Talks, Seminars, Events

Abstract
Weak lensing by large-scale structure is one of the most promising techniques to learn more about the nature of dark energy by mapping the dark matter distribution in the Universe as a function of distance. Weak lensing has also developed into the main tool to determine cluster masses, critical for their use for cosmology, but can also be used to study the dark matter halos of galaxies.I will review the recent progress in this active area of research and discuss the prospects for future projects, such as Euclid.

Abstract
Globular clusters are important tools to help us understand how galaxies form and evolve. Globular cluster formation tells us about the conditions of extreme star formation while their survival from high redshift tells us about the processes of galaxy assembly. Being much brighter than red giant stars, globular clusters allow the stellar populations of galaxies to be studied at much greater distances, and thus a wider range of galaxy masses, environments and morphologies to be studied than can be with resolved stars. Using data from the WAGGS survey of massive star clusters in the Milky Way and its satellite galaxies and the SLUGGS survey of the globular cluster systems of massive early-type galaxies, I will talk about how we can measure the metallicities of globular clusters using the strength of the calcium triplet in integrated light. Using globular cluster metallicity distributions and the relationships between globular cluster colour and metallicity, I will present evidence that different galaxies with similar masses experienced different formation histories. I will compare these observations with the predictions of the E-MOSAICS cosmological simulations of the formation of globular cluster systems. I will also talk about how the E-MOSAICS simulations have allowed to us to understand how globular cluster colour distributions vary

Abstract
The chromosphere is the interface between the interior of the Sun, where its magnetic field is generated, and the hot outer corona that drives the solar wind and causes space weather. It is difficult to understand: the chromosphere is the interface where the dynamics change from gas-pressure driving to magnetic-force driving, radiation transport changes from optically thick to optically thin, the gas state changes from neutral to ionised and from local thermodynamic equilibrium (LTE) to non-equilibrium conditions, and the MHD approximation is not sufficient to fully describe its physics. In this talk I will give an overview of the major open questions on understanding the solar chromosphere, and dicuss how observations, simulations, and advanced analysis tools can be used to shed light on these questions.

Abstract
We will review the role of classical variables crossing the Instability Strip in our understanding of galaxy formation and evolution. We will discuss the fundamental role of photometric surveys of resolved stellar populations in our Galaxy and beyond. We have recently analysed the old stellar populations, as traced by RR Lyrae stars, observed in low density environments such as the Galactic halo and dwarf galaxies surrounding the Milky Way. This investigation has revealed that small satellites can not have had a major role in building up this old component of the Galaxy, whereas the contribution of more massive dwarf galaxies like the Large Magellanic Cloud (LMC) and/or the still merging Sagittarius dwarf spheroidal galaxy can be significant. In the near future thanks to Gaia and to the Large Synoptic Survey Telescope these kind of studies will gain much more detail and will be largely extended to the the outskirts of our Local Group (distances 1Mpc).

Abstract
Type Ia supernovae are the incredibly luminous deaths of white dwarfs in binary systems. They play a vital role in chemical enrichment, galaxy feedback, stellar evolution, as well as being instrumental in the discovery of dark energy. However, what are their progenitor systems, and how they explode, remains a mystery. There is increasing observational evidence that there are multiple ways in which white dwarfs can explode. I will review the status of what we know about the stellar systems that produce Type Ia supernovae, as well as discuss the recently discovered zoo of peculiar transients that are also predicted to result from the explosions of white dwarfs, such as He-shell mergers, tidal disruption events, violent mergers. Distinguishing between these explosion scenarios and understanding their diversity is vital for producing the best samples for future precision measurements of the cosmological parameters.

Abstract
I will present the three-dimensional structure of the Magellanic System using over 9000 classical Cepheids (CCs) and almost 23000 RR Lyrae (RRL) stars from the OGLE Collection of Variable Stars. The vast coverage of the OGLE-IV data and the high completeness of the sample allowed us to study the Magellanic System in great detail. The CC distribution shows that, contrary to previous results, the LMC bar is not offset from the galaxy's plane. Moreover, the northern arm is located closer to us than the overall sample. The CCs in the SMC have a non-planar distribution that can be described as an ellipsoid extended almost along the line of sight. RRL stars reveal a very regular distribution in both Magellanic Clouds. In the Magellanic Bridge, there is no evidence of an actual physical connection between the Clouds in the RRL distribution. We only see the two halos overlapping. The few CCs in the Bridge seem to form a genuine connection between the Clouds. Their on-sky locations are well correlated with young stars and the neutral hydrogen distribution.

