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.