Talks, Seminars, Events

Abstract
Radio galaxies have emerged as a new very high energy (VHE) gamma-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of active galactic nuclei (AGN) in general. Up to now there are six radio galaxies detected at gamma-ray energies above 100 GeV. With their jets not directly pointing towards us (i.e. ”misaligned blazars”), they offer a unique tool to probe into the nature of the fundamental (and often ”hidden”) radiation processes in AGN. The detection of fast flux variability in the TeV energy regime, for example, offers important information for our understanding of particle acceleration in AGN. In this talk I will discuss some of the observational results in the TeV range and the theoretical implications from recent years, including, e.g., the extreme short-term variability as seen in IC 310 and NGC 1275.

Abstract
TBD

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.

Abstract
Everywhere we look, the Universe is threaded with magnetism. These magnetic fields are surprisingly organised and coherent, and are vital to many of the fundamental processes that astronomers take for granted. However, the mechanisms that create and then sustain magnetism in the Universe are not understood, in no small part because magnetic fields are usually not directly observable. I will present innovative new observations of radio polarimetry and Faraday rotation, and will explain how these data sets provide a unique view of magnetic fields in the Milky Way, in distant galaxies, and in the intergalactic medium. I will conclude by showcasing the powerful new generation of radio telescopes that are at last fully opening the window to the magnetic Universe.

Abstract
nn

Abstract
TBD

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.

Abstract
nn

Abstract
nn

Abstract
TBA

Abstract
Stellar clusters are present in the local Universe in a variety of environments, from the current cluster formation sites in the disks of the Antennae galaxies to the old globular cluster population that mostly populates the halo of the Milky Way, implying that their evolution is tightly linked to that of their host galaxy. I will discuss how these clusters form and co-evolve alonside their host galaxies over cosmic time and I will present results from state-of-the-art hydrodynamical cosmological simulations.

Abstract
nn

Abstract
TBD

Abstract
On October 19, 2017 the Pan-STARRS1 telescope discovered a rapidly moving object. Additional astrometry obtained with pre-discovery observations on October 18 through data obtained with the Canada-France-Hawaii-Telescope on October 22 showed that the object had the highest hyperbolic eccentricity ever detected, confirming that this object clearly originated from outside the solar system. 1I/2017 U1 passed perihelion on September 9, 2017 and had made its Earth close approach at 63 lunar radii on October 14. The official name of ‘Oumuamua, meaning visitor from the distant past, was approved by the IAU on November 6. Beginning on October 22 there was an intense effort to secure observing resources to characterize the object. Because it was receding rapidly from the Earth and Sun, within a week of discovery the brightness had dropped by a factor of 10 and in less than a month it had dropped by a factor of 100. Thus, there was a period of just over a week where the target could be relatively easily characterized. Deep images of ‘Oumuamua showed no hint of cometary activity, with limits on the amount of dust that could be present at less than 7-8 orders of magnitude that of a typical comet at similar distances. Light curve observations showed that the object was rotating with an instantaneous rotation period of 7.34 hours, and a light curve range of 2.5 magnitudes, implying an extremely elongated axis ratio perhaps as large as 10:1, but certainly larger than 5:1. Spitzer observations suggest an average diameter somewhere between 98-440 km depending on model-dependent surface thermal properties. As more time series data were obtained, it was evident that ‘Oumuamua was in an excited spin state with the long axis precessing around the total angular momentum vector with an average period of 8.67±0.34 hr. The timescale for damping an excited spin in a body this size is very long, so the spin state may reflect the violent process of ejection of ‘Oumuamua from its host planetary system. The color of ‘Oumuamua was found to be quite red with a spectral slope of 23%±3% per 100 nm, consistent with comet surfaces, the dark side of Iapetus, and other minerals. Precision astrometric measurements obtained from the Hubble Space Telescope and the ground allowed us to do a detailed study of the orbit. Analysis of 207 astrometric positions showed that the orbit cannot be fit by a purely gravity-only trajectory, but are well matched (at the 30-sigma level) by the addition of a radial acceleration. We explored several explanations for the non-gravitational motion, and found that cometary outgassing is the most physically plausible, but requires that ‘Oumuamua has a somewhat different nature from solar system comets. Many attempts were made to trace ‘Oumuamua back to it’s home system, the most detailed after the release of the Gaia 2 catalog, but no convincing candidates have been found. Whether this will ever be feasible depends on how long ago it was ejected. ‘Oumuamua has challenged many of our assumptions about what small bodies from another solar system would look like, and has triggered an avalanche of papers, some highly speculative. In this talk I’ll share the story of the discovery of ‘Oumuamua and discuss what we know about our first known interstellar visitor – including new information from papers in press.

