paper

A Role for Turbulence in Circumgalactic Precipitation

arXiv:1803.06036 · doi:10.3847/1538-4357/aae8e2

Abstract

Abundant observational evidence indicates that the cooling time t_cool of the hot ambient medium pervading a massive galaxy does not drop much below 10 times the freefall time t_ff at any radius. Theoretical models have accounted for this finding by hypothesizing that cold clouds start to condense out of the ambient medium when t_cool/t_ff < 10 and fuel a strong black-hole feedback response that reheats the ambient gas, but those models have not yet been able to provide a simple explanation for the origin of the critical t_cool/t_ff ratio. This paper explores a heuristic model for condensation that links the critical ratio to turbulent driving of gravity-wave oscillations. In the linear regime, internal gravity waves are thermally unstable in a thermally balanced medium. Buoyancy oscillations in a balanced medium with t_cool/t_ff therefore grow until they saturate without condensing at an amplitude that depends on t_cool/t_ff. However, in a medium with 10 < t_cool/t_ff < 20, turbulence with a velocity dispersion roughly half the galaxy's stellar velocity dispersion can drive those oscillations into condensation. Intriguingly, this is indeed the gas-phase velocity dispersion observed among galaxy-cluster cores that contain multiphase gas. It is therefore possible that both the critical t_cool/t_ff ratio for condensation of ambient gas and the level of turbulence in that gas are determined by coupling between condensation, feedback, and turbulence. Such a system can converge to a well-regulated equilibrium state, as long as the fraction of feedback energy that goes into turbulence is significantly less than the fraction that goes more directly into heat.

ApJ, in press, 23 pages, 9 figures (v3 corresponds to published version)

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