In this thesis we make a new measurement of the primordial deuterium abundance, and analyse five other systems selected as possible D/H candidates. We also undertake an investigation of systematic errors in a system where an existing deuterium measurement has been made. We measure the number of hydrogen components and their velocity distributions in a moderate and high redshift sample of Lyman limit systems in one Angstrom resolution spectra. We present a new measurement of the deuterium-to-hydrogen ratio, D/H, at z=3.2560 in a newly-discovered low metallicity absorption system towards the quasar PG1937-1009. We attempt to account for any systematic effects that could influence the D/H measurement. We find a 1 sigma range for D/H*1e5 of 1.6 (+0.25) (-0.30). Using high resolution spectra from the Hubble Space Telescope and the Very Large Telescope, we assess the suitability of five D/H candidate absorption systems. These systems were selected as candidates using lower resolution spectra. We measure the neutral hydrogen column density, identify metal lines and analyse the velocity structure of each system, and show them to be unsuitable for measuring D/H. We also investigate the systematic errors in the absorber at z=0.701 towards quasar PG1718-4801 that was initially thought to show a high primordial D/H value. We analyse the dependence of the putative deuterium line's parameters on wavelength calibration errors in the HST spectra and present a revised deuterium measurement. We examine the velocity widths of two samples of one Angstrom resolution quasar spectra showing Lyman limit absorption systems. The first sample is at high redshifts, taken from the Sloan quasar data release 3 catalogue. The second is at intermediate redshifts, compiled from a survey for UV quasar absorption systems taken with the Hubble Space Telescope. We use a modified version of the Voigt profile fitting program, VPFIT, to estimate the number of hydrogen velocity components and column density in the Lyman limit systems. We compare the velocity distributions of the higher and lower redshift samples. We find the distributions are consistent with other measures of the velocity spread in absorption systems, and find no compelling evidence for evolution between the redshift samples.

When we look throughout the Universe, we can see the stages of galaxy evolution across cosmic time; however there are still many unanswered questions about the details of galaxy evolution. How did galaxies like our Milky Way assemble? Do the first galaxies look different than our own? What makes galaxies stop forming stars? Many of these questions can be addressed by studying the detailed chemistry of gas located in and around galaxies. Absorption lines imprinted on quasar spectra probe this hard-to-see gas within and surrounding galaxies, giving an luminosity-unbiased census of gas from z~0 to the epoch of the most distant quasars. In this thesis, I present two samples of high resolution spectra of quasars obtained from both ground- and space-based observatories to study the evolution of galaxies through their gas-phase absorption lines. The first of the two samples presented in this thesis consists of the 100 quasar sightlines from the XQ-100 legacy survey completed with the X-Shooter spectrograph on the Very Large Telescope in Chile. The XQ-100 survey provides a blind sample of over 350 HI absorption line systems associated with galaxies with column densities 18.8<=logN(HI)<=21.5. Using this sample, I investigated the evolution of neutral gas reservoirs from z~4.5 to z~2.0. I demonstrate that the lower column density sub-damped Lyman alpha systems (with column densities 19.0<=logN(HI)<20.3) contribute ~20% of the HI observed in galaxy gas reservoirs compared to the rarer but high column density damped Lyman alpha systems (DLAs; logN(HI)>=20.3). Furthermore, I show that using the presence of metal lines (particularly MgII absorption) to identify and select absorbing systems can potentially bias the properties of the sample; absorbers selected to contain strong metal line absorbers tend to exclude low metallicity and low HI column density systems. I demonstrate that the systems missed by metal-selected searches can have a significant impact on the study of the cosmic evolution of neutral gas reservoirs. In addition to the HI content, the metal abundances for 13 elements in the 41 DLAs of the XQ-100 sample are presented. In concert with my literature compilation of 280 DLA metal abundance measurements, I studied the dust-corrected [Zn/Fe]. This work emphasizes that near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and TiII lines (at redshifts 3-4) to provide additional evidence for subsolar [Zn/Fe] ratio in DLAs, a chemical signature that DLAs can be high-redshift dwarf galaxy analogues. Furthermore, the XQ-100 DLA sample consists of several unique systems that probe the effects of environment on the chemical evolution of the Universe, as well as the chemical makeup of the first generations of stars. I demonstrate that DLAs close to their background quasar (within 5000 km/s) with logN(HI)<21.0 show lower [S/H] and [Fe/H] (relative to intervening systems with similar redshift and N(HI)), whilst higher [S/H] and [Si/H] are seen in these proximate systems with logN(HI)>21.0. Contrary to previous studies, DLAs within 10,000 km/s of another DLA show no difference in [alpha/Fe] relative to single DLAs matched in metallicity and redshift. In addition, I present follow-up high-resolution data of J0034+1639, a sightline containing three DLAs, including a metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to date) at z=4.25. In the latter part of this thesis, I study the circumgalactic medium (CGM) of galaxies that host an active galactic nucleus (AGN). AGN are thought to play a critical role in shaping galaxies, but their effect on the gaseous reservoirs surrounding galaxies is not well studied. I present results from the COS-AGN survey: 19 quasar sightlines that probe the gas surrounding 20 optically-selected AGN host galaxies observed over 65 hours with the Hubble Space Telescope. Absorption lines from a variety of species are measured and compared to a stellar mass and impact parameter matched sample of sightlines through non-AGN galaxies. Amongst the observed species in the COS-AGN sample (HI, CII, SiII, SiIII, CIV, SiIV, NV), only HI shows a high covering fraction whilst many of the metal ions are not detected in individual sightlines. A sightline-by-sightline comparison between COS-AGN and the control sample yields no significant difference in equivalent width distribution. However, stacked spectra of the COS-AGN and control samples show significant enhancements in the equivalent width of both Lya and SiIII at high impact parameters (>164 kpc) by a factor of +0.45+/-0.05 dex and >+0.75 dex respectively. The lack of detections of both high-ionization species near the AGN and strong kinematic offsets between the absorption systemic galaxy redshifts indicates that neither the AGN's ionization nor its outflows are the origin of these differences. Instead, I suggest the observed differences could result from either AGN hosts residing in haloes with intrinsically distinct gas properties, or that their CGM has been affected by a previous event, such as a starburst, which may also have fuelled the nuclear activity. / Graduate

