Papers › The Local Group L-Band Survey: The First Measurements of Localized Cold Neutral Medium...
The Local Group L-Band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-Metallicity Dwarf Galaxy NGC 6822
Nickolas M. Pingel, Hongxing Chen, Snežana Stanimirović, Eric W. Koch, Adam K. Leroy, Erik Rosolowsky, Chang-Goo Kim, Julianne J. Dalcanton, Fabian Walter, Michael P. Busch, Ryan Chown, Jennifer Donovan Meyer, Cosima Eibensteiner, Deidre A. Hunter, Sumit K. Sarbadhicary, Elizabeth Tarantino, Vicente Villanueva, Thomas G. Williams
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Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insight into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (HI) 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 HI emission) and spectral (0.4 \kms) resolution. We introduce LGLBS and describe a custom pipeline to search for HI absorption at high angular resolution and extract associated HI 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 non-detections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the HI absorption profile encompassing the CO (2$-1) emission, indicating coincident molecular gas and a depression in high-resolution HI emission. We also analyze a nearby sightline with deep, narrow HI self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and H\alpha$ emissions suggests a close link between the colder, denser HI phase and star formation in NGC 6822.
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