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1. A New Constraint on the Relative Disorder of Magnetic Fields between Neutral Interstellar Medium Phases

   https://doi.org/10.3847/1538-4357/ad5ade  

   Lei, Minjie; Clark, S E (August 2024, The Astrophysical Journal)

   Utilizing Planck polarized dust emission maps at 353 GHz and large-area maps of the neutral hydrogen (Hi) cold neutral medium (CNM) fraction (f_CNM), we investigate the relationship between dust polarization fraction (p₃₅₃) andf_CNMin the diffuse high latitude ( $\left(b\right)>30°$) sky. We find that the correlation betweenp₃₅₃andf_CNMis qualitatively distinct from thep₃₅₃–Hicolumn density (N_Hi) relationship. At low column densities (N_Hi< 4 × 10²⁰cm⁻²) wherep₃₅₃andN_Hiare uncorrelated, there is a strong positivep₃₅₃–f_CNMcorrelation. We fit thep₃₅₃–f_CNMcorrelation with data-driven models to constrain the degree of magnetic field disorder between phases along the line of sight. We argue that an increased magnetic field disorder in the warm neutral medium (WNM) relative to the CNM best explains the positivep₃₅₃–f_CNMcorrelation in diffuse regions. Modeling the CNM-associated dust column as being maximally polarized, with a polarization fractionp_CNM∼ 0.2, we find that the best-fit mean polarization fraction in the WNM-associated dust column is 0.22p_CNM. The model further suggests that a significantf_CNM-correlated fraction of the non-CNM column (an additional 18.4% of the Himass on average) is also more magnetically ordered, and we speculate that the additional column is associated with the unstable medium. Our results constitute a new large-area constraint on the average relative disorder of magnetic fields between the neutral phases of the interstellar medium, and are consistent with the physical picture of a more magnetically aligned CNM column forming out of a disordered WNM. 

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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. Constraining the H ₂ column densities in the diffuse interstellar medium using dust extinction and H I data

   https://doi.org/10.1051/0004-6361/202347968  

   Skalidis, R; Goldsmith, P F; Hopkins, P F; Ponnada, S B (February 2024, Astronomy & Astrophysics)

   Context. Carbon monoxide (CO) is a poor tracer of H₂in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions. Aims. We present a novel, indirect method for constraining H₂column densities (N_H2) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction,A_V(H₂), derived from dust emission and the H Icolumn density (N_H I) are due to the presence of molecular gas. Methods. We employed archival (N_H2) data, obtained from the UV spectra of stars, and calculatedA_V(H₂) toward these sight lines using 3D extinction maps. The following relation fits the data: logN_H2= 1.38742 (logA_V(H₂)⁾³− 0.05359 (logA_V(H₂)⁾²+ 0.25722 logA_V(H₂) + 20.67191. This relation is useful for constrainingN_H2in the diffuse ISM as it requires onlyN_H Iand dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed aN_H2map of our Galaxy and compared it to the CO integrated intensity (W_CO) distribution. Results. We find that the average ratio (X_CO) betweenN_H2andW_COis approximately equal to 2 × 10²⁰cm⁻²(K km s⁻¹)⁻¹, consistent with previous estimates. However, we find that theX_COfactor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions withN_H2≳ 10²⁰cm⁻², we estimate that the average H₂fractional abundance,f_H2= 2N_H2/(2N_H2+N_H I), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (A_V≥ 1 mag), hence limitingf_H2in these directions. Conclusions. More than 50% of the lines of sight withN_H2≥ 10²⁰cm⁻²are untraceable by CO with aJ= 1−0 sensitivity limitW_CO= 1 K km s⁻¹. 

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4. Revisiting the Vertical Distribution of H i Absorbing Clouds in the Solar Neighborhood. II. Constraints from a Large Catalog of 21 cm Absorption Observations at High Galactic Latitudes

   https://doi.org/10.3847/1538-4357/ad79f7  

   Rybarczyk, Daniel R; Wenger, Trey V; Stanimirović, Snežana (October 2024, The Astrophysical Journal)

   The cold neutral medium (CNM) is where neutral atomic hydrogen (H I) is converted into molecular clouds, so the structure and kinematics of the CNM are key drivers of galaxy evolution. Here we provide new constraints on the vertical distribution of the CNM using the recently developed kinematic_scaleheight software package and a large catalog of sensitive H I absorption observations. We estimate the thickness of the CNM in the solar neighborhood to be σ z ∼ 50–90 pc, assuming a Gaussian vertical distribution. This is a factor of ∼2 smaller than typically assumed, indicating that the thickness of the CNM in the solar neighborhood is similar to that found in the inner Galaxy, consistent with recent simulation results. If we consider only structures with H I optical depths τ > 0.1 or column densities N(H I) > 1019.5 cm‑2, which recent work suggests are thresholds for molecule formation, we find σ z ∼ 50 pc. Meanwhile, for structures with τ < 0.1 or column densities N(H I) < 1019.5 cm‑2, we find σ z ∼ 120 pc. These thicknesses are similar to those derived for the thin- and thick-disk molecular cloud populations traced by CO emission, possibly suggesting that cold H I and CO are well mixed. Approximately 20% of CNM structures are identified as outliers, with kinematics that are not well explained by Galactic rotation. We show that some of these CNM structures—perhaps representing intermediate-velocity clouds—are associated with the Local Bubble wall. We compare our results to recent observations and simulations, and we discuss their implications for the multiphase structure of the Milky Way's interstellar medium. 

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5. 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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