The stable thermal unstable medium and its effect on cold filament in multi-phase ISM

The ubiquity of very thin and lengthy cold neutral medium (CNM) has been reported by multiple authors in the H I community. Yet, the reason of how the CNM can be so long and lengthy is still in debate. In this paper, we recognize a new type of instability due to the attractive nature of the pressure force in the unstable phase. Furthermore, we demonstrate that the presence of turbulence can stabilize the originally unstable phase through its mixing effects. We provide a new estimation of the average CNM filament aspect ratio with the consideration of force balances at the phase boundary, which is roughly 5–20 in common CNM environment. We show that most of the cold filaments are less filamentary than what usually predicted via MHD turbulence theory or inferred from observations: The average length of CNM filament is roughly 1/2 of that in isothermal MHD turbulence with similar turbulence conditions. This suggests that the ‘cold filaments’ that are identified in observations might not be in pressure equilibrium or generated via other mechanisms.