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

Precipitation of cold gas due to thermal instability in both galaxy clusters and the circumgalactic medium may regulate active galactic nucleus 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(cool)/t(ff)less than or similar to 5. Our simulations with a ratio of t(cool)/t(ff)similar to 10 do not precipitate under any circumstances, perhaps because the thermostat mechanism we use maintains a significant non-zero entropy gradient.