and uses tools accessible to a high-school student. Unfortunately, it turns out that this solution is actually wrong! I will discuss why, and derive the correct result. The audience is encouraged to think about the solution in advance!

2017-12-11
11:15
The impact of tracer bias on voids
Giorgia Pollina (Universitäts-Sternwarte München)
ITA "blackboard" Colloquium
Philosophenweg, 12, 105
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Abstract
The large-scale structure of the Universe can only be observed directly via luminous tracers of the underlying matter density field. However, luminous tracers, such as galaxies, do not precisely mirror the clustering statistic of the bulk of the cold dark matter distribution: their correlation function (or power spectrum) is biased and depends on various properties of the tracers themselves. Although on small scales this bias is an unestablished function of space and time, on very large scales it results in a constant offset in the clustering amplitude, known as linear bias. In this talk we focus on the bias of luminous objects within and around cosmic voids, enormous under-dense regions of the Universe that occupy the vast majority of its volume. As a remarkable result, we find that, within voids, the relation between matter and galaxy density is always linear and determined by a multiplicative constant. Furthermore, the value of this constant decreases with the increase of the size of voids and asymptotes to the linear bias. This result opens to the possibility of using such simple relation in other voids studies, allowing to extend our theoretical understanding of voids (typically defined as depressions in the matter density field) to voids that are identified using galaxies as tracers of the matter density. Ultimately we aim to test these possibility with observations, using the Dark Energy Survey data currently available.

2017-12-04
11:15
tpd
Fabian Klein (ZAH/ARI)
ITA "blackboard" Colloquium
Philosophenweg 12, 106

2017-12-04
11:15
Disk Fragmentation as a Planet Formation Scenario
Hans Baehr (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Circumstellar disks are likely gravitationally unstable early in their lifetimes when massive and still accreting from their surrounding envelopes. These gravitational instabilities will form spirals and possibly fragment into dense clumps of gas which may result in gas giant planets and low-mass stars. My research uses hydrodynamic simulations to focus on what disk conditions result in the formation of planets and do so over various resolutions to ensure convergence.

2017-11-27
11:15
Cosmic ray driven galactic winds and their dependence on halo mass
Jacob Svenja (H-ITS)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Galactic winds are commonly observed in starburst galaxies and play an important role in modelling realistic galaxies. However, the mechanism that drives these winds remains poorly understood. Cosmic rays provide one possibility to launch the outflows. To improve our understanding of cosmic ray driven winds, I use a set of hydrodynamic simulations of isolated galaxies to study how the wind properties change with halo mass.

2017-11-20
11:15
EMPIRE: Probing the dense, star-forming gas across nearby disk galaxies
Maria Jesus Jimenez (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
I will present results from a comprehensive and systematic study of the fraction of dense gas and its star formation efficiency as traced by several high critical density molecular lines. The results are based on the IRAM-30m large program EMPIRE, a ~500h survey that provides full maps of high critical density tracers like HCN or HCO+ across the entire star forming disks of 9 nearby disk galaxies. My thesis work aims to understand how dense gas fractions and star formation efficiencies vary across and among galaxies. Another particular focus is to relate the fraction of star forming gas and its ability to form stars to local ISM conditions such as stellar surface density or ISM pressure. I furthermore present results of analyzing optically thin isotopologues (e.g. H13CN) and high signal-to-noise 13CO and C18O in EMPIRE and ancillary ALMA data, to constrain optical depths of high critical density tracers and abundance variations in our disk galaxies. Both quantities are crucial to understand the ISM density distribution

2017-11-13
11:15
Testing Galaxy Formation Models Using Lyman-Alpha Absorption Profiles of Galactic Halos
Daniele Sorini (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 106
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Abstract
Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflows from the IGM and outflows from supernovae or AGN feedback. The average Lyman-alpha (Ly-a) absorption profile around foreground galactic halos at different transverse separations from background quasars represents a powerful tool to probe their gaseous environments. I compare predictions of state-of-the-art hydrodynamic cosmological simulations with the observed absorption around foreground quasars, damped Ly-a systems, and Lyman-break galaxies. I show that large-scale BOSS (Baryon Oscillation Spectroscopic Survey) and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance (20kpc - 20Mpc). Far from galaxies (>2 Mpc), the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the LCDM model successfully describes the ambient IGM, and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations are present on scales (20kpc - 2Mpc), illustrating the challenges of accurately modelling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as ~2 Mpc, indicating that the sphere-of-influence of galaxies could extend to approximately ~20 times the halo virial radius (~100 kpc). Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. I demonstrate that the Ly-a absorption profile is primarily sensitive to the underlying temperature-density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.

2017-11-06
11:15
Action-based Dynamical Modeling for the Milky Way
Wilma Trick (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
In the era of large Galactic surveys like Gaia, we require sophisticated dynamical modeling tools to turn the amount and quality of the observed stellar data (positions and velocities) into reliable constraints on the Milky Way’s overall structure. In this blackboard talk, I introduce RoadMapping, a dynamical modeling machinery for the Galactic disk using action-based distribution functions that I developed during my PhD. I outline the idea behind the technique, the tests I performed to demonstrate its robustness, and – last but not least – I present brand-new measurements of the Galactic gravitational potential using data from Gaia DR1 and RAVE.

2017-10-23
11:15
Galaxy luminosity modulations as a probe of large-scale flows and cosmic growth
Martin Feix (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Spatial modulations in the observed galaxy luminosity distribution (derived from redshifts as distance proxies) trace the radial peculiar velocity field. Using spectroscopic redshift surveys, this signal can be used to obtain independent bounds on large-scale flows and cosmic growth, complementary to traditional methods (such as clustering analysis). Considering SDSS galaxies at z=0.1, I will present recent measurements of the cosmic growth rate based on this approach and discuss ongoing developments that aim at extending the general methodology for other purposes such as applications to photometric redshift catalogs.

2017-10-09
11:15
Rings in protoplanetary disks: planets or ring instabilities?
Cornelis Dullemond (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The famous ALMA image of the disk around HL Tau shows that that protoplanetary disk consists of numerous concentric nearly perfect rings. In the mean time several new disk sources have revealed similar multi-ring patterns. These patterns have been interpreted by many astronomers as being caused by several low mass planets opening gaps in the disk. However, the planets themselves have not yet been detected, raising the possibility that perhaps these rings might have a completely different (and equally interesting) origin: that they are caused by disk-internal slow instabilities. The slowness is important, because a fast instability leads to a clumpy/turbulent disk. A sufficiently slow instability, however, can allow communication between gas parcels 360 degrees along their orbits, thus allowing density fluctuations to tidally smear out, coordinate and merge into rings. I will discuss two such potential instabilities, both related to the dust in the disk, and whether they might be at the origin of these rings.

2017-07-24
11:15
Constraining the physics of the Circum Galactic Medium with Ly-Alpha absorbtion
Daniele Sorini (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2017-07-17
11:15
How galactic feedback regulates the angular momentum content of galaxies
Jolanta Zjupa (h-its)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
In the early universe all matter that later forms gravitationally bound structures acquires an initial angular momentum through interaction with the gravitational tidal field. This initial spin is preserved during structure formation and determines the rotation of dark matter haloes at all redshifts. However, galaxies at low redshift are observed to have substantially higher spin than the surrounding dark matter. This originates in the impact of supernova and AGN feedback on the gas content of the galaxy and its decisive role in shaping galaxy morphology. In this talk I will discuss the main feedback mechanisms at play and explain how they regulate the angular momentum content of galaxies. Using insight from the hydrodynamical cosmological simulations Illustris and IllustrisTNG I will show evidence for the spin of galaxies to be independent of the detailed feedback prescription.

2017-07-10
11:15
tbd
tba
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2017-07-03
11:15
tbd
tba
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2017-06-26
11:15
Testing Lorentz invariance of dark matter with satellite galaxies
Dario Bettoni (ITP)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Lorentz symmetry is a fundamental pillar of General Relativity and Particle Physics. Nonetheless, certain theories of quantum gravity involve some degree of Lorenz violation which may have significant consequences on all scale. It is hence worthwhile to explore the observable signatures of Lorentz violations on the dynamics of the low energy spectrum of the theory. I will consider the case of a Lorentz violating vector field acting on the dark matter component of a satellite galaxy orbiting in a host halo and discuss the key observational signatures such as modifications to the line of sight velocity dispersion, mass profiles and shapes of satellites.

2017-06-19
11:15
The 'Missing Link' in Planet Formation Theory and why the size distribution of asteroids and Kuiper Belt objects is so similar
Andreas Schreiber (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
The most promising way around the direct growth hindering m-sized barrier is in-situ planetesimal formation via gravitational collapse of massive particle clouds. In this talk, I present the circumstances under which planetesimals, which we believe are the same as cometesimals, and its remnants we today call asteroids, are thought to form and how we were able to close the missing link between mm and km sized objects. For the presented work, I performed hydro-simulations of the so called streaming instability within the environment that a collapse is expected to happen in. This instability is known mainly for its ability to concentrate dust, but I will introduce its turbulent diffusive behavior as the limiting factor for this in-situ collapse to occur. We further find this diffusive process to be pre-determining the planetesimal birth size of around 100 km independent of semi-major axis! With this we were the first to find an explanation why observed asteroid and classical Kuiper-belt size distribution, as well as Jupiter Trojans sizes and Pluto impact crater statistics, have a kink exactly at this size precise size. Moreover, we could derive from first principles a collapse criterion, similar to the Jeans criterion for stars but for planetesimals, giving us a prediction on the initial planetesimal size and find this in agreement with our simulations.

2017-06-12
11:15
Gravity emerging from entropy?
Matthias Bartelmann
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
A recent paper by E. Verlinde on emerging gravity (https://arxiv.org/abs/1611.02269) has attracted considerable attention because he claims there that entropy density is a primary characteristic of space-time rendering space-time elastically responsive to the presence of matter. If gravity should indeed emerge as an effect of the entropy content of space-time, the gravitational law must be modified in a way similar to Modified Newtonian Dynamics, thus potentially avoiding dark matter. In the talk, I will review Verlinde's main arguments and describe his derivation of a modified gravitational law. Recent measurements testing this modification arrive at mixed conclusions.

2017-05-29
11:15
Detecting giant gravitational arcs in wide field surveys
Mauricio Carrasco
ITA "blackboard" Colloquium
Philosophenweg, 12, 106

2017-05-22
11:15
Periodicity makes galactic shocks unstable
Mattia Sormani (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Roberts (1969) first realised that stationary spiral shocks can arise in the interstellar medium of a disk galaxy as a response to an externally imposed spiral gravitational potential. This has broad consequences: for example these are thought to be important sites of star formation. But are these shocks stable? We find that boundary conditions are critical in determining whether the solutions are stable or not. Shocks are unstable if periodic boundary conditions are imposed. The instability disappears if boundary conditions are imposed such that the upstream flow is left unperturbed as in the classic analysis of D’yakov and Kontorovich. This reconciles apparently contradictory findings of previous authors. This also shows that the instability is distinct from the Kelvin-Helmholtz instability. We suggest that instability is a general characteristics of periodic shocks, regardless of the presence of shear, and provide a physical picture as to why this is the case.

