Linear analyses of thermal instability in stratified medium

ABSTRACT Thermal instability in the circumgalactic 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 analysed thermal instability in uniform medium, the instability mechanism in gravitationally stratified medium like CGM has not been fully analysed. This study investigates how gravity affects the behaviour of thermal instability through linear perturbation analyses. We find that in stratified medium, thermal instability can drive overstable modes, a behaviour distinctly different from the monotonic growth of thermal instability in a uniform medium. 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 disc and the typical size of high-velocity cloud driven by thermal instability to be a few $\mathrm{M}_\odot \, {\rm yr}^{-1}$. This gas accretion rate is comparable to the observed star formation rate, and hence, the mass in the Galactic disc can be maintained. Our results provide a theoretical framework for understanding the formation of multi-phase gas, particularly in the CGM.