Abstract
Observations of Galactic dust are a highlight and a lasting legacy of the Planck space mission. Spectacular images combining the intensity of dust emission with the texture derived from polarization data have received world-wide attention and become part of the general scientific knowledge. Beyond this popular success, the dust maps are an immense step forward for Galactic astrophysics, greatly superseding earlier observations. Planck has provided us with the data needed to statistically characterize the structure of the Galactic magnetic field and its coupling with interstellar matter and turbulence. Planck multi-frequency observations have also opened a new perspective on interstellar dust, upsetting existing models. Futrhermore, the astrophysics of dust emission has become inter-connected to a paramount objective of observational cosmology: the quest for curl-like (B-mode) polarization of the cosmic microwave background expected to arise from primordial gravitational waves produced during the inflation era in the very early Universe. I will introduce these science topics and highlight key results and perspectives of on-going research.

Abstract
A cryogenic electrostatic storage ring, the CSR, has been taken into operation at the Max Planck Institute for Nuclear Physics. With laboratory astrophysics studies as one of its goals, the machine was built to store fast ion beams in extremely high vacuum, screened against terrestrial thermal radiation, and over times reaching up to an hour. Through the few-Kelvin cryogenic environment and the extreme suppression of collisional perturbations, in contrast to other ion-trap arrangements, low-energy molecular excitations (rotation, low-frequency vibrations or isomeric conformations) can evolve freely and thermalize purely by radiation. Controlled interaction is applied to the circulating ions by laser radiation or by merged or crossed particle beams (electrons, neutral atoms). Event-by-event multi-fragment detection, even for neutral products, and the high accuracy of charged-particle current measurements enable reaction cross-section and branching ratio measurements. The presentation focuses on laboratory studies of rotational excitation in small molecular ions and on inelastic collisions of molecular ions with cold electrons (dissociative recombination and collisional (de-)excitation). First experimental results, planned studies on polyatomic species relevant in interstellar and circumstellar media, and further projects to study complex molecular ions will be addressed.

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.

Abstract
In 1992 the Ulysses spacecraft detected a stream of interstellar dust grains passing our solar system. The Stardust mission succeeded in collecting and identifying seven particles of likely interstellar origin. They are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from representative models of interstellar dust inferred from astronomical observations and theory, but the relatively large grains found are biased to the high mass tail of the interstellar population and may not be truly representative. In-situ analyses of the Cosmic Dust Analyser on-board the Cassini spacecraft obtained between 2004 and 2013 yielded the first mass spectra of grains from the Local Interstellar Cloud. These 36 interstellar grains can be clearly identified and distinguished from Saturn bound dust by their direction and high velocity, and their mean mass is consistent with the typical ISD size inferred from astronomical observations. Mass spectra and grain dynamics suggest the presence of magnesium-rich grains of silicate and oxide composition, partly with iron inclusions. Major rock-forming elements (magnesium, silicon, iron, and calcium) are present in approximately cosmic abundances, with only small grain-to-grain variations, but sulfur and carbon are depleted. The ISD grains in the solar neighborhood appear to be homogenized, likely by repeated processing in the interstellar medium.

Abstract
Meteoritic evidence shows that the Solar system at birth contained significant quantities of short-lived radioisotopes (SLRs) such as 60Fe (with a half-life of 2.6 Myr) and 26Al (with a half-life of 0.7 Myr) produced in supernova explosions and in the Wolf-Rayet winds that precede them. Proposed explanations for the high SLR abundance include formation of the Sun in a supernova-triggered collapse or in a giant molecular cloud (GMC) that was massive enough to survive multiple supernovae (SNe) and confine their ejecta. However, the former scenario is possible only if the Sun is a rare outlier among massive stars, while the latter appears to be inconsistent with the observation that 26Al is distributed with a scale height significantly larger than GMCs. In this talk, we present a high-resolution chemo-hydrodynamical simulation of the entire Milky Way Galaxy, including stochastic star formation, HII regions, SNe, and element injection, that allows us to measure for the distribution of 60Fe/56Fe and 26Al/27Al ratios over all stars in the Galaxy. We show that the Solar System's abundance ratios are well within the normal range, but that SLRs originate neither from triggering nor from confinement in long-lived clouds as previously conjectured. Instead, we find that SLRs are abundant in newborn stars because star formation is correlated on galactic scales, so that ejecta preferentially enrich atomic gas that will subsequently be accreted onto existing GMCs or will form new ones. Thus new generations of stars preferentially form in patches of the Galaxy contaminated by previous generations of stellar winds and supernovae.