2017-05-15
11:15
Winds and Radiation in Unison: A new 1D Feedback Model for Cloud Dissolution
Daniel Rahner
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Star formation in molecular clouds is inefficient. One possible reason for this is stellar feedback in the form of winds, radiation and supernova explosions which disrupt clouds, drive large shells, and shut off star formation. It is still not clear which of these feedback processes dominates. Since radiative and mechanical feedback are highly interconnected, this question can only be answered if all feedback processes as well as gravity are accounted for in a self-consistent manner and if the coupling between radiation and matter is taken into account properly. To this end, we present a new semi-analytic feedback model which calculates shell dynamics and structure simultaneously. It allows us to scan a large range of physical parameters and to estimate escape fractions of ionizing radiation, the minimum star formation efficiency required to drive an outflow, and re-collapse times for clouds in which feedback fails.

2017-05-08
11:15
AMICO: detection of galaxy clusters in the Euclid survey
Matteo Maturi (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Galaxy clusters allows us to investigate cosmology and in particular the properties of dark energy. In this talk I will describe AMICO (Adaptive Matched Identifier of Clustered Objects), a new code for the detection of Galaxy Clusters in photometric wide field surveys. AMICO is going to be implemented in the data analysis pipeline of the Euclid ESA space mission. This code takes advantage of the statistics of the color, luminosity and photometric redshift of the galaxies belonging to the field and those belonging to clusters. These initial statistical properties are refined through an iterative approach which takes advantage of the newly detected objects. Its construction allows to maximize the signal-to-noise ratio of the structures to be found. In addition it returns a mass proxy for the detected galaxy clusters and a probabilistic association of the galaxies to these structures, a very useful information for the study of the precious objects.

2017-04-24
11:15
On the Physical Origin of Radial Surface Density Profiles in Disk Galaxies
Jakob Herpich (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, 106
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Abstract
Stellar disks are observed to have near-exponential light profiles. More recent deeper observations reveal that many galaxies actually have a two-component exponential profile: truncated and anti-truncated disks. I present results from simulations of isolated disk galaxies that are able to reproduce all of these profiles and show that they correlate with the initial distribution of baryonic angular momentum in the host halo. I find that the formation mechanism of truncated disks is consistent with previous results and link the formation of anti-truncated disks to radial heating of stellar orbits by a strong and persistent central bar. Additionally, I derive an analytic formula for the radial stellar surface density profile using methods of statistical physics.

2017-04-03
11:15
no colloquium today
no colloquium today
ITA "blackboard" Colloquium
Philosophenweg 12, 106

2017-02-06
11:15
Stellar Tidal Streams as Cosmological Diagnostics
Gustavo Morales (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar tidal features of low surface brightness. In this regard, the initial observational effort carried by Martínez-Delgado et al. 2008 has led to the discovery of previously undetected extensive stellar structures in the halos surrounding these galaxies, likely debris from tidally disrupted satellites. In addition, our team confirmed and clarified several enormous stellar over-densities previously reported in the literature, but never before interpreted as tidal streams. Even this pilot sample of galaxies exhibits strikingly diverse morphological characteristics of these extended stellar features: great circle-like features that resemble the Sagittarius stream surrounding the Milky Way, remote shells and giant clouds of presumed tidal debris far beyond the main stellar body, as well as jet-like features emerging from galactic disks. Together with presumed remains of already disrupted companions, our observations also captured surviving satellites caught in the act of tidal disruption. My work has been focused on both quantitative and qualitative comparisons with available simulations set in a Lambda-CDM cosmology (that model the stellar halo as the result of satellite disruption evolution), showing that the extraordinary variety of stellar morphologies detected in this pilot survey matches that seen in those simulations. The common existence of these tidal features around normal disk galaxies and the morphological match to the simulations constitutes new evidence that these theoretical models also apply to a large number of other Milky Way-mass disk galaxies in the Local Volume.

2017-01-30
11:15
tba
tba (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
tbd

2017-01-23
11:15
Direct N-Body simulations of the Galactic Centre
Taras Panamarev (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Galactic nuclei are perfect laboratories for studying physical processes in the vicinity of a supermassive black hole and for exploring the impact of the massive black hole on its stellar and interstellar environment. High resolution observations of the Milky Way nucleus, which is very quiet, provide detailed and specific information that can be compared to numerical models. Our model for the dynamical evolution of a galactic nucleus takes into account the formation and evolution of binaries as well as stellar evolution. We model the tidal disruption and accretion of stars onto the central black hole by a simplified model. In this talk I will present some results from the simulation of the Galactic Centre

2017-01-16
11:15
Momentum-Correlation tensor in cosmological structure formation
Carsten Littek (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Knowledge about the cosmic velocity field is crucial in understanding various aspects in large scale structure formation. Its statistics help modelling distortions in redshift space and interpreting measurements of the kinetic Sunyaev-Zeldovich effect. In standard perturbation theory (SPT) one usually looks at the velocity divergence, since the rotational part decays in the linear regime proportional to the scale factor. In N-body simulations the statistics of the three dimensional field can be measured, but may be prone to low resolution difficulties in low density regions. Based on a kinetic field theory ansatz developed by Bartelmann et al. (2016), I will present a way to assess the statistics of the 3D velocity field and results for the power spectra of the kinetic energy density, velocity divergence, and vorticity field including non-linear scales.

2017-01-09
11:15
Revealing the secrets of Westerlund 2 - A young massive star cluster observed with the HST
Peter Zeidler (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
We present a detailed study of the young massive star cluster Westerlund 2 (Wd2) using a high-resolution multi-band survey observed with the Hubble Space Telescope. With an estimated mass of ~4*10^4 Solar masses, Wd2 is one of the most young (~1 Myr) star clusters in the Milky Way and contains a rich pre-main-sequence population. We show that the mass accretion rate of the pre-main-sequence stars is reduced in proximity to the cluster’s massive O stars, which are eroding their circumstellar disks. We find that Wd2’s mass function is flatter than the canonical slope of the initial mass function. The cluster shows mass segregation, which is likely primordial given its young age.

2016-12-19
11:15
Implementation of magnetic resistivity in GR-MHD simulation
Qian Qian (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Turbulent magnetic diffusivity plays an important role for accretion disks and the launching of disk winds. We have implemented magnetic diffusivity, respective resistivity in the general relativistic MHD code HARM. This colloquium describes the theoretical background of our implementation, its numerical realization, tests and preliminary applications under astronomical context.

2016-12-12
11:15
The enrichment of circum-galactic gas in hydrodynamical simulations
Thales Gutcke (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
I will present the results of the hot and cold circum-galactic medium (CGM) of 86 galaxies of the cosmological, hydrodynamical simulation suite NIHAO. NIHAO allows a study of how the z=0 CGM varies across 5 orders of magnitude of stellar mass using OVI and HI as proxies for hot and cold gas. The cool HI covering fraction and column density profiles match observations well, particularly in the inner CGM. OVI shows increasing column densities with mass, a trend seemingly echoed in the observations. As in multiple previous simulations, the OVI column densities in simulations are lower than observed and optically thick HI does not extend as far out as in observations. I will discuss reasons for the discrepancy between observations and simulations. I also take a look at the collisional ionisation fraction of OVI as a function of halo mass. The chemical enrichment of both halo and disc gas follow expected increasing trends as a function of halo mass that are well fit with power laws.

2016-12-05
11:15
Planets around giant stars: two transiting planets and one S-type planet in an eccentric binary system
Oritz Mauricio (LSW)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
tbd

2016-11-28
11:15
Molecular Clouds: theory and simulations
Javier Ballesteros Paredes (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
tbd

2016-11-21
11:15
Non-linearities in modified gravity theories
Santiago Casas (ITP)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
The non-linear matter power spectrum will be one of the most important tools to discriminate between competing cosmological models in the upcoming years. However, apart from some N-body simulations in specific models, there are few predictions of the non-linear power spectrum for models beyond LCDM (such as scalar-tensor theories) that have the required few-percent accuracy to be tested with future surveys at least in the mildly non-linear regime. In this talk I will review the standard perturbation theory for the Vlasov-Poisson system of equations in the field theory language, give a glimpse of how Renormalized Perturbation Theory (RPT) and the Time Flow formalism work and then will explain how we plan to modify it in such a way that it can handle modified gravity terms, such as the ones obtained from Horndeski's theory in the quasistatic limit.

2016-11-14
11:15
A model for periodic blazars
Mattia Carlo Sormani (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Blazars are powerful gamma-ray sources that are believed to consist in a jet powered by an AGN pointing in the general direction of the Earth. These sources are often claimed to show quasi-periodic oscillations on a variety of timescales, ranging from hours to years. I describe a theoretical scenario to explain blazar periodicities with timescales of ~ few years. The scenario is based on a binary SMBH system in which one of the two SMBH carries a jet. I will discuss the various mechanisms that can cause the jet to precess and produce corkscrew patterns through space with a scale of ~ few pc. I will then apply my scenario to the source PG1553+113, which has been recently claimed to show a T_obs=2.18±0.08 years periodicity. To do this, I complement my scenario with a kinematical jet model that has a spine-sheath structure inspired to the one actually observed in M87. I will show that I am able to reproduce the optical and gamma-ray light curves and multiple synchrotron spectra simultaneously. I also give estimates of the source mass and size.

2016-11-10
11:00
Constraining the early Universe with primordial black holes
Sam Young (Sussex)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Abstract: Primordial black holes may have formed in the radiation dominated epoch in the early universe from the collapse of large overdensities. They are a viable dark matter candidate and may have been responsible for the observed gravitational waves from LIGO. In this talk I will discuss how they can be used to constrain the early Universe, and my research investigating the effect of non-Gaussianity on the abundance of and constraints from PBHs - with a particular focus on how modal-coupling can lead to the formation of dark matter isocurvature modes in the scenario that dark matter is made of PBHs. Such isocurvature modes would be observable in the CMB and are strongly constrained by results from the Planck satellite - which leads to extremely strong constraints on the non-Gaussianity parameters.

2016-11-07
11:15
The Abell 2744 cluster: too much substructure for Lambda-CDM?
Johannes Schwinn (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
The galaxy cluster Abell 2744 is not only one of the most massive objects in the current Universe, it also contains eight very massive (M > 10^14 Msun) substructures within a radius of 1 Mpc from the cluster centre. It thus provides an excellent probe for the Lambda-CDM model. However, the comparison of masses obtained by gravitational lensing analysis with those obtained in simulations comprises many difficulties, such as line-of sight projections or the choice of cosmological parameters in the simulation. I will shortly present ways how to make those masses more comparable to each other and how to include biasing effects. Using these results, I will then summarise the search for Abell 2744-like clusters in the Millennium XXL simulation.

