2024ApJ...974...93P

Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (H I) absorption features in NGC 6822, a low-metallicity (0.2 Z_⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local Group L-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for H I emission) and spectral (0.4 km s^–1) resolution. We introduce LGLBS and describe a custom pipeline for searching for H I absorption at high angular resolution and extracting associated H I emission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 10²⁰ cm^–2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the H I absorption profile encompassing the CO (2-1) emission, indicating coincident molecular gas and a depression in high-resolution H I emission. We also analyze a nearby sightline with deep, narrow H I self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hα emissions suggests a close link between the colder, denser H I phase and star formation in NGC 6822.