| We study the evolution of cold gas in distant galaxies by analyzing observations, semi-analytic models (SAMs), and simulations of star-forming galaxies (SFGs) and damped Lyalpha absorption systems (DLAs). First, we present individual and composite rest-frame ultraviolet (UV) spectra for 81 SFGs where we study the relations among Ly? emission, low and high ionization absorption strength, rest-ultraviolet continuum slope, redshift, and velocity offset. We find that galaxies with R 20A have bluer UV continua, weaker low-ionization interstellar absorption lines, weaker C IV absorption, and stronger Si II nebular emission than those with WLyalpha < 20A. Next, we present our range of models which include "standard," "extended," and merger-based disks as well as a metallicity-dependent and pressure-based prescription for partitioning cold gas into atomic and molecular components. Using these models, we "observe" a catalog of mock DLAs, which we compare to observations. We find that extended disk models reproduce quite well the column density distribution of absorbers over the column density range 19 < log N(HI) < 22.5, the observed line density of DLAs, Hi gas density, the Deltav distribution in the redshift range 2 < z < 3.5, and the evolution of DLA metallicity with redshift. Using these models, we characterize the properties of DLA host galaxies and compare them to model SFGs "observed" in the SAMs. We show that DLA host galaxies exhibit a broad range of galaxy properties spanning several decades in stellar mass, star formation rate, and luminosity and fall upon common galaxy scaling relations. Finally, we analyze the radial profiles and evolution of 15 galaxies in numerical simulations and compare them to predictions from the SAMs. Galaxies' cold gas and stellar components are moderately well-fit by exponential profiles, although both gas partitioning recipes predict more molecular gas and less star formation than is observed in the numerical simulations. Nonetheless, both types of simulations predict similar evolution in galaxy properties from z = 2 to z = 0. |