Abstract
Using images from Subaru and spectroscopy from Keck, the SLUGGS survey aims to understand the formation and evolution of massive early-type galaxies. For a sample of 25 galaxies, we probe the detailed kinematics and metallicities of their field stars to 3-4 effective radii and their globular clusters to 8-15 effective radii. From these data we have derived 2D maps of the halo stellar kinematics and metallicity. We find changing kinematic signatures as we probe from the galaxy inner to halo regions. Using JAM models we have derived the mass density slope and compare it to the latest cosmological simulations. The SLUGGS survey has also collected over 4000 globular cluster radial velocities (the largest sample to date). From this data we have derived the dynamical mass and dark matter fractions for our galaxies. We compare our results with the latest predictions from the Illustris simulations. We also explore the kinematics and scaling relations of globular cluster systems. These results are placed in the context of two-phase galaxy formation. And if time allows, I will briefly mention recent work on ultra-diffuse galaxies using the Keck telescope.

Abstract
Pulsar Winds and the nebulae which they energize (PWN) are among the most enigmatic objects in astrophysics. They consist of a relativistic, magnetized, electron-positron plasma that forms a compact cloud surrounding young pulsars. Their nonthermal synchrotron and inverse-Compton emission is detected from the radio band to very high energy (TeV) gamma-rays, where they are the dominant galactic source population. The radio-to-infrared spectra of PWN are flat, indicating a remarkably efficient particle acceleration mechanism, able to transfer most of the system energy into a tiny fraction of particles. Despite decades of research, the mechanism responsible for accelerating these particles has remained elusive, and poses one of the greatest challenges in particle acceleration theory. In this talk I will give an introduction to the physics of pulsar winds, and describe recent work on the acceleration mechanisms thought to be at work. These include not only variants of the well-known first-order Fermi mechanism, but also "inductive acceleration", which may explain the mysterious gamma-ray flares from the Crab Nebula, discovered in 2011 by the Agile and Fermi satellites.

Abstract
Tracing matter and chemical elements in the Universe is critical for understanding the formation of the first galaxies, the formation and growth of supermassive black holes, and ultimately the evolution of galaxies like our Milky Way. Throughout the history of the universe, large-scale gas flows have moulded the arms of spiral galaxies, formed the bulges of the most massive galaxies in the universe, fed supermassive black holes in the centers of galaxies, fueled generation upon generation of new stars, and enriched the intergalactic medium with metals. The physics and impact of these processes can now be traced through new efficient, wide-field 3D integral field spectrographs. We use multi-object integral field spectroscopy to build the largest local sample of galaxies with wide 3-dimensional imaging spectroscopy. We combine our local results with insights into the early universe probed through gravitational lensing and adaptive optics. I will present the latest results from our large local and high-z 3D surveys to understand the relationship between gas inflows, galactic-scale outflows, star-formation, chemical enrichment, and active galactic nuclei in galaxies. I will finish by discussing how this field will be transformed in the JWST and ELT era.

Abstract
With ALMA and high-contrast optical/IR imaging, protoplanetary disks are revealed to be structured objects. They display rings, spirals, vortices and warps. These structures appear to be extremely well-defined and often have high contrast. This poses the question: what processes cause these conspicuous structures? Are these signs of planet formation? Or do they betray the existence of just-born planets in these disks? In this talk I will discuss these observations and some theoretical models that attempt to explain them. I will show that these structures indicate that dust “pebbles” are being moved around and are trapped in so-called “pressure traps”. I will show that planetary/substellar companions perturb the disk, but that also disk-internal processes can explain some of the ringlike dust traps. Finally I will discuss some ideas to explain the strong warps seen in some of these protoplanetary disks.