How the existence of unstable neutral media restricts the aspect ratio of cold neutral media?

TL;DR

This paper investigates how unstable neutral media influence the aspect ratio of cold neutral media, revealing a new instability mechanism and providing revised estimates that challenge previous turbulence-based predictions.

Contribution

It introduces a new instability due to pressure forces in unstable phases and offers a revised aspect ratio estimate for CNM filaments considering force balances.

Findings

Average CNM filament aspect ratio is 5-20.

Most cold filaments are less filamentary than turbulence predictions.

Observed cold filaments may not be in pressure equilibrium.

Abstract

The ubiquity of very thin and lengthy cold neutral media (CNM) has been reported by multiple authors in the HI 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. 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 is identified in observations might not be in pressure equilibrium or generated via other mechanisms.