The First Direct Calibration of Na D as a Tracer of High-redshift Neutral Outflows

Galactic outflows of neutral gas play a crucial role in regulating galaxy evolution by expelling gas and suppressing star formation. These outflows can be studied via absorption lines of H I against the UV continuum. However, this method is not viable for high-redshift galaxies with a weak UV continuum, such as galaxies that are experiencing a decline in their star formation (possibly due to the neutral outflow). Other tracers such as Na I, Mg II, or Ca II are comparatively easier to observe in the rest-frame near-UV or optical spectrum, but the conversion of the observed column density into the total column density of neutral gas (which is mostly hydrogen) represents a major source of systematic uncertainty. In this thesis, I analyze the unique system J1439, where a sub-damped Lyman-α absorber (sub-DLA) at z = 2.41827, likely ejected by a massive, quiescent galaxy (J1439B) at z = 2.4189, reveals signatures of a neutral outflow observed via H I absorption lines in the optical spectrum of a bright background quasar (QSO J1439+1117) at z = 2.585. Through detailed analysis of new near-infrared spectroscopy obtained with Magellan/FIRE, I identify Na D and Mg II doublet absorptions — rarely observed in distant DLAs — and measure their column densities. The main achievement of this study is the first-ever empirical calibration between the column densities of Na I and H I in a high-redshift environment. This new calibration marks a significant breakthrough in neutral outflow studies, offering a precise tool to estimate the neutral gas content in systems where Lyman-α observations are not feasible. Furthermore, I provide the first constraints on the use of Mg II as a tracer of neutral gas in high-redshift systems. The implications of this work extend beyond the studied system, setting the stage for future research into the characterization of neutral outflows in distant galaxies and their role in quenching star formation.