My dissertation focuses on a relatively new field of study: the region immediately around galaxies known as the circumgalactic medium (CGM). The CGM holds vast quantities of mass and metals, yet its connection to galaxies is not well understood. My work uses cosmological hydrodynamic simulations and comparisons to data from Hubble's Cosmic Origins Spectrograph (COS) to understand the CGM's connection to galaxy evolution, gas accretion, outflows, star formation, and baryon cycling. This includes studies of the CGM's extent and physical conditions; the cause and nature of outflows; gas dynamics, including the first comprehensive study of tracers of inflowing and outflowing gas at low redshift (z=0.25); and direct comparison of theoretical results to observational data. Chapter 1 introduces my research and show its connection to galaxy evolution. Chapter 2 investigates hydrogen and metal line absorption around low-redshift galaxies in cosmological hydrodynamic simulations. This chapter studies different models for stellar outflows, physical conditions, and dependencies on halo mass. Chapter 3 examines the flow of gas into, out of, and around galaxies using a novel particle tracking technique. This chapter examines the baryon cycle in detail for our preferred model of stellar outflows. Chapter 4 compares our model results, including two separate prescriptions for outflows, with data from COS. We contrast these wind models, showing how they cycle baryons differently, and show degeneracies in observational diagnostics. In Chapter 5, I summarize and discuss plans for future research in this field, and how it can be more fully leveraged to understand galaxy evolution.

I investigate the chemical evolution of the Universe in a series of cosmological hydrodynamic simulations with the purpose of finding a self-consistent evolutionary scenario of galaxy formation able to reproduce key observables focusing on the enrichment of the intergalactic medium (IGM). The most successful models I run and analyze use the scalings of momentum-driven feedback whereby UV photons generated during the Main Sequence stage accelerate dust-driven winds while providing a significantly larger energy budget than from supernovae alone. The success of this outflow model relies on its ability to drive highly mass-loaded winds from small galactic haloes. These feedback relations, supported by observations of local starburst, are inserted into simulations at all epochs, reproducing observables including the C IV column density and linewidth distributions at z=6->1.5 and the O VI forest at z=0-0.5. Outflows at z>=5 propagate early nucleosynthetic products traced by C IV and lower ionization species into an otherwise metal-free IGM. Continual outflows at the peak ages of star formation (z=5->1.5) produce a non-evolving cosmic mass density of C IV despite continual enrichment increasing IGM metallicity by a factor of ten. The z=0-0.5 O VI forest is composed of weaker absorbers tracing photo-ionized diffuse IGM metals, sometimes injected by primordial galaxies, and stronger absorbers tracing recently injected metals, often unable to escape their parent haloes and sometimes collisionally ionized. Tracking the individual histories of metals in outflows shows the average outflow travels ~100 physical kpc and returns to galaxies on an average timescale of 1-2 Gyr; this result implies metals in superwinds do not remain in the IGM for a Hubble time and are more likely to rejoin galaxies. Metal absorbers aligned with Lyman-alpha are examined in detail, finding that the two often trace different phases of gas with the former tracing an inhomogeneous distribution of metals exhibiting turbulence imparted during the outflow phase dissipating on a Hubble timescale. I find this is the first model to self-consistently reproduce the wide range of IGM observables spanning the history of heavy metal production while being consistent with key galaxy observables. The link between star formation and galactic superwinds requires that a successful model of galaxy formation reproduces both the evolution of galaxies and the IGM.

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