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

Abstract

We investigate the atomic-to-molecular gas transition in the molecular formation cloud HLCG 92-35. Using the ROHSA algorithm to decompose GALFA-H I data, we find the Lukewarm Neutral Medium (LNM) to be the dominant mass component, indicating a state driven out of thermal equilibrium by turbulence or past shocks. Spatial analysis reveals an inverse correlation between the phase distributions, with small-scale Cold Neutral Medium (CNM) structures embedded within an extended LNM envelope.Using Astrodendro, we identified 2,214 CNM clumps with sub-parsec scales. While the CNM mass spectrum steepens at high masses, its intermediate-mass slope matches that of CO clumps, suggesting that molecular clouds inherit the hierarchical structure of the CNM. Significant non-thermal linewidths and localized CNM-CO velocity offsets imply that the CNM consists of subsonic cloudlets moving collectively as aggregates. Our results show that these sub-parsec CNM structures are the fundamental building blocks of the cold interstellar medium, driven by thermal instability and turbulent compression.