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1. Probing the Conditions for the H i-to-H ₂ Transition in the Interstellar Medium

   https://doi.org/10.3847/1538-4357/ace164  

   Park, Gyueun; Lee, Min-Young; Bialy, Shmuel; Burkhart, Blakesley; Dawson, J R; Heiles, Carl; Li, Di; Murray, Claire; Nguyen, Hiep; Hafner, Anita; et al (September 2023, The Astrophysical Journal)

   Abstract We investigate the conditions for the Hi-to-H₂transition in the solar neighborhood by analyzing Hiemission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with¹²CO(1–0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly Hi-dominated. In addition, we find that the CO-dark H₂, not the optically thick Hi, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between Hiand CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ∼0.6, andV-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H₂formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the Hi-to-H₂transition. 

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2. A Neutral Hydrogen Absorption Study of Cold Gas in the Outskirts of the Magellanic Clouds Using the GASKAP-H i Survey

   https://doi.org/10.3847/1538-3881/adc67c  

   Chen, Hongxing; Stanimirović, Snežana; Pingel, Nickolas M; Dempsey, James; Buckland-Willis, Frances; Clark, Susan E; Dénes, Helga; Dickey, John M; Gibson, Steven; Jameson, Katherine; et al (April 2025, The Astronomical Journal)

   Abstract Cold neutral hydrogen (Hi) is a crucial precursor for molecular gas formation and can be studied via Hiabsorption. This study investigates Hiabsorption in low column density regions of the Small and Large Magellanic Clouds (SMC and LMC) using the Galactic-ASKAP Hi(GASKAP-Hi) survey, conducted by the Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC directions in the outer regions and 18 LMC directions, with four in the outskirts and 14 within the main disk. Using the radiative transfer method, we decompose the emission and absorption spectra into individual cold neutral medium (CNM) and warm neutral medium (WNM) components. In the SMC, we find Hipeak optical depths of 0.09–1.16, spin temperatures of ∼20–50 K, and CNM fractions of 1%–11%. In the LMC, optical depths range from 0.03–3.55, spin temperatures from ∼10–100 K, and CNM fractions from 1%–100%. The SMC’s low CNM fractions likely result from its low metallicity and large LOS depth. Additionally, the SMC’s outskirts show lower CNM fractions than the main body, potentially due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell motions dominate the kinematics of the majority of CNM clouds in this study and likely supply cold Hito the Magellanic Stream. In the LMC, high CNM fraction clouds are found near supergiant shells, where thermal instability induced by stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide has been detected, enhanced dust shielding in these areas helps maintain the cold Hi. 

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3. CO-dark Molecular Gas Traced by HCO ⁺ in the Diffuse Interstellar Medium

   https://doi.org/10.3847/1538-4357/ae76d4  

   Rybarczyk, Daniel R; Busch, Michael P; Dawson, J R; Lee, Min-Young; Luo, Gan (June 2026, The Astrophysical Journal)

   Abstract A classic problem in the study of the interstellar medium (ISM) is the near-invisibility of molecular hydrogen (H₂) in cold environments. Observations of CO emission are typically used to indirectly trace H₂, but a significant fraction of H₂in the diffuse ISM is not associated with any detectable CO emission (“CO-dark” molecular gas). Meanwhile, observations of HCO⁺absorption trace nearly all of the H₂in diffuse directions. In particular, a kinematically broad HCO⁺absorption signature traces extremely diffuse, CO-dark H₂. We have used sensitive observations of HCO⁺, CO, and atomic hydrogen (HI) in absorption to constrain the properties of such diffuse molecular gas in five directions. The diffuse molecular gas revealed by broad HCO⁺absorption has a lower fraction of cold H I ( $f_{\mathrm{CNM}}=0.38_{-0.27}^{+0.28}$) and a lower fraction of hydrogen in H₂( $f_{\mathrm{mol}}=0.09_{-0.03}^{+0.06}$) than gas traced by CO in the same directions. We detect almost no CO absorption from the gas traced by broad HCO⁺absorption. We constrain the CO abundance relative to H₂to be ≲10⁻⁶–10⁻⁵for gas traced by both broad and narrow HCO⁺absorption, consistent with chemical model predictions for the diffuse ISM. We further show that neither CO emission nor absorption is likely to be detected whereN(H2) ≲ few × 10¹⁹cm⁻²—a result of both the low CO abundance and the low H₂column—while HCO⁺absorption is readily detected forN(H2) ≳ few × 10¹⁸cm⁻². These results demonstrate that even modest amounts of cold H I can bear H₂, providing critical constraints on the HI-to-H₂transition in the ISM. 

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

   https://doi.org/10.3847/1538-4357/ad6604  

   Pingel, Nickolas M; Chen, Hongxing; Stanimirović, Snežana; Koch, Eric W; Leroy, Adam K; Rosolowsky, Erik; Kim, Chang-Goo; Dalcanton, Julianne J; Walter, Fabian; Busch, Michael P; et al (October 2024, The Astrophysical Journal)

   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. 

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5. Neutral Gas Phase Distribution from H I Morphology: Phase Separation with Scattering Spectra and Variational Autoencoders

   https://doi.org/10.3847/1538-4357/ae044a  

   Lei, Minjie; Clark, S E; Morel, Rudy; Allys, E; Butsky, Iryna S; Redshaw, Caleb; Fielding, Drummond B (October 2025, The Astrophysical Journal)

   Abstract Unraveling the multiphase structure of the diffuse interstellar medium as traced by neutral hydrogen (Hi) is essential to understanding the lifecycle of the Milky Way. However, Hiphase separation is a challenging and underconstrained problem. The neutral gas phase distribution is often inferred from the spectral line structure of Hiemission. In this work, we develop a data-driven phase-separation method that extracts Hiphase structure solely from the spatial morphology of Hiemission intensity structures. We combine scattering spectra (SS) statistics with a Gaussian-mixture variational autoencoder model to (1) derive an interpretable statistical model of different Hiphases from their multiscale morphological structures, and (2) we use this model to decompose the 2D channel maps of GALFA-Hiemission in diffuse high-latitude (∣b∣ > 30°) regions over narrow velocity channels (Δv = 3 km s⁻¹) into cold neutral medium (CNM), warm neutral medium (WNM), and noise components. We integrate our CNM map over velocity channels to compare it to an existing map produced by a spectrum-based method. We find that the two maps are highly correlated, but ours recovers more spatially coherent structures at small scales. Our work illustrates and quantifies a clear physical connection between the Himorphology and Hiphase structure, and it unlocks a new avenue for improving future phase-separation techniques by making use of both Hispectral and spatial information to decompose Hiin 3D position–position–velocity space. These results are consistent with a physical picture where processes that drive Hiphase transitions also shape the morphology of Higas, imprinting a sparse, filamentary CNM that forms out of a diffuse, extended WNM. 

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