2016-10-24
11:15
Model-independent characterisation of strong gravitational lenses
Jenny Wagner (ITA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
We develop a new approach to extracting model-independent information from observations of strong gravitational lenses. The approach is based on the generic properties of images near the fold and cusp catastrophes in caustics and critical curves. Observables used are the relative image positions, the magnification ratios, ellipticities and orientation angles of extended images, and time delays between images with temporally varying intensity, if available. These observables constrain (ratios of) the surface mass density and its derivatives near a critical curve, enable to reconstruct the critical curve in the vicinity of the images, can determine the source position, and an estimate of the absolute magnifications of multiple images. Applying our approach to lenses of galaxy-cluster scale thus yields local characteristics of the lensing potential in the strong lensing regime that can serve as constraints in addition to weak lensing, SZ-effect and X-ray information in model-free reconstruction algorithms like SaWLens for full cluster analyses. As a standalone, directly relating measurable observables to (ratios of) potential derivatives in a set of equations, it efficiently provides information about the lensing potential in the vicinity of images, while model-fits often require a high degree of (manual) fine-tuning and introduce additional degeneracies when adding substructure to reproduce the observed image configurations.

2016-07-25
11:15
Intrinsic alignment in modified gravity
Tim Tugendhat (ARI)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Weak gravitational lensing is a powerful tool in observational cosmology. Large surveys of cosmic shear aim to constrain the cosmological parameters such as the mass density parameter $\Omega_m$ or the effective equation of state parameter $w$. Upcoming surveys, in particular EUCLID, will measure such a massive amount of galaxy shapes to such precision that systematical physical effects need to be accounted for. Ellipticity correlations are not necessarily only due to gravitational lensing but can also be caused by an intrinsic alignments of those galaxies, which can be thought of as nuisance “noise" on top of the lensing “signal". I will present one alignment model which our group uses to predict the intrinsic alignment of elliptical (i.e. early type) galaxies and discuss its implications on future surveys. Furthermore, I will show how we can turn that noise into signal — by comparing the intrinsic alignment correlations of early type galaxies to the weak lensing signal we can directly probe for gravitational slip, thereby conducting a test on general relativity on the largest scales.

2016-07-18
11:15
The Response of Dark Matter haloes to Galaxy Formation
Aaron Dutton (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
The distribution of dark matter in galaxies provides a non-linear scale cosmological test of dark matter models. However, the utility of this test is at present limited by our lack of knowledge about how dark matter haloes respond to galaxy formation. I will discuss recent theoretical progress from the NIHAO galaxy simulations, which zoom-in on 100 haloes of dwarf to Milky Way masses. Our simulations exhibit a wide range of halo responses, from expansion to contraction, primarily varying with the global stellar-to-halo mass ratio (equivalent to integrated star formation efficiency). I will show how the diversity of halo response can be captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). This regularity in the galaxy formation process provides a step towards a fully predictive model for the structure of cold dark matter haloes.

2016-07-11
11:15
Chasing LyAlpha Haloes at z>6
Emanuele Paolo Farina (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Luminous quasars have been detected well into the epoch of reionization, probing the existence of super-massive black holes (SMBH) less than a Gyr after the Big Bang, and setting strong constraints on the the first crucial phases of the build-up of SMBHs and their host galaxies. To form a SMBH in such a short time scale, the first quasars need to be embedded in abundant cold gas reservoirs able to continuously feed the growth of the first black holes. This key ingredient of the galaxy formation could be mapped at z>6 through the detection of an extended and diffuse LyAlpha emission that appears when the cold gas is fluorescently illuminated by the emission of a bright quasar and/or by the UV-photons generated by an intense star formation event. I will present our efforts to directly observe this feature and thus to probe the environment where supermassive black holes can form in the early Universe.

2016-07-06
11:00
Morphometry of random spatial structures in physics (SPECIAL EDITION)
Michael Klatt (KIT)
ITA "blackboard" Colloquium
Philosophenweg 12, 105
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Abstract
From the large-scale structure of the universe to exotic states in nuclear matter: complex and dis- ordered spatial structures appear ubiquitously on nearly all length scales in very different physical or chemical systems. Often physical insight can best be obtained by a rigorous structure characte- rization. How can we sensitively and comprehensively quantify the geometry of this variety of very different random spatial structures? Such a versatile and rigorous shape analysis is provided by a family of intuitive structure characteristics, the so-called Minkowski functionals. We use them to analyze im- portant mathematical models as well as exotic states of matter from nuclear physics to condensed matter systems. They can also detect structural deviations and features in background noise. This can be used from medical tomography to gamma-ray astronomy.

2016-07-04
11:15
The Hubble parameter revisited
Valeria Pettorino (ITP)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
I will discuss preliminary results of a reanalysis of the Hubble parameter from direct measurements, using hyperparameters and compare with the recent results by Planck and by Adam Riess.

2016-06-27
11:15
Extraction of the thermal Sunyaev-Zel'dovich effect from multi-frequency observations of the microwave sky
Guillaume Hurier (CNRS)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Galaxy clusters are good tracers of the large scale distribution of matter in the Universe. Consequently, they are a privileged probe for cosmological models. They can be observed through several processes: galactic emissions, bremsstrahlung, gravitational lensing, Sunyaev-Zel'dovich effects. The thermal Sunyaev-Zel'dovich (tSZ) effect has proven to be a efficient way to detect high-z and high-mass galaxy clusters (see e.g., Planck, ACT, SPT galaxy cluster catalogs). First, I will present a method, MILCA, optimized for the extraction of the tSZ effect from multi-frequency observations of the microwave sky. Then, I will discuss the advantages and drawbacks of such reconstructed tSZ emission. Finally, I will present a validation method based on artificial neural network that have been used inside the Planck collaboration to build a high-purity galaxy cluster sample. The production of such high-purity galaxy cluster sample is a key step in order to set robust constraints on cosmological models.

2016-06-20
11:15
Molecular clouds properties in a turbulent interstellar medium
Juan Camilo Ibañez Mejia (ZAH, Heidelberg Univesity)
ITA "blackboard" Colloquium
Philosophenweg, 12, R106
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Abstract
Understanding the formation and evolution of molecular clouds is critical for understanding the star formation process. I will talk about the properties of a simulated molecular cloud population self-consistently formed in a galactic environment. I will concentrate on discussing where supersonic turbulent motions inside MCs come from, how is their interaction with their environment and how fast they collapse.

2016-06-13
11:15
Scale dependence analysis of Large Scale Structures
Robert Lilow (ITA-ZAH)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106

2016-06-06
11:15
Emission line Diagnostics: the multiphase ISM of the local universe
Eric Pellegrini (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106

2016-05-30
11:15
Can we go from microscopic particles to halo density profiles? An analytic journey
Elena Kozlikin
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
For a few decades now the density profiles of dark matter halos have been studied both through observation and numerical simulations. While the shape and some key features like universality of the density profile were analysed in great detail the question about the how and why density profiles exhibit these shapes and properties still stands. I will talk about a few interesting properties of density power spectra, which are closely related to density profiles of dark matter halos, which we have observed in the scope of a microscopic, non-equilibrium, statistical field theory for cosmic structure formation recently published by Bartelmann et al. It avoids the difficulties of standard perturbation theory by construction and allows to proceed deeply into the non-linear regime of density fluctuations. I will also show how to obtain density profiles of dark matter halos analytically from this approach and what we can learn about the properties that determine the shape and universality feature of these profiles.

2016-05-23
11:15
Making galaxies passive
Miguel Querejeta (MPIA)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
The rapid suppression of star formation, or quenching, is thought to be an important process in the evolution of the most massive galaxies, but the mechanisms involved are still hotly debated. I will discuss two agents that control star formation and can ultimately lead to its suppression: AGN feedback and galaxy mergers. I will first focus on the interplay between stellar structure, nuclear activity, and molecular gas in the context of AGN feeding and feedback, with particular attention to M51. I will show that there is sufficient molecular gas inflow to feed the AGN in M51, and feedback effects which include a nuclear molecular outflow, as well as a large-scale radio jet which pushes molecular gas laterally, inducing shocks and turbulence. In the second part of the talk, I will address the role of galaxy mergers in the buildup of a passive population of lenticular galaxies (or S0s). Using numerical simulations I will show that even major mergers of spiral galaxies can result in disc-dominated lenticulars, with a photometric bulge-disc coupling and kinematics in agreement with observations. Globally, our results imply that both internal processes (transport of gas and AGN feedback) and external mechanisms (mergers) have the ability to regulate and eventually suppress star formation in galaxies.

2016-05-09
11:15
The molecular ISM: How much H2 is in the Milky Way? And how is it distributed?
Ralf Klessen (ITA/ZAH, Heidelberg)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
Stars and star clusters form by gravitational collapse in regions of high density in the complex multi-phase interstellar medium (ISM). Stellar birth is most strongly correlated with the molecular phase of the ISM. Thus, it is interesting to get an estimate of the total amount of H2 gas in our Galaxy and to find out how it is distributed. I report some recent results obtained from numerical simulations and recent observations.

2016-05-02
11:15
Scale-dependent analysis of cosmic structure formation
Robert Lilow
ITA "blackboard" Colloquium
Philosophenweg, 106
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Abstract
At first sight, the growth of cosmic structures clearly shows a strongly scale-dependent behaviour – linear evolution of large-scale fluctuations, non-linear evolution on smaller scales. But at the same time, observations and simulations suggest the existence of scale-independent features. Most prominently, it appears that collapsing dark matter forms halos characterized by a universal density profile. Unfortunately, it has proven extremely difficult to investigate this within conventional analytical frameworks, as their validity breaks down when going beyond the mildly non-linear regime of structure growth. We hope to overcome these problems by using a framework based on particle dynamics in combination with non-perturbative methods, in particular the so-called renormalization group (RG). In this talk I will briefly introduce our framework as well as the RG and explain how they could be used to perform scale-dependent analyses of cosmic structure formation.

2016-04-25
11:15
Spherical collapse with tidal shear
Robert Reischke (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
Despite its simplicity the (spherical) collapse model has turned into a valuable tool to describe the evolution of non-linear objects such as galaxy clusters. Especially when deriving number counts of such objects the collapse model enters as an essential part of the story. However, as galaxy clusters or other collapsed objects form in overdense regions there might be effects due to surrounding tidal shear fields, which can influence the collapse. I will briefly introduce the spherical collapse model and discuss how tidal shear can be implemented into the evolution equations using first order Lagrangian perturbation theory.

