Linear Analyses of Thermal Instability in Stratified Medium

TL;DR

This paper analyzes how gravity influences thermal instability in the stratified circum-galactic medium, revealing over-stable modes and estimating gas accretion rates relevant to galaxy evolution.

Contribution

It provides a linear perturbation analysis of thermal instability in gravitationally stratified media, highlighting new over-stable modes and their implications for CGM dynamics.

Findings

Thermal instability can drive over-stable modes in stratified media.

Gravity and thermal instability together produce multiple unstable modes.

Estimated gas accretion rate matches observed star formation rates.

Abstract

Thermal instability in the circum-galactic medium (CGM) can be responsible for the existence of cold clouds (e.g., high-velocity clouds) embedded in a hot diffuse medium (e.g., X-ray emitting gas). While many previous studies have analyzed thermal instability in uniform medium, the instability mechanism in gravitationally stratified medium like CGM has not been fully analyzed. This study investigates how gravity affects the behavior of thermal instability through linear perturbation analyses.We find that in stratified medium, thermal instability can drive over-stable modes, a behavior distinctly different from the monotonic growth of thermal instability in a uniformmedium. Furthermore, we find that the combination of buoyancy and thermal instability drives other two unstable modes. Applying our results to a simplified model of the CGM, we estimate the gas accretion rate from the CGM to the Galactic disk and the typical size of high-velocity cloud driven by thermal instability to be a few solar masses per year. This gas accretion rate is comparable to the observed star formation rate, and hence, the mass in the Galactic disk can be maintained. Our results provide a theoretical framework for understanding the formation of multi-phase gas, particularly in the CGM.