Vorträge, Seminare, Ereignisse

Abstract
The redshift 2 universe is one of the most interesting epochs of galaxy evolution. It is the era with the peak of the cosmic star formation rate. Between redshift 3 and 1 the total stellar mass density in galaxies increased fromm15% to 70%. It is also the time of rapid galaxy assembly and the epoch where galaxy morphology was determined. Observations of z=2 star-forming galaxies reveal physical properties that are unparalleled in the z=0 Universe. Gas-rich, extended, fast rotating and highly turbulent disks have been found with star formation rates that are a factor of 10 to 100 larger than in present-day Milky-Way type galaxies. Kpc-sized, massive gas clumps dominate the appearance of these galaxies. Even more interesting are recent observations of declining rotation curves in the outer parts of these disks and dynamical masses, inferred from their rotation velocities that are equal to the observed baryonic mass leaving no room for dark matter. I will summarize the newest observations and the puzzles and challenges that they generate for our theoretical understanding of cosmic galaxy formation and galactic dynamics.

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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.

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Dynamically important magnetic fields have been shown to play pivotal roles in processes that are closely linked to galaxy evolution. However, how galaxies and their magnetic fields have co-evolved since the early Universe remains an unsolved fundamental question in astro-plasma physics and cosmology. In this talk, I will describe how the advent of broadband radio polarimetry is revolutionizing the field of cosmic magnetism by enabling unambiguous and precise polarization measurements. Then, I will highlight several innovative studies on mapping galactic magnetic fields near and far: from the Milky Way/ nearby galaxies to distant galaxies. I will conclude by discussing the exciting prospects of decoding the origin and evolution of cosmic magnetic fields with Square Kilometre Array pathfinders and the next generation radio telescopes.

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Pulsating variable stars play an important role in the study of the Galaxy's history since they are excellent tracing populations in uniquely identifiable evolutionary phases. RR Lyrae stars, in particular, are known Population II objects that can also be used as precise distance indicators. These stars have been recently used to reveal the early assembly history of the Milky Way, since the early phases of massive galaxy evolution are believed to have been dominated by the accretion of smaller galaxies containing old stars. Moreover, they are thought to be potential tracers of faint satellite systems hardly detectable by the traditional methods. In this talk I will present the results from an RR Lyrae search using data from the High cadence Transient Survey (HiTS), which was carried out with the Dark Energy Camera (DECam). HiTS is a campaign primarily aimed at detecting early supernovae explosions in real-time with the deep optical images DECam provide. However, the cadence and the strategy followed for the survey are well matched for RR Lyrae detection as well. Using data from HiTS we were able to detect new RR Lyrae stars out to 200 kpc from the Sun. I will discuss the results of the search for RR Lyrae stars using HiTS’ data, their connection with known or previously undiscovered satellite systems and halo substructures, as well as further implications of these findings.

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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).

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