2016-04-18
11:15
Fueling SMBHs: Mergers vs Smooth Accretion
Jillian Bellovary (American Museum of Natural History)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
There is debate regarding whether massive black holes (MBHs) are primarily fueled by major merging events, or whether more quiescent "cold flow" accretion can efficiently fuel them as well. I will describe results from cosmological hydrodynamic simulations of galaxies at a range of masses and redshifts, in which massive black holes accrete gas from both mergers and cold flows. In general, a SMBH does not exhibit a preference for one or the other; the gas it accretes mirrors the composition that its host galaxy contains. However, in one instance we find an alternative result, hinting that major merger-driven fueling events may be responsible for some of the dominant phases of SMBH growth.

2016-04-11
11:15
Ram Pressure Stripping and Internal Feedback in Low Mass Milky Way Satellites
Andrew James Emerick (Columbia University)
ITA "blackboard" Colloquium
Philosophenweg, 12, R 106
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Abstract
The combination of ram pressure stripping, tidal stripping, and internal feedback from massive stars dominates the evolution of infalling dwarf satellites of the Milky Way. In the context of recent observational and theoretical work placing constraints on the quenching timescales of dwarf satellites, we investigate the ability for ram pressure stripping and internal feedback to explain the quenching process in the smallest satellites of the Milky Way using three-dimensional, high resolution, wind tunnel simulations. Using initial conditions appropriate for a dwarf galaxy like Leo T, we investigate whether or not environmental gas stripping and internal feedback together can quench these low mass galaxies on the expected timescales, shorter than 2 Gyr. Our results show that ram pressure stripping operates less efficiently than expected on low mass dwarfs, requiring additional physical processes to account for the whole picture of dwarf satellite quenching. I will discuss these results and the potential additional physics needed to quench low mass dwarf galaxies.

2016-04-04
11:15
The Survival and Destruction of Galactic Gas in Clusters
Rukmani Vijayaraghavan (University of Virginia)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
The discovery of long-lived, ubiquitous, X-ray emitting hot galactic coronae in groups and clusters by Chandra poses a challenge to our understanding of galactic ISM survival in harsh environments. These coronae are unique probes of ICM and ISM microphysics, since their survival depends on a delicate balance between external ICM physical processes that can alternatively destroy or replenish these coronae and internal galactic physics that can replenish them. In this talk, I will summarize the effects of external ICM phenomena, specifically tidal and ram pressure stripping, shielding by magnetic fields, and isotropic and anisotropic conduction on the survival of these coronae.

2016-02-01
11:15
Second Generation Star Formation in Primordial Supernova Remnants
Katharina Wollenberg
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
I will give an insight into our research on chemical abundances in second-generation stars. Explaining their actual metallicities requires an understanding of how the metals produced by the first supernovae are diluted in the early IGM and later taken up into the second generation of stars. With this project we want to explain the chemical abundance of the metal-poor star J031300 with a maximum Fe abundance of 10^(-7.1) solar. Additionally we examine the chemical evolution of dwarf galaxy Leo IV which might be a result of the nucleosynthesis from a single or few Pop III supernova explosions. We study these processes by modeling mixing in primordial supernova remnants and their fragmentation into new stars using high-resolution numerical simulations.

2016-01-25
11:15
Autoencoding Astronomical Time Series for Visualisation
Nikos Gianniotis (H-ITS)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
We present a dimensionality reduction method that caters for the visualisation of time-series. Often, however, the temporal nature of such data is only superficially taken into account as the data are treated as vectors. This means that latent dynamics present in the time-series are ignored and therefore interesting aspects characterising the series are lost. In this work, we propose the use of a dimensionality reduction algorithm that is aware of the temporal nature of the data, hence leading to more informative visualisations. The algorithm proposed involves an autoencoder coupled to a modified objective function. We apply the proposed algorithm on data originating from the Kepler survey. In particular, we focus on objects that have not been classified before by selecting objects that are unlikely to display periodic behaviour. Interestingly, the proposed visualisation displays a strong correlation between the variability of the objects and their physical properties.​

2016-01-18
11:15
Mass reconstruction in galaxy clusters: parametric and non-parametric methods
Mauricio Carrasco (ITA/BBC)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Gravitational lensing has developed into one of the most powerful tools for analysis of the dark universe because it is sensitive to the lens' total matter content, independent of its composition and under a minimum number of assumptions. this talk, I will summarize the basic procedure for modelling strong lensing galaxy clusters by using both parametric and non-parametric methods. I will show the advantages and disadvantages of both approaches and how to rank models by using the observational lensing features and Bayesian evidence.

2016-01-11
11:15
Model-independent characterisation of strong gravitational lenses
Jenny Wagner (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
I will present a new approach to extracting model-independent information from observations of strong gravitational lenses. The approach is based on the generic properties of images near the fold and cusp catastrophes in caustics and critical curves. Observables used are the relative image positions, the magnification ratios and ellipticities of extended images, and time delays between images with temporally varying intensity. I will show how these observables constrain derivatives and ratios of derivatives of the lensing potential near a critical curve. Based on these measured properties of the lensing potential, classes of parametric lens models can then easily be restricted to such parameter values compatible with the measurements, thus allowing fast scans of large varieties of models. Applying the approach to a representative galaxy (JVAS B1422+231) and a galaxy-cluster lens (MACS J1149.5+2223), model-independent information can be extracted in those cases and I will demonstrate that the parameters obtained by this method for known parametric lens models agree well with those found by detailed model fitting.

2015-12-14
11:15
Physics in a candle flame
Matthias Bartelmann (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg 12, 105
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Abstract
>>> Only for this time in SR 105 Phil 12 <<< Why is a candle flame blue at the bottom and red at the top? Fitting to Christmas, I will discuss this phenomenon and explain why the usual explanation does not work. The answer will lead us directly into the physics of emitting gases and of emission in thermal equilibrium.

2015-12-07
11:15
Light and shadow on gravitational flexion
Agnese Fabris (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Weak gravitational lensing has turned into one of the most promising tools to analyse dark cosmological structures. So far, almost exclusively weak gravitational shear has been exploited. It is caused by the gravitational tidal field and gives rise to elliptical distortions. Important information should also be contained in the derivative of the gravitational tidal field, the so-called gravitational flexion. However, the reliable measurement of flexion in presence of shear has not been demonstrated yet. If flexion became routinely measurable, it would allow improving the spatial resolution of dark-matter maps and thus testing one of the fundamental predictions of the standard CDM cosmogony, namely that galaxy clusters should be highly substructured. Thus, is gravitational flexion reliably measurable in realistic observational conditions? To address this question we undertook a detailed investigation into the problem of estimating flexion. I will present in this talk the conclusions we drew from such an investigation.

2015-11-30
11:15
Migration traps in disks around supermassive black holes
Mordecai-Mark Mac Low (Dep. Physics. American Museum of Natural History)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Accretion disks around supermassive black holes (SMBHs) in active galactic nuclei contain stars, stellar mass black holes, and other stellar remnants. These objects perturb the disk gas gravitationally. The resulting density perturbations in turn exert torques on the embedded masses causing them to migrate through the disk in ways strongly analogous to the behavior of planets in protoplanetary disks. We determine the strength and direction of these torques using an empirical analytic description dependent on local disk gradients, applied to two different analytic, steady-state disk models of SMBH accretion disks. We find that there are radii in such disks where the gas torque changes sign, trapping migrating objects. Our analysis shows that major migration traps generally occur where the disk surface density gradient changes sign from positive to negative, around 100--300R_g, where R_g=2GM/c^2 is the Schwarzschild radius. At these traps, massive objects in the AGN disk can accumulate, collide, scatter, and accrete. Intermediate mass black hole (IMBH) formation is likely in these disk locations, which may lead to preferential gap and cavity formation at these radii. Our model thus has implications for SMBH growth as well as gravitational wave emission.

2015-11-23
11:15
Characterizing transiting extrasolar planets
Simona Ciceri (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The detection of the first planets outside our solar system two decades ago has spurred growing efforts to detect new planets. Thanks to improvements in spectroscopic and photometric technology, more and more extrasolar planets have been found showing an astounding diversity of physical and orbital characteristics. In this talk I will present the work carried out during my PhD, which is pointed on one side towards the identification and confirmation of new exoplanets (e.g. HATS-15 b, HATS-16 b and Kepler-432 b), on the other side towards the characterization of known transiting exoplanetary systems.

2015-11-16
11:15
X-ray and SZ observations of the outskirts of the galaxy cluster Abell 2142
Céline Tchernin (Geneva University)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
In the concordance model, small structures form first and merge to create larger structures. In this framework, galaxy clusters are the latest structures to have formed and they continue to grow through accretion of matter. The processes of accretion can be studied in the cluster outskirts. Galaxy clusters are filled with hot gas, which can be observed both in X-rays, through Bremsstrahlung emission and emission lines, and in milimetric, through the Sunyaev Zel'dovich effect. However, the observations of cluster outskirts with X-rays are difficult because of the very low density of gas in this region. In the work I will present today, I will combine the milimetric and X-ray information collected respectively with the Planck and XMM satellite, to study the outskirt of the cluster Abell 2142. By accretion of matter, we expect inhomogeneities in the density distribution («clumpiness»). Such clumpiness affects more the X-ray than the SZ observations due to their different dependence on gas density. Therefore, I will analyze the X-ray data with an appropriate statistical method which takes this clumpiness into account. I will show some gas properties of the outskirt of the cluster Abell 2142. For instance, I will show that an entropy flattening feature similar to the one observed by Suzaku, and which would induce an out of equilibrium state of the gas, disappears when clumpiness is taken into account.

2015-11-09
11:15
Free Dynamics of CDM particles in SFTCP and Eulerian SPT
Felix Fabis (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
>>> FROM NOW ON ALWAYS AT 11:15 <<< We have recently developed an alternate field theory (SFTCP) approach to cosmological structure formation which allows us to calculate the time evolution of the statistics of the cosmological density field based on the underlying microscopic dynamics of particles. In contrast the well-known Eulerian Standard Perturbation Theory (SPT) is based on the macroscopic dynamics of the density field. This leads to the question at which level of perturbation theory the two approaches give the same results and where they start to differ from one another. This question becomes a lot easier to answer if one considers the evolution of a non-interacting system. Concerning this, I will report my findings both from analytical calculations as well as comparisons to N-body simulations, where the latter seem to indicate that the omission of velocity dispersion in SPT restricts its validity on small scales when compared with SFTCP.

2015-11-02
11:15
No ITA BBC
no colloquium
ITA "blackboard" Colloquium
none, none

2015-10-26
11:00
Extreme value statistics in cosmology
Robert Reischke (ITA/ZAH)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The statistics of extreme values is a powerful tool which describes very rare events in a set of random numbers of some underlying distribution. Given this distribution information about the behaviour of those extreme values can be extracted rather easily. I will give a brief introduction into the theory of extreme value statistics and the related concept of order statistics. Furthermore I will give an example on how this concept can be applied to cosmology in the case of weak lensing peak counts.

