Fueling Galaxy Growth Through Gas Accretion in Cosmological Simulations

Abstract

Despite significant advances in the numerical modeling of galaxy formation and evolution, it is clear that a satisfactory theoretical picture of how galaxies acquire their baryons across cosmic time remains elusive. In this thesis we present a computational study which seeks to address the question of how galaxies get their gas. We make use of new, more robust simulation techniques and describe the first investigations of cosmological gas accretion using a moving-mesh approach for solving the equations of continuum hydrodynamics.

We focus first on a re-examination of past theoretical conclusions as to the relative importance of different accretion modes for galaxy growth. We study the rates and nature of gas accretion at z=2, comparing our new simulations run with the Arepo code to otherwise identical realizations run with the smoothed particle hydrodynamics code Gadget. We find significant physical differences in the thermodynamic history of accreted gas, explained in terms of numerical inaccuracies in SPH. In contrast to previous results, we conclude that hot mode accretion generally dominates galaxy growth, while cold gas filaments experience increased heating and disruption.

Next, we consider the impact of feedback on our results, including models for galactic-scale outflows driven by stars as well as the energy released from supermassive black holes. We find that feedback strongly suppresses the inflow of "smooth" mode gas at all redshifts, regardless of its temperature history. Although the geometry of accretion at the virial radius is largely unmodified, strong galactic-fountain recycling motions dominate the inner halo. We measure a shift in the characteristic timescale of accretion, and discuss implications for semi-analytical models of hot halo gas cooling.

To overcome the resolution limitations of cosmological volumes, we simulate a suite of eight individual 10^12 solar mass halos down to z=2. We quantify the thermal and dynamical structure of the gas in and around these halos. A radial sightline analysis allows us to measure the angular variability of halo gas properties, and demonstrate its increasing complexity at higher numerical resolution. We study the presence and characteristics of a strong virial shock, and make the link to recent observations of the circumgalactic medium surrounding galaxies.

We conclude with a technically oriented presentation of the full public data release of the Illustris simulation. Our goal is to facilitate a new era of robust comparisons, between state of the art theoretical models of galaxy formation and the many rich observational surveys of galaxy populations across cosmic time. We describe the data itself, as well as the comprehensive interface and set of tools we have developed for its analysis. We discuss scientific issues relevant when interpreting the simulations, technical details of the release effort, and future goals.