We are using ultraviolet and optical spectra of distant background sources (quasars and other active galactic nuclei) to study the absorption of metal ions in diffuse gas clouds in the halo of the Milky Way ("circumgalactic" gas). From this, in combination with semi-analytic models of the gas distribution around Milky-Way type galaxies, we obtain crucial information on the total mass of the Milky Way's circumgalactic medium and its physical state. These studies further allow us to derive the Milky Way's gas-accretion rate and to characterize the role of the CGM for the evolution of our Galaxy (project leader: Philipp Richter).
Small-scale structure in the Milky Way's interstellar medium is mapped using VLT/MUSE data of thousands of stars in Galactic globular clusters. The method of 3D-spectroscopy allows us to study variations in the interstellar line strengths of optical transitions of metal ions and in diffuse interstellar bands on linear scales of milli-pc. The MUSE data are complemented by high-resolution spectra in the optical and in the UV from various different instruments. Studies like these are essential to improve our understanding of the distribution of interstellar gas at small spatial scales (project leader: Martin Wendt).
We are mining the VLT/UVES and Keck/HIRES data archives to use high-resolution, high signal-to-noise optical spectra of distant quasars to systematically explore the physical conditions intervening metal-absorption line systems at high redshift. Multi-component Voigt profile fitting is applied to these spectra to obtain accurate information on the metal-abudances, ionization conditions (via ionization modeling), and incidence rate of the different absorber classes and to determine their origins and their role in the large-scale structure formation. In addition, column-density ratios of metal ions are systematically investigated to set constraints to the redshift-evolution, intensity, and hardness of the overall UV background that permeates the Universe (project leader: Cora Fechner).
Optical and ultraviolet spectra are used to study the reionization history of hydrogen and helium in the Universe. As for the other above mentioned projects, the method of absorption spectrocopy is used for this task. A particular focus lies on the HElium Reionization Survey (HERS), which aims at exploring the epoch of helium reionization with HST spetral data. We are also interested in the radiative feedback of quasars onto their surroundings and the relative contributions of quasars and star-forming galaxies to the UV background that keeps the intergalactic medium highly ionized (project leader: Gabor Worseck).
We are using state-of the art cosmological, hydrodynamical simulations (e.g., from the Illustris, CLUES, OWLS projects), to investigate the circumgalactic gas flow around galaxies, gas accretion processes, and the evolution of galaxies. Our current and future focus lies on the gas distribution in the Local Group (studied with CLUES) and the local Cosmic Web as well of the morphological evolution of galaxy disks of the merging galaxies (studies with Illustris), for which we use a sample of simulated major merger galaxies in full cosmological setups (project leader: Martin Sparre, in cooperation with Christoph Pfrommer, AIP).