2015-10-19
11:15
Planet-disk interactions in non-isothermal disks
Aiara Lobo Gomes (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
>> only for this time at 11:15 << Protoplanetary disks control the formation and evolution of planets, in reaction the planets also influence the disk structure. Disk gas and dust are the building materials of planets. Tidal forces between planet and disk determine the radial movement of the planet (migration); the planets simultaneously influence the disk, possibly carving out a gap. The interplay between planets and disks is important to understand the variety of exoplanets observed and constrain planet formation theories. This thesis studies new aspects of planet-disk interactions in non-isothermal disks; an important step to a better description of this subject. To this end, radiative-hydrodynamical simulations of planet-disk interactions are performed. Firstly, a Newtonian cooling law is used to investigate the dynamics of vortices, which are generated by the planet and amplified by the convective overstability. The formation of density rings beyond the position of a planetary gap carved out by a high mass planet is studied, as well as the migration rates of low mass planets. Finally, radiative-hydrodynamical simulations are performed. The coupling of radiation transfer to hydrodynamics yields a more accurate determination of the disk temperature in the planet's vicinity, which has a strong influence on the formation and properties of gaps in disks.

2015-10-12
11:00
The prediction of weak-lensing peak counts from the statistics of perturbations in the cosmic gravitational potential
Christian Angrick (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The prediction of weak-lensing peak counts from the statistics of perturbations in the cosmic gravitational potential Predicting accurately the number of peaks in weak-lensing maps as function of the signal-to-noise ratio is highly desireable since it provides an opportunity to constrain cosmological parameters. In this talk, I will summarise the basics of an analytic derivation invoking both the contributions from large-scale structure and individual galaxy clusters parametrised by their mass. By focussing on their gravitational potential, I will show how cluster masses could be avoided in the derivation since they present a poorly defined quantity.

2015-07-20
11:00
The prediction of weak-lensing peak counts from the statistics of perturbations in the cosmic gravitational potential
Christian Angrick (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Predicting accurately the number of peaks in weak-lensing maps as function of the signal-to-noise ratio is highly desireable since it provides an opportunity to constrain cosmological parameters. In this talk, I will summarise the basics of an analytic derivation invoking both the contributions from large-scale structure and individual galaxy clusters parametrised by their mass. I will show how cluster masses could be avoided by focussing on their graviational potential to avoid the mass in the derivation, which is a poorly defined quantity.

2015-07-06
11:00
Hydrodynamics in non-equilibrium statistical field theory for classical particles and applications to cosmology
Celia Viermann (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
In cosmic structure formation, an important topic is the prediction of the power spectrum, especially on small scales where non-linear effects occur and the different properties of dark and baryonic matter become important. Conventionally, the only way to tackle this task analytically is a perturbative approach based on the hydrodynamical equations. However, these equations are unable to correctly describe collisionless dark matter. Its properties are better described within the scope of a non-equilibrium statistical field theory for classical particles first developed by Das and Mazenko. Within this approach, first results on the dark matter power spectrum look very promising. To calculate the all-matter power spectrum, the theory must be expanded to include baryonic matter which shows fluid-like behavior. After a short introduction to the field theoretical approach I will try to explain the implementation of a hydrodynamical model in the theory and how it might be used to set up a two-component ensemle with gravitationally coupled baryonic and dark matter which hopefully allows access to the full-matter power spectrum.

2015-07-06
11:00
Hydrodynamics in non-equilibrium statistical field theory for classical particles and applications to cosmology
Celia Viermann (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
In cosmic structure formation, an important topic is the prediction of the power spectrum, especially on small scales where non-linear effects occur and the different properties of dark and baryonic matter become important. Conventionally, the only way to tackle this task analytically is a perturbative approach based on the hydrodynamical equations. However, these equations are unable to correctly describe collisionless dark matter. Its properties are better described within the scope of a non-equilibrium statistical field theory for classical particles first developed by Das and Mazenko. Within this approach, first results on the dark matter power spectrum look very promising. To calculate the all-matter power spectrum, the theory must be expanded to include baryonic matter which shows fluid-like behavior. After a short introduction to the field theoretical approach I will try to explain the implementation of a hydrodynamical model in the theory and how it might be used to set up a two-component ensemle with gravitationally coupled baryonic and dark matter which hopefully allows access to the full-matter power spectrum.

2015-06-29
11:00
The discovery and characterization of the most distant quasars
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
High-redshift quasars provide unique information about the evolution of supermassive black holes, their host galaxies, and the intergalactic medium at early cosmic time. Numerous studies have established a sample of ~60 quasars at 5.5<z<7.1. These studies demonstrated the existence of massive black holes less than a Gyr after the Big Bang and indicated that the end of cosmic reionization occurred at z~6. These findings suggest that fundamental changes are happening in the intergalactic medium at 6<z<7. The discovery and characterization of a statistically significant sample of quasars in this redshift range is crucial to further study this important era in the history of the universe. I will present how in less than 3 years we doubled the number of known z>5.5 quasars using the Pan-STARRS1 survey. The quasar sample spans a factor of ~20 in luminosity and shows a diverse range of properties, including a number of weak-line and radio-loud quasars. I will also discuss some of the surprises revealed by this quasar sample as well as our initial follow up studies, which are the first steps towards a statistical characterization of the high-redshift quasar population.

2015-06-22
11:00
Analysis of Sgr A* with HESS II
Helen Poon (MPI)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Sgr A* has always been a main target of observations of H.E.S.S. Previous observations have shown that the Galactic Centre is a point-like gamma ray source above 100 GeV. It has exhibited rapid flares in lower energy wavelengths but not in gamma ray. A 600 m^2 dish, HESS II, has been added to the array. This significantly increases sensitivity and lowers the energy threshold to below 100 GeV. In this work, preliminary results of the GC are presented.

2015-06-15
11:00
Structure formation in dark energy cosmologies on galactic scales
Camilla Penzo (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
I will review the basis of dynamical dark energy and coupled dark energy models, to then introduce zoom-in simulations, an extremely useful tool in order to investigate collapsed structures with a much higher resolution and, at the same time, maintain the cosmological context. Firstly, I will then show highlights from the Dark MaGICC project, which aims to investigate the effect of dynamical Dark Energy on galaxy formation via hydrodynamical cosmological simulations. Then, I will summarize the results regarding zoom-in dark matter only simulations of Milky-Way size halos in coupled dark energy cosmologies and describe the effects of the coupling on dark matter halo substructures.

2015-06-08
11:00
Chemical fingerprinting of the birthplaces of exoplanet and brown dwarf companions.
Taisiya Kopytov (MPIA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Abstract: The formation mechanism and formation location of exoplanets and brown dwarfs in circumstellar disks is still intensely debated. The two main scenarios for for formation of sub-stellar companions in circumstellar disks are gravitational instability (GI) and core accretion (CA). Based on observational studies of exoplanet systems, it seems that massive giant planet and brown dwarf companions form at large (> 40 AU) separations while less-massive planets originate closer-in. However, various migration mechanisms can affect present-day observed positions of sub-stellar companions, which complicates the picture. A way out is chemical fingerprinting. As Carbon and Oxygen have different condensation temperatures, the absolute (and relative) amounts of gaseous Carbon and gaseous Oxygen change with disk radius. Hence, the C/O ratio might provide the crucial hint for identifying the correct formation scenario of exoplanets and brown dwarf companions. However, the C/O ratio might also be subject to various degeneracies and uncertainties. For instance, the classical search for the best fit to models might result in a set of unphysical parameters, or to reveal several models that describe observation equally well. In this case, the use of a sequential (i.e. "Bayesian") approach should give more reliable results. The way the models fit to data also plays a significant role. In my talk I will discuss the current approach we are using to define parameters of exoplanets.

2015-06-01
11:00
How to classify light curves
Dennis Kuegler (H-ITS)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
The classification of irregularly sampled light curves has been a huge challenge in databases of photometric surveys. The difference in sampling and photometric errors makes the comparison of different light curves a challenging task. So far, people have extracted handcrafted (and possibly biased) features which are considered to be effective descriptors of the presented light curves. In this presentation, a more principled way to tackle the classification problem is shown. The data points are therefore treated as probability densities, and this constitutes a new representation of the static information available of a light curve. Consequently, the classification results are more general and even yield a better classification result, if biased features are neglected.

2015-05-18
11:00
Large Scale Structure Statistics from Reformulated Kinetic Theory
Felix Fabis (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Standard approaches to large scale structure formation usually start out from the collisionless Vlasov equation and introduce a priori approximations to reduce it to the macroscopic equations of pressureless hydrodynamics, limiting validity on smaller scales from the outset. This talk will present a different approach based on the microscopic trajectories of particles in phase-space described in terms of a path integral as found in QFT. I will argue why this should allow us to obtain the time evolution of statistical information like the density power spectrum at smaller scales compared with standard approaches.

2015-05-11
11:00
A new/old model of chondrule formation: splashes of lava droplets after planetesimal collisions
Cornelis Dullemond (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, 106
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Abstract
Meteorites are parts of relatively recently broken-up asteroids. They are interesting because they give us a peek in the interiors of asteroids, which are "messengers" from the planet formation era in our own solar system. When one cuts a meteorite in half, one often sees that its interior consists of ~millimeter size spherical "beads" called chondrules. These little stones are much bigger than the dust from the interstellar medium (~0.1 micron). They are solidified molten lava droplet that must have cooled down quite rapidly after they were formed (in a matter of hours). Apparently during the formation of the solar system there must have been processes going that could heat silicates to temperatures of ~2000 K, form small lava droplets, and then quickly cool these droplets back to the environmental temperature of ~200 K. It has been a debate for over a century what this process could have been. For a long time a particular class of models (collisions between molten planetesimals) were discarded. Recently, however, evidence for this scenario is growing, and a rising number of scientist start to revisit this scenario. One of the main problems is the complexity, making it hard to make quantitative calculations that can be compared with observations. In this talk I will show a super-simplified model that allows actual numbers to be computed. I will argue why, in spite of extreme simplifications, the computed numbers are still expected to be accurate enough to be compared with chondritic data, and how this comparison pans out.

2015-05-03
11:00
NO blackboard Colloquium
none
ITA "blackboard" Colloquium
none, none

2015-04-27
11:00
Detecting Ancient Supernovae with CLASH
Daniel Whalen (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
Supernovae are important probes of the properties of stars at high redshifts because they can be detected at early epochs and their masses can be inferred from their light curves. Finding the first cosmic explosions in the universe will only be possible with JWST, WFIRST and the next generation of extremely large telescopes. But strong gravitational lensing by massive clusters, like those in the Cluster Lensing and Supernova Survey with Hubble (CLASH), could reveal such events now by magnifying their flux by factors of 10 or more. We find that CLASH will likely discover at least 2 - 3 core-collapse supernovae at 5 < z < 12 and perhaps as many as a dozen. Future surveys of cluster lenses similar in scope to CLASH by JWST might find hundreds of these events at z ~ 15 - 17. Besides revealing the masses of early stars, these ancient supernovae could also constrain cosmic star formation rates in the era of first galaxy formation.

