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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. 

Abstract Observational studies of Hiiregion–molecular cloud interactions constrain models of feedback and quantify its impact on the surrounding environment. A recent hypothesis proposes that a characteristic spectral signature in ground state hyperfine lines of hydroxyl (OH)—the OH flip—may trace gas that is dynamically interacting with an expanding Hiiregion, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array observations of three Galactic Hiiregions, G049.205−0.343, G034.256+0.145, and G024.471+0.492, in 1–2 GHz continuum emission, all four 18 cm ground-state OH lines, and multiple hydrogen radio recombination lines. A Gaussian decomposition of the molecular gas data reveals complex OH emission and absorption across our targets. We detect the OH flip toward two of our sources, G049.205−0.343 and G034.256+0.145, finding agreement between key predictions of the flip hypothesis and the observed multiwavelength spectra, kinematics, and morphology. Specifically, we demonstrate a strong spatial and kinematic association between the OH flip and the ionized gas of the Hiiregions—the first time this has been demonstrated for resolved sources—and evidence from¹³CO(1–0) data that the expected OH component originates from the nondisturbed gas of the parent cloud. While we detect no flip in G024.471+0.492, we do find evidence of interacting molecular gas traced by OH, providing further support for OH’s ability to trace Hiiregion–molecular cloud interactions. 

Abstract We present the first localized detections of the cold neutral medium (CNM) in IC 10, offering a rare view of dense atomic gas in a low-metallicity (Z/Z_⊙ ∼ 0.27) dwarf galaxy. As a low-metallicity starburst, IC 10’s interstellar medium conditions could reflect small scale physics conditions that mirror those of early galaxies, providing a unique window into the heating and cooling processes that shaped the interstellar medium in early-Universe environments. Leveraging the high angular (<5″ ∼ 15 pc) and spectral (0.4 km s⁻¹) resolution of the Local GroupL-band Survey, we searched for HIabsorption against nine continuum radio sources and detected absorption along three sightlines corresponding to internal radio emission sources within IC 10. Using Gaussian decomposition and radiative transfer, we characterize the CNM, deriving spin temperatures of ∼30–55 K, column densities of (0.6–3.0) × 10²¹cm⁻², cold HIfractions of ∼21%–37%, and line widths of ∼5.6–13.6 km s⁻¹. For each individual detection of HIabsorption, we find corresponding molecular emission from¹²CO (J= 1–0), HCO⁺(J= 1–0), and HCN (J= 1–0) at similar velocities and with comparable line widths, indicating a well-mixed cold atomic and molecular medium. In IC 10, the CNM shows a clear kinematic connection to the high-density ISM, implying a stronger dynamical coupling with molecular gas than in the Milky Way, in line with expectations for low-metallicity environments. At the ∼15 pc scales probed by slightly extended HIIregions in IC 10, unresolved CNM clouds likely contribute to line blending, so the observed broad HIline widths may partly reflect spatial and kinematic averaging. 

The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new high-sensitivity absorption line observations of the four ground state hyperfine-splitting transitions of OH at 18 cm toward four Galactic and extragalactic continuum sources as follow-up to the THOR survey. We compared these to deep observations of the [C II] 158 µm line at 1.9 THz obtained with the upGREAT instrument on SOFIA, observations of the neutral atomic hydrogen (H I) 21 cm line with the VLA, and CO (J= 2–1) lines obtained with the APEX PI230 receiver at the APEX 12 m sub-mm telescope. We trace OH over a large range of molecular hydrogen column densities of 7.9 × 10¹⁹cm^-2to 4.7 × 10²²cm^-2, and derive OH abundances with respect to molecular and total hydrogen column densities ofX_OH,H2=N_OH/N_H2= 1.2_−0.2^+0.3× 10⁻⁷andX_OH,H=N_OH/N_H= 4.8_−0.8^+0.9× 10⁻⁸, respectively. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features with the lowest column density detected atN_OH= 3.7 × 10¹³cm^-2. The increase in sensitivity is a factor of five in direct comparison at the resolution the OH observations in the THOR survey (1.5 km s^-1). We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions (f_mol≤ 0.8). A potential association of [C II] 158 μm emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears to be tracing only the upper end of the distribution of CO-dark OH features found by previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it is not an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions. 

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. 

We present the largest Galactic neutral hydrogen H i absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30 arcsec. This survey, GASKAP-H i, unbiasedly targets 2714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between $$-45^{\circ }$$ and $$-25^{\circ }$$). We detected strong H i absorption along 462 lines of sight above the 3$$\sigma$$ threshold, achieving an absorption detection rate of 17 per cent. GASKAP-H i’s unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of H i optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighbourhood environments, indicating that cold H i properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for $$\sim$$30 per cent of the total H i gas, with the CNM fraction increasing with column density towards the two filaments. Our analysis reveals an anticorrelation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature. 

The Magellanic Stream (MS), a tail of diffuse gas formed from tidal and ram pressure interactions between the Small and Large Magellanic Clouds (SMC and LMC) and the Halo of the Milky Way, is primarily composed of neutral atomic hydrogen (HI). The deficiency of dust and the diffuse nature of the present gas make molecular formation rare and difficult, but if present, could lead to regions potentially suitable for star formation, thereby allowing us to probe conditions of star formation similar to those at high redshifts. We search for HCO+ ,HCN,HNC,andC2H using the highest sensitivity observations of molecular absorption data from the Atacama Large Millimeter Array (ALMA) to trace these regions, comparing with HI archival data from the Galactic Arecibo L-Band Feed Array (GALFA) HI Survey and the Galactic All Sky Survey (GASS) to compare these environments in the MS to the HI column density threshold for molecular formation in the Milky Way. We also compare the line of sight locations with confirmed locations of stars, molecular hydrogen, and OI detections, though at higher sensitivities than the observations presented here. 

Abstract We investigate the kinematic properties of Galactic H ii regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array at 4–10 GHz and the Jansky Very Large Array at 8–10 GHz. Our H ii region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position–velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m s − 1 arcsec − 1 . About 15% of the sources also have an RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic plane. We craft H ii region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, >40 m s − 1 arcsec − 1 , velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most H ii region complexes are rotating and may have inherited angular momentum from their parent molecular clouds.