The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies

TL;DR

This study uses hydrodynamical simulations to investigate how cool gas clouds survive in the hot circumgalactic medium of Milky Way-like galaxies, revealing that larger clouds can persist for hundreds of millions of years due to thermal conduction effects.

Contribution

It provides new insights into the longevity and fragmentation of cool gas clouds in the CGM, highlighting the role of thermal conduction in their survival.

Findings

Large clouds (>250 pc) survive over 250 Myr despite partial destruction.

Thermal conduction prevents hydrodynamical instabilities, maintaining cloud integrity.

Simulated column densities match observed ion distributions in the CGM.

Abstract

Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM). To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4 cm^-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~ 50-150 kpc). We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc) and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc) the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations are compatible with those observed for low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.