2015-04-20
11:00
Star formation in the galactic center
Erik Bertram (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
The galactic center (GC) is a fascinating region for many reasons. It provides an accessible laboratory for studying astrophysical processes under extreme conditions (high densities, high temperatures, high velocity dispersions, etc.), such as those that also occur in distant starburst galaxies. The inner 500pc region is known as the "Central Molecular Zone" (CMZ) and contains the largest reservoir of dense gas in the Milky Way. Theoretically, we would expect to find a very high star formation efficiency (SFE) in such a dense environment. However, the measured SFE in the CMZ is significantly smaller and seems to be suppressed by a factor of > 10 compared to the SFE measured in solar neighbourhood-like clouds. Therefore, the central question is which physical processes regulate the formation of stars in the central few parsecs of our Galaxy. First, I will give a brief overview of the physical environment found in the GC and highlight the current research challenges on this field. Second, I will present a recent set of numerical simulations using the AREPO code and discuss the impact of varying environmental parameters (the density, the turbulence level and the external radiation field) on the SFEs of typical molecular clouds found in the GC.

2015-04-15
14:00
How do Massive Stars accrete? The Lyman excess problem
Michael Smith (University of Kent)
ITA "blackboard" Colloquium
Philosophenweg 12, tba
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Abstract
Some evidence as to how massive stars form in molecular clumps will be discussed. Recent unbiased surveys in the radio provide constraints on the extreme ultraviolet ionising radiation (the Lyman flux or Lyman continuum luminosity) while infrared surveys yield the total bolometric luminosity. It is found that for about 1/3 of the young HII regions, the Lyman continuum luminosity appears to be greater than the value expected for a zero-age main-sequence star with the same bolometric luminosity. I have proposed that this is a result of UV photons emitted from shocked material infalling onto the star itself and/or a circumstellar disk. This raises the question as to how a massive star is constructed.

2015-02-23
11:00
What caused star formation in dwarf galaxies to cease?
Thorsten Lisker (ZAH/ARI)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
The gas disks of galaxies can be partially stripped off when moving through the thin hot plasma in galaxy clusters. Understanding whether this ram pressure stripping played a major or a minor role in removing the star formation fuel from the abundant red dwarf galaxies would be of high relevance for modelling the past environmental impact on galaxies. With our ongoing deep imaging survey of the nearby Fornax galaxy cluster, we can compare its low-mass galaxy population to that of the Virgo cluster, which has a deeper gravitational potential, a denser plasma causing ram pressure, but a similar central mass density. This provides us with the exciting opportunity to search for environmental differences between the red galaxy populations that still show residual star-formation activity: is their abundance and distribution mainly a result of ram pressure, of tidal forces, or of the large-scale environment's galaxy supply?

2015-02-16
11:00
The [CII] 158 micron emission in the disk and bulge of M31
Maria Kapala (MPIA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
The [CII] 158 micron line is typically the brightest far-IR emission line from star-forming galaxies. To use this line as a tracer of star-formation and a diagnostic of ISM conditions, we must understand which phases of the ISM and what gas heating sources are contributing to it. To address these issues, we have assembled a unique set of observations. As a close by massive galaxy, M31 is perfect to understand [CII], as we can resolve individual star-forming regions in the galaxy, but it is a representative of more distant galaxies. The observations includes: [CII] 158 micron and [OI] 63 micron emission line maps from Herschel PACS in the disk, and [CII] individual pointings from ISO in the bulge; fully sampled optical integral field spectroscopy from PPAK on the Calar Alto 3.5m in the disk, and Herschel dust continuum mapping from 70-500 microns. These observations span a range of conditions across Andromeda. We present results on how [CII] correlates with the far-IR continuum on ~50 pc scales. In particular, we find that star-forming regions in M31 do not exhibit a "[CII] line deficit" even in regions where the dust is very warm. Using the optical line emission, we determine the fraction of [CII] emission spatially associated with star-forming regions. Our method implies a high fraction ~20-90% of [CII] emission is coming from diffuse regions. These diffuse regions appear to dominated by the UV interstellar radiation field, which we infer from the Pan-Chromatic Hubble Andromeda Treasury data to be dominated by B stars. We consider as well photon leakage from the SF regions. We investigate relation between [CII] emission and SFR surface density. On ~50pc scales it is sub-linear in most of the fields, but it approaches 1-1 relation when averaged over whole fields (~700pc scales), which is in agreement with other extragalactic studies on similar scales. Interestingly, [CII]/TIR decreases with radius by a factor of ~3 between 16 and 7 kpc, but then it rise up again in the bulge. Interestingly, [CII] emission as bright as our fainter/diffuse regions adjacent to the SF regions in the disk, yet, there is no proof of any SF activity in the bulge based on Halpha, dust and stellar studies. We discuss possible explanations.

2015-02-09
11:00
Title: Investigating molecular gas in nearby galaxies through its velocity dispersion
Anahi Caldu Primo (MPIA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
Constraining the physical properties of molecular gas is essential to understand the processes involved in star formation, and ultimately understand the present day galactic morphologies. Observations of nearby galaxies allow us to study the properties of this gas phase from an outside perspective, and in this way escape the projection effects present in Galactic studies of molecular gas. By analyzing the velocity dispersions of molecular gas on scales of ~500 pc we find evidence that points to the existence of a more diffuse component, which would not be inside giant molecular clouds, and therefore, would not necessarily be directly involved in the star formation process.

2015-02-02
11:00
Forecasting with higher order Fisher matrices
Elena Sellentin (ITP)
ITA "blackboard" Colloquium
Philosphenweg 12, 106

2015-01-26
11:00
Physical characterization of brown dwarfs
Elena Manjavacas (MPIA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
The initial mass function describes the distribution of masses for a population of stars and substellar objects when they are born. It defines the evolution of a population of stars and provides constrains on the star formation theory. The determination of the initial mass function in the substellar regime is still an open question in Astrophysics. Brown dwarfs do not have enough mass to sustain hydrogen fusion. As a consequence, mass and age are degenerate for these objects. An older high mass object may be indistinguishable from a younger low mass object. In this PhD thesis, through the characterization of brown dwarfs using several observational methods, I work towards solving the general problem of constraining the substellar initial mass function. In my first project, I calculated trigonometric parallaxes of a sample of six cool brown dwarfs. I determined the luminosity for our objects and I found that one of them might be a brown dwarf binary. In my second project, I confirmed the youth of seven brown dwarfs (ages between 1 and 150 Myr) using spectroscopic data. In the last project of this PhD thesis, I aimed to refine the brown dwarf binary fraction using spectroscopic data in the optical and in the near infrared for 22 brown dwarfs. I found six new brown dwarf binary candidates, two of them were previously known. The determination of distances, ages and the refinement of the brown dwarf binary fraction in this PhD thesis will contribute to the determination of the initial mass function. In the next years, the Gaia satellite, the James Webb Space Telescope and the E-ELT will provide new data, allowing the discovery of new brown dwarf binaries, the constraining of atmospheric and evolutionary models, and the refinement of the intial mass function.

2015-01-19
11:00
A semi-analytic prediction for the cluster X-ray temperature function without reference to mass
Christian Angrik (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106

2015-01-12
11:00
Planets and Binary Star Systems
ITA "blackboard" Colloquium
Philosophenweg 12, 106

2014-12-15
11:00
Chemical abundances and kinematic properties of Galactic globular clusters
Nikolay Kacharov (ZAH/LSW)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
High-resolution spectroscopic studies of globular clusters (GC) have been a major break-through in our understanding of these ancient stellar systems. The observations carried out in the last decade revealed a very heterogeneous population of objects -- not only have GCs undergone a complex star formation history resulting in large chemical inhomogeneities common amongst all of them, but there are also non-negligible cluster-to-cluster differences in both chemistry and kinematics that are not yet fully understood. I will present the first ever high-resolution studies of two poorly known GCs M75 and NGC4372. M75 is a massive, relatively metal-rich, outer halo GC with extremely broad horizontal branch. We found that, besides the typical light-element variations, it shows a marginal spread in metallicity and likely has a small s-process rich population. Surprisingly for its metallicity, the majority of the analysed stars lack s-process enrichment, which hints to a formation in an environment that built metals very quickly. On the other hand, NGC4372 is one of the most metal-poor GCs in the Galaxy. It is found in the inner halo and has experienced multiple disk crossings. The chemical analysis revealed it as a standard representative of the old, metal-poor halo group. More interesting are its structural and kinematic properties as the cluster has an unusually high intrinsic rotation for its metallicity and appears to be rotationally flattened.

2014-12-08
11:00
Evolution of linear perturbations in Lemaîre-Tolman-Bondi models
Sven Meyer (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
We try to apply linear perturbation theory to a special class of inhomogeneous cosmological models, namely Lemaîre-Tolman-Bondi (LTB) models. They describe inhomogeneous dust solutions of Einstein's field equations that are isotropic around a central point, but anisotropic everywhere else. Those models became attractive since they allow to fit the distance-redshift relation of Type Ia-Supernovae without need of a cosmological constant or dark energy fluid. In this context, our galaxy is assumed to be located close to the centre of a Gpc-scale underdense region, a so called void. I'll try give an overview on LTB models, point out a possibility to set-up linear perturbations and show first numerical results for their time evolution.

2014-12-01
11:00
Probing spatial homogeneity with LTB models
Matthias Reidlech (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
Probing spatial homogeneity with LTB models\$Is our Universe spatially homogeneous on the largest observable scales? To investigate this question, we developed a flexible method based on spherically symmetric, but radially inhomogeneous Lemaître-Tolman-Bondi (LTB) models that allowed us to study a wide range of non-Copernican cosmological models. We employed a Monte Carlo sampler to systematically vary the shape of the (local) matter density profile and determined the likelihood of the sampled models given a selected set of observational data. After analysing non-Copernican models with and without cosmological constant, we conclude that the observational data considered provide no statistical evidence for deviations from spatial homogeneity on large scales. However, more accurate constraints are required to ultimately confirm the validity of the Copernican principle.

2014-11-24
11:00
Transforming gravity
Miguel Zumalacarregui (ITP)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
I will outline recent developments in alternative theories of gravity from the point of view of understanding their properties and characterizing their cosmological implications.

