I am interested in understanding how galaxies and many of their properties evolve over cosmic history: Which processes determine their sizes, regulate their star formation rates, or shape their morphology? In fact, nobody really knows. However, compared with just 10 years ago, we are now much closer to answering those questions. This remarkable development is largely a result of two major advances. First, with the advent of large scale digital galaxy surveys the quantitative analysis of millions of galaxies became possible -- a development that is still revolutionizing our understanding of galaxy evolution. Secondly, the continuous increase in computing power enables astrophysicists like myself to develop and evaluate increasingly sophisticated theoretical models. This is required because galaxy evolution is an inherently complex problem and requires numerical models of high dynamical range. In my work I often rely on state-of-the-art hydrodynamics and gravity solvers such as the SPH-code GASOLINE or the AMR-code ART to study how galaxies evolve in a cosmological setting.

Interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way provide the majority of the gamma rays observed by the Fermi Gamma Ray Space Telescope. In addition to the gas which is densely concentrated along the Galactic Disk, hydrodynamical simulations and observational evidence favor the presence of a halo of hot (T~10^6 K) ionized hydrogen, extending with non-negligible densities out to the virial radius of the Milky Way. We show that cosmic ray collisions with this circum-galactic gas should be expected to provide a significant flux of gamma rays, on the order of 10% of the observed isotopic gamma ray background at energies above 1 GeV. In addition, gamma rays originating from the extended halos of other galaxies along a given line-of-sight should contribute to this background at a similar level.

A star formation efficiency per free fall time that evolves over the life time of giant molecular clouds (GMCs) may have important implications for models of supersonic turbulence in molecular clouds or for the relation between star formation rate and H2 surface density. In the paper we discuss observational data that could be interpreted as evidence of such a time variability. In particular, we investigate a recent claim based on measurements of H2 and stellar masses in individual GMCs. We show that this claim depends crucially on the assumption that H2 masses do not evolve over the life times of GMCs. We exemplify our findings with a simple toy model that uses a constant star formation efficiency and, yet, is able to explain the observational data.

Many physical processes may be responsible for making elliptical morphologies and quenching star formation, but the mainly responsible mechanisms, and the epochs and timescales at/on which they operate have not been yet identified. In this paper we use a simulation of the formation of a group of galaxies with sufficient resolution to track the evolution of gas and stars of about a dozen galaxy group members over cosmic history. Ellipticals form, as suspected, through galaxy mergers. In contrast with what has often been speculated, however, these mergers occur at z>1, before the merging progenitors enter the virial radius of the group and before the group is fully assembled. Quenching of star formation in the still star-forming elliptical galaxies lags behind their morphological transformation, but, once started, is taking less than a billion years to complete.

In this paper we trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.

We use a set of high-resolution N-body simulations of binary galaxy mergers to show that the morphologies of the tidal features that are seen around a large fraction of nearby, massive ellipticals in the field cannot be reproduced by equal-mass dissipationless mergers; rather, they are well explained by the accretion of disk-dominated galaxies. The minor cold-accretion events that explain the presence, brightness, and structural and color properties of the tidal debris cause only a modest mass and luminosity increase in the accreting massive elliptical. These results, coupled with the relative statistical frequencies of disk- and bulge-dominated galaxies in the field, suggest that massive ellipticals assemble most of their mass well before their tidal debris forms through the accretion of relatively little, kinematically cold material rather than in very recent, dissipationless major mergers.