How Is the Galactic Weather?: Tracing the Baryon Cycle and Gas Flows in the Milky Way's Inner Atmosphere

Abstract

A galaxy's atmosphere is a key factor in its evolution, presenting the cycle of gas flowing between its star-forming disk and the surrounding multiphase gaseous halo. The metallicities, ionization states, and kinematics of these flows determine the gas supply for future star formation and its impact on Galactic chemical evolution. While external surveys reveal the average climate of the circumgalactic medium across many galaxies, only the Milky Way allows individual halo clouds to be resolved spatially, kinematically, and chemically on the scales most directly relevant to the disk. In this dissertation, I report the current weather of the Milky Way's inner atmosphere and forecast its impact on the Galaxy. Using HST/COS UV absorption spectroscopy combined with Voigt profile fitting and Bayesian photoionization modeling, I measure the metallicity, ionization, and kinematics of halo clouds. First, I introduce a method that uses pulsar dispersion measures from globular clusters paired with UV-bright halo stars to constrain the ionized hydrogen column density, providing a precise ionization correction to the total hydrogen column. The metallicities of the diffuse ionized gas span a wide range and do not separate cleanly into metal-poor inflow and metal-rich outflow, pointing to complex origins. Second, I spatially resolve the metallicity, ionization, and kinematics of a Galactic fountain flow across 27 sight lines through the intermediate-velocity cloud Complex K. The majority of the gas is metal-rich (<Z> ~ 2.4 Z_sol), with the most metal-rich components showing the slowest infall and progressively lower metallicities at higher infall velocities. It indicates fountain-driven mixing with the metal-poor corona in action and forecasts that fountain accretion returns gas to the disk well above the metallicities assumed in chemical evolution models. Last, I search the underexplored low-latitude sky and report the discovery of ionized Magellanic Stream gas in a northern extension, whose mass and location refine constraints on the Stream's total gas budget and trajectory toward the disk.