Title: Gas in Galaxies in Different Environments across Cosmic Time
Description: Cold gas is fundamental in understanding galaxy formation and evolution since it provides the fuel for star formation. In addition, the atomic gas can be used to probe the internal properties of galaxies, their halos, and their environment. Several of the remaining questions in galaxy evolution can be addressed by studying the gas properties in galaxies, in particular, the following three: (1) How do galaxies get their gas? (2) How do galaxies change over time? (3) How are galaxies affected by the environment? The work presented in this thesis addresses these questions. The thesis is divided into three parts that cover a range of topics related to gas in galaxies, including the fate of gas in a merger remnant, the evolution and distribution of halo gas, and how the gas properties of galaxies change as a function of redshift and environment. Part I consists of two chapters that present the atomic and molecular gas properties of a wet merger remnant (NGC 34). Chapter 2 is an analysis of the HI distribution and kinematics in NGC 34.

We find that the progenitors of NGC 34 were gas-rich. The kinematics of the tidal tails suggest that some of the gas is returning to the central regions and forming an outer disk. In addition, we find puzzling absorption near the systemic velocity against the radio continuum. Chapter 3 is a follow-up study consisting of CO observations done with CARMA and new VLA data with a large velocity coverage to search for outflows. We detect CO concentrated in the inner regions that matches the velocity range of the HI in absorption, indicating that there is a circumnuclear disk in the central regions of molecular and atomic gas. We do not detect the outflow seen in the optical spectrum in CO or HI, but are able to place upper limits on both. Part II is an analysis of halo gas in a Milky Way mass galaxy. We use a cosmological high resolution hydrodynamic simulation to study the distribution, origin, and evolution of halo gas. At z=0, we find that the amount (~ 10⁸ M_sun), covering fraction and distribution are consistent with existing observations.

The origin of halo gas is a combination of filamentary and satellite material. In addition, we find that the amount of halo gas is roughly constant between z=0.3 to z=0, but increases at earlier times. Part III presents results from the COSMOS HI Large Extragalactic Survey (CHILES), an HI deep field done with the VLA. These observations show how galaxies grow in different environments across cosmic time. We are using the expanded capabilities of the VLA to probe HI in part of the COSMOS field with a 5" resolution. Chapter 5 presents results from the pilot that was observed during commissioning. We observed for 60 hr and covered the redshift range 0<z<0.19 in one setting. We report 33 direct detections in different environments across the redshift range, and a stacked HI mass of (1.8 ± 0.3) x 10⁹ M_sun for galaxies in a wall at z=0.12. The pilot demonstrated that the VLA was ready to carry a full HI deep field. The full survey will be 1002 hr spread over several B array configurations. Chapter 6 presents preliminary results for the first 178 hr of the survey (Phase I).

We describe the data reduction from Phase I, lessons for upcoming configurations, and present preliminary results. We detect very extended HI disks in nearby dwarf galaxies, and present the highest redshift detection to date (z=0.376).