2014-11-17
11:00
A new method for the construction of N-body galaxy models in collisionless equilibrium"
Denis Yurin (MPIA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106

2014-11-10
11:00
Relationship between density structure and evolutionary class of molecular clouds
Jorge Abreu-Vicente (MPIA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
We perform the first Galactic scale study of the relationship between the column density (N(H2)) as traced by dust emission and their evolutionary status as measured by their stellar content. We analyze a sample of 195 molecular clouds located at distances < 5.5 kpc, identified from the ATLASGAL 870 &#956;m data. We define three evolutionary classes within this sample: starless clumps (SLCs), protostellar clouds (PSCs), and Hii regions. We find a strong dependence between the N(H) probability density function (N-PDFs) shape and the evolutionary class of a molecular cloud: the N-PDFs of SLCs are log-normal functions while PSCs and Hii regions exhibit log-normal components at low N(H) values and power-law components at high N(H) values. The N-PDFs suggest evolutionary time-scales of &#8764;0.1Myr for SLCs, &#8764;0.3Myr for PSCs, and &#8764;0.7Myr for Hii regions. Finally, we show that the integral of the N-PDFs, the dense gas mass fraction, depends on the total mass of the regions as measured by ATLASGAL: more massive clouds contain greater relative amounts of dense gas across all evolutionary classes.

2014-11-03
11:00
Alternatives to particle Dark Matter
Juri Smirnov
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
Several models will be discussed, which have the potential of describing the dark matter component of the universe but do not contain elementary particles. Drawbacks and advantages of this models will be considere.

2014-10-27
11:00
On the effective field theory for cosmological clustering
Gerasimos Rigopopulos (ITP)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
I will discuss an effective field theory for cosmological large scale structure where an effective gravitational viscosity damps the initial power and a stochastic force accounts for power generated from very short, highly non-linear scales that is uncorrelated with the initial power spectrum. The theory should arise in the context of standard CDM but with a coarse graining introduced and, contrary to standard perturbation theory, is renormalizable.

2014-10-20
11:00
Statistical properties of dark matter mini-haloes at z >= 15 & the criterion for HD formation in the early universe
Mei Sasaki (ZAH/ITA)
ITA "blackboard" Colloquium
Philosphenweg 12, 106
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Abstract
Understanding the formation of the first objects in the Universe critically depends on knowing whether the properties of small dark matter structures at high redshift (z >= 15) are different from their more massive lower-redshift counterparts. To clarify this point, we performed a high-resolution N-body simulation of a cosmological volume 1 h-1 Mpc comoving on a side, reaching the highest mass resolution to date in this regime. We compare our results with literature results and discuss differences and similarities with more massive dark matter haloes. If time permitted, I would also like to discuss conditions for HD formation in gas clouds inside such dark matter mini-haloes.

2014-07-21
11:00
Pebble-Pile Planetesimal Formation
Joanna Drazkowska (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Planets form in disks surrounding young stars from micron-sized grains that are already present in the interstellar medium. The planet forming processes are ubiquitous: there is almost 2000 exoplanets already known and indirect methods indicate that every star in our Galaxy has at least one planet. However, how the planets form remains a mystery. Analytical and numerical models of dust evolution aim to explain the growth of the primordial grains through 40 orders of magnitude in mass to >1000 km-sized planets. However, the growth encounters serious obstacles already at the beginning of the size range, at mm-sizes, and these are known as growth and drift barriers. I will briefly review the issues and then focus on the possibility of planetesimal formation by gravitational collapse of dense dust clumps produced by two-fluid instability known as the streaming instability. It is a potentially very efficient way of overcoming both growth and drift barriers. However, it was shown that the strong clumping, which can lead to planetesimal formation, requires relatively abundant population of mm to cm-sized aggregates: pebbles. State-of-the-art streaming instability models do not take into account realistic size distributions resulting from dust collisional evolution. I investigate whether sufficient amount of large grains can be produced by dust aggregates sticking and what is the interplay of the coagulation and planetesimal formation in this scenario.

2014-07-14
11:00
CO chemistry in molecular cloud simulations
László Szűcs (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
TBD

2014-07-07
11:00
Correlation and Causation strike back
Andrea V. Macciò (Max-Planck Institute for Astronomy)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Correlations between global galaxy properties, sometimes known as scaling relations, have been extensively used by both theorists and observers to better understand the path of galaxy formation. In my last blackboard colloquium I showed that correlation does not always imply causation, as for example in the case of the bulge-black hole mass relation. In this talk, I will look at the other side of the causation-correlation problem. Namely, I would like to check if a clear causal relation between two physical quantities in a given galaxy formation model always provides the expected relation between the derived observable quantities. I will demonstrate that a lot of care should be taken before drawing conclusions on the underlying physics of galaxy formation from simple scaling relations.

2014-06-30
11:00
Resolving the statistical properties of supersonic turbulence in numerical simulations
Lukas Konstandin (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Observations show that supersonic turbulence is prevalent in star forming clouds. The density and velocity field in these clouds indicate complex, chaotic, and filamentary structures, where turbulent motions interact with shocks. Therefore, understanding the properties of turbulence is a prerequisite for developing a comprehensive theory of star formation in the ISM. We have studied three-dimensional numerical simulations of driven, isothermal, turbulence with r.m.s. Mach numbers ranging from the subsonic to the highly supersonic regime. We focused on the influence of two extreme cases for the driving mechanism by applying a purely solenoidal (divergence-free) and a purely compressive (curl-free) forcing field to drive the turbulence. Incompressible and compressible turbulence theories predict a power-law for the energy-density power spectrum in the inertial range, where the scaling exponent plays a key role to distinguish between the different cases. I will introduce a hierarchical Bayesian method for estimating the parameters of the power spectrum and compare it with classical linear regression methods. The Bayesian measurements reveal that the scaling exponents span the whole range of theoretical predictions depending on the definition of the fitting/inertial range. In addition, I will talk about the interplay between the highly turbulent velocity field and the resulting statistical properties of the density distribution.

2014-06-16
11:00
Dirty lensing
Matteo Maturi (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
All probes to constrain cosmological models have their points of strength and weakness. For instance, in measuring luminosity distances of supernovae we have to deal with absorption, not very well understood calibrations, etc.; in measuring the baryonic acoustic oscillations we have to assume a fiducial cosmology in first place, this is to recover physical scales out of angles, and deal with non-linearities of structure formation; in observing the CMB power spectra diverse galactic foregrounds, jpg decompression, etc. have to be confronted. This applies also for gravitational lensing. Although it is based on a very clean physics, its actual measure is complicated by intervening spurious contributions and observational effects. In this talk I am going to discuss the main aspects that have to be faced to recover this weak and yet powerful signal.

2014-06-02
11:00
Saturation and Efficiency of the Turbulent Dynamo
Jennifer Schober (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Magnetic fields are ubiquitous in the present-day Universe. The high field strengths result probably from weak seed fields that have been amplified by magnetohydrodynamical dynamos. In my talk I will summarize the main properties of the turbulent small-scale dynamo, which amplifies magnetic fields by randomly stretching, twisting and folding the field lines. I will focus on the saturation process that can be modelled theoretically via an increasing magnetic Reynolds number. With a simple model for the evolution of the turbulent and magnetic energy spectra, we can estimate the amount of turbulent kinetic energy that is converted into magnetic energy by small-scale dynamo action.

2014-05-26
11:00
Small scale structures and turbulence of the interstellar medium
Kengo Tachihara (Nagoya University)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Interstellar turbulence is believed to be one of the most important but unknown factor controlling morphological and physical evolutions of clouds. The origin of the interstellar turbulence is not well described yet, but a promising theoretical model of the two-phase medium with thermal instability has been proposed. Our observations have detected several 1000 AU scale cloudlets at a cloud surface, suggesting that they are formed as a result of the thermal instability in compressed medium. Similarly, recent high resolution HI 21 cm observations revealed a few times ~0.1 pc scale filamentary and clumpy structures of cold atomic hydrogen gas. We suggest that cold HI gas undergoes thermal instability and form dense condensations with turbulent random motion where molecular gas is being formation.

2014-05-19
11:00
The Flea Circus
Christoph Pfrommer (HITS)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
This is a fun talk originally designed as a habilitation talk - and delivered here upon great demand. The central question I try to answer is: how high can a flea jump in comparison to a human being or other animals such as a locust? I will develop a simple model, which does an astonishing job in explaining the measured jump heights. However, this model suffers from a large power requirement for small animals. Click beetles overcome this problem by storing energy in their exoskeleton. To further improve upon the simple model, we need to consider the air resistance. I show that this is the limiting factor for the jump heights of small animals, which have a large surface area-to-volume ratio. Using dimensional analysis by means of Buckingham's Pi theorem enables us to obtain a general description of the air resistance. However, during the flea jump, the characteristics of the flow transits from a turbulent to a laminar flow pattern. This effect modifies the jump height in accordance with the data. To account for the audience, I spice this talk up with a fun astrophysical application and show which drag forces are relevant for a spiral galaxy orbiting in a galaxy cluster.

2014-05-12
11:00
Principal Component Analysis of Molecular Clouds: Can CO reveal the dynamics?
Erik Bertram (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Turbulence plays a fundamental role in the star formation process. Observations have shown that the velocity dispersion of molecular clouds is correlated with its size and mass, following a power-law. We use Principal Component Analysis (PCA) to measure the line width size relation and to study ISM dynamics in numerical simulations of typical molecular clouds with time-dependent chemical and radiative transfer. We investigate the slope of the pseudo structure function computed by PCA for individual chemical components: the total density, H2 density, 12CO density, 12CO J = 1 -> 0 intensity and 13CO J = 1 -> 0 intensity. We estimate power-law slopes for different chemical species that range from 0.5 to 1.4, in good agreement with observational results, and demonstrate that optical depth effects can influence the PCA. We show that when the PCA succeeds, the combination of chemical inhomogeneity and radiative transfer effects can influence the observed PCA slopes by as much as +/-0.1. The method can fail if the CO distribution is very intermittent, e.g. in low-density clouds where CO is confined to small fragments.

2014-05-05
11:00
The search for faint dwarf galaxies in the Local Volume with tiny telescopes
David Martinez-Delgado (Zentrum für Astronomie, Astronomisches Recheninstitut, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
The L-CDM paradigm predicts a large number of dwarf galaxies in the Local Volume, only a small fraction of which have so-far been discovered. It is thus of broad interest for galaxy formation theory to carry out a full inventory of the numbers and properties of dwarfs, both satellites and isolated. Small aperture (10-15 cm) telescopes, combined with the new generation of commercial CCD cameras, can be valuable instruments for finding faint dwarf companions around the Local Group spirals. The short focal ratio of these telescopes allows these stellar systems to be traced as unresolved, diffuse light structures -- a similar approach was sucessfully used to discover stellar tidal streams around nearby spirals. In this talk, I discuss this new project to search for faint nearby satellites with amateur telescopes. I also present our first discovery: DGSAT I, a probable isolated dwarf spheroidal galaxy projected on a halo field of M31, that was previously missed by surveys based on resolved star counts and on the H-I radio line.

2014-04-28
11:00
A recipe for disk photoevaporation
Jonathan Ramsey (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
The study of the dispersal of protoplanetary disks is important for, among other things, constraining the conditions of planet formation. In this talk, I will discuss one such process for disk dispersal: photoevaporation due to far-UV stellar radiation, and our progress towards self-consistent models.

