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arXiv · 2603.14967

Mapping of the Cold Neutral Medium via HI Phase Separation in an Atomic Cloud Undergoing Molecular Cloud Formation

Yamato Matsuzuki·Hiroaki Yamamoto·Kengo Tachihara
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Abstract

We investigate the atomic-to-molecular gas transition in the molecular cloud formation region HLCG 92-35 using GALFA-H I and NANTEN 12CO(J=1-0) observations. The HI spectra were decomposed into Cold Neutral Medium (CNM), Lukewarm Neutral Medium (LNM), and Warm Neutral Medium (WNM) components with the ROHSA algorithm, and CNM structures were identified using Astrodendro. The derived HI mass ratio is CNM:LNM:WNM = 43:47:10, indicating that a substantial fraction of the gas resides in the intermediate-temperature phase. We identify 1792 CNM clumps with characteristic sizes of ~1-2 pc and average densities of ~ 80 cm^{-3}, broadly consistent with theoretical predictions for CNM structures formed through thermal instability. The CNM mass spectrum exhibits a power-law slope similar to that of CO clumps, suggesting that molecular clouds inherit the hierarchical structure established in the atomic phase. Significant non-thermal linewidths and localized CNM-CO velocity offsets imply that the CNM consists of small-scale cloudlets with distinct kinematic properties. These results provide observational support for a scenario in which small-scale CNM structures produced by thermal instability and turbulent flows represent an early stage of molecular cloud formation.

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Yamato Matsuzuki, Hiroaki Yamamoto, Kengo Tachihara. 2026-03-16. Mapping of the Cold Neutral Medium via HI Phase Separation in an Atomic Cloud Undergoing Molecular Cloud Formation. https://arxiv.org/abs/2603.14967

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