Precipitation possible: turbulence-driven thermal instability with constrained entropy profiles

TL;DR

This study uses extensive 3D hydrodynamic simulations to explore how turbulence-driven thermal instability leads to cold gas precipitation in galaxy cluster-like environments, emphasizing the role of turbulence type and entropy profiles.

Contribution

It provides a comprehensive parameter study of turbulence-driven thermal instability in the circumgalactic medium, highlighting the impact of turbulence type and entropy profiles on cold gas formation.

Findings

Cold gas precipitates when turbulence is vertical and $t_{cool}/t_{ff} \, \lesssim 5$.

No precipitation occurs at $t_{cool}/t_{ff} \sim 10$ due to entropy gradient effects.

Precipitation depends strongly on turbulence driving mechanism and entropy profile.

Abstract

Precipitation of cold gas due to thermal instability in both galaxy clusters and the circumgalactic medium may regulate AGN feedback. We investigate thermal instability in idealized simulations of the circumgalactic medium with a parameter study of over 600 three-dimensional hydrodynamic simulations of stratified turbulence with cooling, each evolved for 10 Gyr. The entropy profiles are maintained in a steady state via an idealized `thermostat' process, consistent with galaxy cluster entropy profiles. In the presence of external turbulent driving, we find cold gas precipitates, with a strong dependence whether the turbulent driving mechanism is solenoidal, compressive, or purely vertical. In the purely-vertical turbulent driving regime, we find that significant cold gas may form when the cooling time to free-fall time $t_{\rm cool} / t_{\text{ff}} \lesssim 5$. Our simulations with a ratio of $t_{\rm cool} / t_{\text{ff}} \sim 10$ do not precipitate under any circumstances, perhaps because the thermostat mechanism we use maintains a significant non-zero entropy gradient.