Unveiling The Circumgalactic Medium Using A Cloud By Cloud Multiphase Bayesian Ionization Modeling Approach

This thesis investigates the physicochemical nature of the gaseous structures in the vicinity of galaxies. Intervening absorption systems seen in the spectra of background quasars probe the circumgalactic medium (CGM) around galaxies and can provide insights into the nature of the gas. The CGM is a dynamic and multiphase interface between a galaxy and its surroundings. Unraveling the origin of the multiphase gas in the CGM is important because it potentially allows us to learn about the processes that supply inflowing gas to the galaxy, enrich the surroundings with metal-rich outflows, and send enriched material back to the galaxy as recycled accretion. First, I present a new method aimed at improving the efficiency of component-by-component ionization modeling of intervening quasar absorption line systems. I carry out cloud-by-cloud, multiphase modeling making use of CLOUDY and Bayesian methods to extract physical properties from an ensemble of absorption profiles. As a demonstration of the method, I focus on four weak, low ionization absorbers at low redshift, because they are multi-phase but relatively simple to constrain. We place errors on the inferred metallicities and ionization parameters for individual clouds and show that the values vary from component to component across the absorption profile. This method requires user input on the number of phases and relies on an optimized transition for each phase, one observed with high resolution and signal-to-noise. The measured Doppler parameter, b, of the optimized transition provides a constraint on the Doppler parameter of HI, thus providing leverage in metallicity measurements even when hydrogen lines are saturated. I present several tests of this methodology, demonstrating that I can recover the input parameters from simulated profiles. I also consider how the model results are affected by which radiative transitions are covered by observations (for example how many HI transitions) and by uncertainties in the $b$ parameters of optimized transitions. I discuss the successes and limitations of the method, and consider its potential for large statistical studies. This improved methodology will help to establish direct connections between the diverse properties derived from characterizing the absorbers and the multiple physical processes at play in the CGM. Next, I present an absorption line study of the physical and chemical properties of the Leo HI Ring and the Leo I Group as traced by 11 quasar sightlines spread over a ~ 600 kpc x 800 kpc region. Using HST/COS G130/G160 archival observations as constraints, I couple cloud-by-cloud, multiphase, Bayesian ionization modeling with galaxy property information to determine the plausible origin of the absorbing gas along these sightlines. I find absorption plausibly associated with the Leo Ring towards five sightlines. The absorption along these five sightlines is stronger in metal lines than expected from individual galaxies, indicative of multiple contributions, and of the complex kinematics of the region. Along three other sightlines, I find absorption likely to be associated with individual galaxies, intragroup gas, and/or large-scale filamentary structure. I also identify three sightlines within a 7° x6° field around the Leo Ring, along which I do not find any absorption. I find that the metallicities associated with the Leo Ring are generally high, with values between solar and several times solar. The inferred high metallicities are consistent with the origin of the ring as tidal debris from a major galaxy merger. Next, I analyze archival ultraviolet quasar spectra from HST/COS covering 47 absorption line systems produced by the CGM of galaxies that have galaxy imaging with HST with known impact parameters and orientations. I conducted a large statistical study with this sample to determine if the metallicities of any of the multiple structures depend on orientation. Cloud-by-cloud, multiphase Bayesian modeling was applied to provide constraints on metallicity, density, and temperature of multiple regions along the sightline. I find that the high metallicity clouds span a large range of velocities while the low metallicity clouds are found close to the systemic velocity of the galaxy. I also find that clouds close to the systemic velocity show a full range of metallicities. High-velocity clouds, on the other hand, show a tendency for tracing high metallicities. I do not find metallicity trends with azimuthal angle, inclination, impact parameter, or galaxy type. Despite the lack of a link between azimuthal angle and metallicity, the independent effects of inflows and outflows are seen in absorption systems using cloud-by-cloud modeling. Finally, I employ the same techniques that I have developed with real absorption systems to investigate absorption systems in cosmological simulations in order to assess the efficacy of the methods in extracting true properties from simulations. I find that the assumption of photoionization thermal equilibrium should be relaxed, particularly for the high ionization gas phases, and this assumption only holds in a particular regime of the CGM phase space. For a discrete distribution of clouds, the inferred posterior distributions contain the actual values in the simulations. For a complex and continuous distribution of clouds in simulations, I find that the properties of the best-constrained clouds agree well with the true values. These findings strengthen our confidence in reliably extracting the properties of the CGM from observational datasets.