Abstract
nn

Abstract
TBA

Abstract
nn

Abstract
The giant molecular cloud (GMC) lifetime provides an upper bound on the local time-scale for star-formation, linking cloud-scale and sub cloud-scale physics to galaxy-scale trends in the star formation rate. Conversely, the galactic environment plays an important role in setting the cloud lifetime, leading to a complex interplay of physical mechanisms over a range of scales in the interstellar medium (ISM), from galactic dynamics to small-scale turbulence and feedback. Previous theories of GMC lifetimes have made predictions based on just one mechanism of cloud evolution, relevant only in a fraction of Galactic and extragalactic star-forming environments. That approach is inconsistent with recent observations, which show that a diverse range of entities are observationally-identifiable as clouds, and reveal environmentally-driven correlations between their gravitational boundedness and the galaxy-scale star formation rate. I present an analytic theory for GMC lifetimes, dependent on the large-scale dynamical environment of the ISM, including its local gravitational stability, cloud-cloud collisions, epicyclic perturbations, galactic shear, and interaction with galactic bars and spiral arms. Our analytic predictions depend on just five observable properties, accessible through measurements of the rotation curve, surface density and velocity dispersion of the host galaxy, and are applicable over a wide range of redshifts. In this contribution, I will present predicted cloud lifetimes and properties across a range of galactic dynamical environments. I will compare these results to hydrodynamic simulations performed using the moving-mesh code Arepo, where the influence of dynamics is combined with sub-cloud physics such as supernova feedback, HII-region feedback, and ISM chemistry. These theoretical and numerical results are consistent with pioneering observational results currently obtained with ALMA. Together, this combination of analytic, numerical and observational results show that the galactic dynamic environment plays a crucial role in determining GMC lifecycles and thus the star formation rates of their host galaxies.

Abstract
nn

Abstract
TBD

Abstract
nn

Abstract
nn

Abstract
Theories of planet formation predict the existence of a population of free-floating planets that are not gravitationally tethered to any host star. Gravitational microlensing provides a unique tool for studying these objects. The first results of Sumi et al. (2011) claimed that Jupiter-mass free-floating planets are as common as main-sequence stars. However, these results disagree with censuses of substellar objects in young clusters and star-forming regions and with predictions of planet formation theories. I will present new results of the analysis of a ten times larger sample of microlensing events discovered by the OGLE-IV survey during the years 2010-2018, which shed new light on the population of free-floating planets. I will also discuss prospects for detecting free-floating planets with the future missions, like Euclid and WFIRST.

Abstract
nn

Abstract
In the course of their accretion phase, massive (proto)stars impact their natal environment in a variety of feedback effects such as thermal heating, MHD-driven protostellar jets and outflows, radiation forces, and photoionization / HII regions. Here, I present our most recent simulation results in terms of the relative strength of the feedback components and the size of the reservoir from which the forming stars gain their masses. For the first time, these simulations include all of the feedback effects mentioned above which allows us to shed light on the physical reason for the upper mass limit of present-day stars. Furthermore, we predict the fragmentation of massive circumstellar accretion disks as a viable road to the formation of spectroscopic massive binaries and the recently observed strong accretion bursts in high-mass star forming regions.

To advertise our latest code development, I will also overview the most recent results obtained in a variety of other astrophysical research fields from the formation of embedded Super-Earth planets' first atmospheres (Cimerman et al. 2017, MNRAS) to the formation of the progenitors of the first supermassive black holes in the early universe (Hirano et al. 2017, Science).

Abstract
TBD