CLASSY. XI. Tracing Neutral Gas Properties Using UV Absorption Lines and 21 cm Observations

Rest-frame far-ultraviolet (FUV) observations from JWST are revolutionizing our understanding of the high-z galaxies that drove reionization and the mechanisms by which they accomplished it. To fully interpret these observations, we must be able to diagnose how properties of the interstellar medium (ISM; e.g., column density, covering fraction, and outflow velocity) directly relate to the absorption features produced. Using the high-signal-to-noise and high-resolution FUV spectra of 45 nearby star-forming galaxies from the Cosmic Origins Spectrograph Legacy Spectroscopic Survey, we present the largest uniform, simultaneous characterization of neutral and low-ionization state (LIS) interstellar UV absorption lines (O I, Si II, S II, C II, and Al II) across a wide range of galaxy properties. We also present 21 cm H I observations for 35 galaxies, multiple of which are gas-poor or nondetected, possibly indicating the onset of a post-starburst phase. We find that our simultaneous one-component Voigt profile fits are capable of accurately modeling the LIS absorption for ∼75% of galaxies, mitigating challenges associated with saturation, infilling, and degeneracies. While the most massive galaxies require additional components, our one-component fits return average properties of the absorbing gas and follow the scaling relations described by a single gas cloud. We explore connections between LIS absorption and direct tracers of the neutral ISM (O I, Lyα, and H I 21 cm), finding that C II most closely traces the neutral gas trends, while other ions exhibit weaker correlations. Given the challenges with directly observing H I at higher-z, we demonstrate that LIS absorption can be a powerful means to study the neutral ISM and present empirical relationships for predicting neutral gas properties. ^∗ Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.