2014-04-14
11:15
Cosmological constant, dark energy and Lovelock's theorem
Matthias Bartelmann (Zentrum für Astronomie, Institut für Theoretische Astrophysik)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
In this talk, I shall not present new results, but remind of results achieved by David Lovelock in 1971 and 1972. Reduced to some degree, Lovelock proved that any sufficiently simple metric theory of gravity in four dimensions must equate the energy-momentum tensor to a linear combination of the Einstein and metric tensors. This implies that, under the assumptions made, Einstein's general relativity is unique, that gravity may naturally have a repulsive component and that any such theory must have two coupling constants. One of these constants is the familiar Newtonian gravitational constant, the other is the cosmological constant. I intend to discuss Lovelock's theorems, their implications and their possible consequences for dark energy.

2014-02-03
11:00
Black Hole scaling relations - overview and recent developments
Ronald Laesker (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
I will give an overview of the current status of the scaling relations between central Supermassive Black Holes (SBHs) and their host galaxies' global properties. These relations are thought to reflect non-gravitational interactions as well as simultaneous merging of SBHs and galaxies in the LCDM hierarchy. The balance and details of these processes are under debate but likely essential for our understanding of galaxy evolution and the (yet unknown) origin of SBHs. Although the scaling relations are generally considered as established and significant, their empirical characterization continues to evolve and appears still uncertain. I will review recent developments in the field with a focus on the SBH mass - (bulge) galaxy mass relation, relevant observational techniques and challenges, and touch on theoretical as well as statistical considerations.

2014-01-27
11:00
Chondrule Formation in Large Scale Shock Waves
Sebastian Stammler (ZAH/ITA)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
Chondrules are small millimeter-sized crystalline spherules found mainly in chondrites - a class of meteorites. From experiments we know that they must have been formed in a high temperature event often referred to as 'chondrule forming event'. The exact mechanism of this event is still unknown and under debate. Among the many proposed scenarios, one suggests the processing of dust in nebular shock waves. I will present a model for processing dust in large-scale shock waves, such as those produced by gravitational instabilities, and I will discuss the consistency with chondrule formation of the resulting temperature history of the dust.

2014-01-20
11:00
Approaching peculiar velocities at redshift z∼0.1
Martin Feix (Israel Institute of Technology)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
Peculiar motion introduces systematic variations in the observed luminosity distribution of galaxies. As recently proposed, this allows one to measure cosmological bulk flows and to reconstruct the peculiar velocity field from large galaxy redshift surveys. Technically, this is achieved by maximizing the probability to estimate a galaxy's absolute magnitude given its observed apparent magnitude and redshift. Thus the approach is independent of galaxy bias and traditional distance indicators, which are prone to systematic errors. Using the NYU value-added galaxy catalog based on the SDSS DR7 data release, we apply this method to measure the cosmological bulk flow out to a depth of around 400 Mpc/h. Including higher-order moments and using the corresponding posterior likelihood, we further estimate the (angular) velocity power spectrum in different redshift bins. Finally, we attempt to constrain cosmological parameters such as sigma_8 directly from the SDSS data.

2014-01-13
11:00
Mixing of Supernova Ejecta into the Intracloud Medium
Christian Baczynski (Institut für Theoretische Astrophysik, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
A long standing question is the efficiency with which metals, ejected by SN, are mixed into the surrounding medium. I very briefly review the current understanding of the mixing process and modeling efforts from the stellar interior to the ISM. Finally I'll discuss a work in progress on how we plan to model the mixing of ejected metals on a molecular cloud scale.

2013-12-16
11:00
Christmas Special Colloquium: The "Science" of Politics or the "Politics" of Science?
Erik Bertram (ITA and Christlich Demokratische Union Deutschlands)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
I will discuss similarities and differences between the two subjects "Science" and "Politics". What can Scientists learn from Politics? What can Politics learn from Scientists? I will argue that political processes play a non-negligible role, and should not be ignored, in a common scientific life, i.e., when promoting our own scientific work. What is the Politics behind Science? Most of the results that I will present are derived by the relatively new discipline of "social physics". I will try to connect the new knowledge on this field with my own experience in Politics to a Physicist's daily work.

2013-12-09
11:00
Dynamical evolution of Star-Forming Regions
Richard Parker (Eidgenössische Technische Hochschule, Zürich, Switzerland)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
I will discuss the dynamical evolution of star-forming regions and highlight the initial conditions that lead to bound (later open) star clusters, and those that lead to unbound associations. The distinction is important because bound clusters are less likely to contribute stars to the Galactic field, as opposed to unbound associations. I will present a new method of distinguishing these two scenarios based on the spatial structure of the stars and the local surface density around stars.

2013-12-02
11:00
A Bayesian approach to Open Cluster Distance determination
Max Palmer (University of Barcelona)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
With the launch of GAIA approaching, we can expect vast quantities of precise parallax data to become available in the coming years. The use of parallaxes is non-trivial, as there are numerous statistical biases and sample selection effects which must be taken into account. I will present a Bayesian method for open cluster distance determination, based on Maximum Likelihood Estimation, which utilizes all available information on a sample and avoids the need for posteriori corrections of statistical biases or selection effects.

2013-11-25
11:00
Gamma-Ray Emission from AGN and Radio Galaxies
Frank Rieger (MPIK)
ITA "blackboard" Colloquium
Philosophenweg 12, Seminar Room R 106 (1st floor)
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Abstract
In the last couple of years, radio galaxies have emerged as a new gamma-ray emitting source class on the extragalactic sky. Harboring jets that are substantially inclined with respect to the observer, their appearance is not much affected by relativistic Doppler boosting effects. As such they offer the unique potential to probe some of the fundamental physical processes in Active Galactic Nuclei that appear otherwise masked in classical blazar-type sources. I will highlight some of the key questions in the field, discuss the relevance of the observed gamma-ray characteristics and, taking the archetypal M87 as an example, report on the theoretical progress achieved within recent years.

2013-11-18
11:00
Constraints on the Stellar Initial Mass Function and Dark Halo Response in Early-Type Galaxies from the Fundamental Plane
Aaron A. Dutton (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
The form of the stellar initial mass function (IMF) and the response of dark matter haloes to galaxy formation are two fundamental unknowns, which are important in many areas of astrophysics. For example, the IMF is needed in order to convert observations of integrated stellar light into stellar masses and star formation rates (two fundamental parameters in galaxy evolution studies), and for modelling the supernova rates, chemical evolution, and production of ionizing photons in galaxies. Furthermore, the form of the IMF itself contains clues to the physics of star formation. The response of dark matter haloes to galaxy formation is needed in order to constrain the nature of dark matter from observations of the structure of dark matter haloes as well as for interpreting indirect dark matter detection experiments. For many years the IMF and dark halo response were thought to be universal: The IMFs of external galaxies are the same as measured in the Milky Way and dark matter haloes contract adiabatically in response to galaxy formation. However, recent observations and numerical simulations have cast doubt on these assumptions. In this talk I will show how the Fundamental Plane can be used to place constraints on the variation of the IMF and dark halo response in early-type galaxies.

2013-11-11
11:00
On the fraction of stars forming in clusters as function of galactic environment
Angela Adamo (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Recent observations, as well as theoretical studies, have suggested that stellar cluster formation may depend on local and global environmental properties. In particular, the fraction of stars that form within long-lived bound clusters (Γ) may depend on environment, with indications that it may be higher in the more extreme environments of high star formation rate density galaxies. How Gamma varies has important implications on the use of clusters to determine the star formation histories of galaxies as well as our understanding of the star formation process itself. In my contribution I will focus on the role of the galactic environment in determining the fraction of stars forming in bound clusters. If cluster formation would only be dominated by size-of-sample effect, one would expect Gamma to be constant (the number of clusters increase as function of increasing star formation rate, SFR, but overall the fraction of star formation happening in star clusters is the same). I will present the results achieved with the study of the cluster populations in the face-on spiral galaxy M83. Gamma has been derived for the whole system as well as in different regions of the same galaxy. Overall we find that Gamma decreases strongly as a function of galactocentric radius, by a factor of ~5 over the inner ~8 kpc, in agreement with recent theoretical predictions and decreasing trends observed in the gas surface density of the galaxy.

2013-11-04
11:00
Embedded and open clusters in the inner Galaxy
Esteban Morales (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
Stars are born within dense clumps of giant molecular clouds, and constitute young stellar agglomerates known as embedded clusters, which only evolve into bound open clusters under special conditions. I will present some of the results from a forthcoming paper (Morales et al. 2013, accepted, arXiv:1310.2612), in which we statistically study a compiled sample of 695 embedded and open clusters in the inner Galaxy (|l| < 60 deg and |b| < 1.5 deg), in particular investigating their interaction with the surrounding molecular environment and the differences in their evolution. I will discuss how the distribution of the molecular material as traced by the ATLASGAL submm continuum survey can be used to observationally define an open cluster or/and embedded cluster, and what is the physical meaning of this classification. Based on this definition, in our paper we study the distance distribution, completeness, and age distribution for embedded and open clusters separately, and quantify the dissolution fraction of embedded clusters.

2013-10-28
11:00
The Properties of Molecular Gas and Star Formation in Galaxy Centers
Karin Sandstrom (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
The efficiency at which interstellar gas is converted into stars is one of the major factors governing the evolution and observable properties of galaxies at all redshifts. In the Milky Way we can study the star formation process in great detail, but only over a limited range of environmental conditions. We must move to nearby galaxies to expand this range. I will present the results of recent work using resolved maps of interstellar gas and dust to trace the amount of molecular gas in nearby galaxies on ?kiloparsec scales. One of the key results from this study is the decrease in the "CO-to-H2" conversion factor (Xco) in some galaxy centers. These galaxy centers also appear to have enhanced star-formation efficiency. I will outline some of the possible drivers for these changes in molecular gas and star formation properties and summarize the evidence we currently have for the various scenarios. Finally, I will discuss the future observational studies with ALMA that will provide key insights into what is happening on the scale of individual star-forming clouds in the centers of nearby galaxies.

2013-10-14
11:00
Searching for old open clusters
Stefan Schmeja (Astronomisches Recheninstitut, Heidelberg, Germany)
ITA "blackboard" Colloquium
Philosophenweg 12, R 106
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Abstract
There is a striking lack of old (t > 1 Gyr) open clusters in the solar neighbourhood (d < 1 kpc). Old open clusters show a larger scale height, so in combination with their small distance they may also lie at higher Galactic latitudes, whereas all systematic open cluster searches have been restricted to areas around the Galactic plane, and having a large angular extent, they do not stand out prominently as overdensities from the field. These are probably the reasons why those clusters have not been detected so far. Our systematic search for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue at Galactic latitudes |b| > 18.5° resulted in the discovery of 137 new open clusters with ages between 0.2 and 5 Gyr and distances < 3 kpc. Nevertheless, the volume < 1 kpc is still expected to contain about 60 unknown old clusters that probably have escaped our search.

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