2018/03/31 by G. Mark Voit, G. M. Voit
Engineering · Physics and Astronomy · #Astrophysics #Astrophysics and Star Formation Studies #Buoyancy #Condensation #Fluid Dynamics and Turbulent Flows #Galaxies: Formation, Evolution, Phenomena #Galaxy #Galaxy cluster #Mechanics #Physics #RADIUS #Thermodynamics #Turbulence #Velocity dispersion #astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/aae8e2
ApJ, in press, 23 pages, 9 figures (v3 corresponds to published version)
arxiv created 2018/11/13 · openalex publication_date 2018/11/27 · arxiv updated 2018/12/05 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
Abstract 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, but they have not yet provided a simple explanation for the critical t cool / t ff ratio. This paper explores a heuristic model for condensation linking 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 ≫ 1 therefore grow until they saturate without condensing at an amplitude depending 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 multiphase galaxy cluster cores. 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, if the fraction of feedback energy going into turbulence is subdominant.