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Abstract 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 (Hi) absorption features in NGC 6822, a low-metallicity (0.2Z_⊙) 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 GroupL-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for Hiemission) and spectral (0.4 km s⁻¹) resolution. We introduce LGLBS and describe a custom pipeline for searching for Hiabsorption at high angular resolution and extracting associated Hiemission. 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 × 10²⁰cm⁻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 Hiabsorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hiemission. We also analyze a nearby sightline with deep, narrow Hiself-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hαemissions suggests a close link between the colder, denser Hiphase and star formation in NGC 6822. 

Abstract Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of ≤170 pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and the Surveying the Whirlpool at Arcseconds with NOEMA. The data set includes the (1–0) and (2–1) rotational transitions of¹²CO,¹³CO, and C¹⁸O isotopologues. The observations cover ther< 3 kpc region, including the center and part of the disk, thereby ensuring strong detections of the weaker¹³CO and C¹⁸O lines. All observations are convolved in this analysis to an angular resolution of 4″, corresponding to a physical scale of 170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H₂conversion factor (α_CO). We employ two approaches to study the molecular gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to analytically determine the CO column density andα_CO, and (ii) using non-LTE modeling withRADEXto fit physical conditions to observed CO isotopologue intensities. We find that theα_COvalues in the center and along the inner spiral arm are ∼0.5 dex (LTE) and 0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTEα_COis 2.4 ± 0.5M_⊙pc⁻²(K km s⁻¹)⁻¹. While both methods show dispersion due to underlying assumptions, the scatter is larger for LTE-derived values. This study underscores the necessity for robust CO line modeling to accurately constrain the molecular interstellar medium’s physical and chemical conditions in nearby galaxies. 

Abstract We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately (∼1 kpc) spatially resolved measurements of CO (1–0) and CO (2–1) intensity,I_CO, and mid-IR intensity,I_MIR, at 8, 12, 22, and 24μm. TheI_COversusI_MIRrelationship is reasonably described by a power law with slopes 0.7–1.2 and normalizationI_CO∼ 1 K km s⁻¹atI_MIR∼ 1 MJy sr⁻¹. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO (1–0) and CO (2–1) allow us to infer that $R_{21}\propto I_{\mathrm{MIR}}^{0.2}$, in good agreement with other work. The 8μm and 12μm bands, with strong polycyclic aromatic hydrocarbon (PAH) features, show steeper CO versus mid-IR slopes than the 22 and 24μm, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass (M_⋆) and anticorrelates with star formation rate/M_⋆. At ∼1 kpc resolution, the first four PHANGS–JWST targets show CO-to-mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities.