The Local Group L -band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-metallicity Dwarf Galaxy NGC 6822

Journal article

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.

Authors: Pingel, NM; Chen, H; Stanimirović, S; Koch, EW; Leroy, AK

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 974
• Issue: 1
• Article number: 93
• Publication date: 2024-10-07
• Acceptance date: 2024-07-18
• DOI: 10.3847/1538-4357/ad6604
• EISSN: 1538-4357
• ISSN: 0004-637X
• Language: English
• Keywords: Dwarf galaxies; Interstellar medium; Interstellar absorption; Interstellar atomic gas