PREPRINT
431E7D02-C5F5-4ED2-8686-B5138824F5DF

# Cooling driven coagulation

Max Gronke, S. Peng Oh
arXiv:2209.00732

Submitted on 1 September 2022

## Abstract

Astrophysical gases such as the interstellar-, circumgalactic- or intracluster-medium are commonly multiphase, which poses the question of the structure of these systems. While there are many known processes leading to fragmentation of cold gas embedded in a (turbulent) hot medium, in this work, we focus on the reverse process: coagulation. This is often seen in wind-tunnel and shearing layer simulations, where cold gas fragments spontaneously coalesce. Using 2D and 3D hydrodynamical simulations, we find that sufficiently large ($\gg {c}_{\mathrm{s}}{t}_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}$), perturbed cold gas clouds develop overstable sound waves which ensure cold gas mass growth over an extended period of time ($\gg r/{c}_{\mathrm{s}}$). This mass growth efficiently accelerates hot gas which in turn can entrain cold droplets, leading to coagulation. The attractive inverse square force between cold gas droplets has interesting parallels with gravity; the `monopole' is surface area rather than mass. We develop a simple analytic model which reproduces our numerical findings.

## Preprint

Comment: 13 pages, 11 figures; submitted to MNRAS

Subject: Astrophysics - Astrophysics of Galaxies