The stable "Unstable Natural Media" due to the presence of turbulence

Abstract

The term "unstable neutral media" (UNM) has traditionally been used to describe the transient phase formed between the warm and cold neutral hydrogen (HI) phases and has not been the focus of HI studies. However, recent observations suggest that the UNM phase not only has a significantly longer-than-expected lifetime but also occupies at least 20\% to 40\% of both the volume and mass fraction of HI. In this paper, we argue that the existence and dominance of the UNM can be explained by the presence of strong turbulence using an energy balance argument. The mass fraction of UNM is directly proportional to the turbulent velocity dispersion $\sigma_v$: mass fraction of UNM $\propto \sigma_v^{\frac{2n}{1+n}}$, where $n$ is the absolute value of the adiabatic index in the unstable phase. We discuss the implications of long-lived unstable thermal phases on ISM physics, including cold dense filament formation, cosmic ray acceleration, and measurement of galactic foreground statistics.