Cooling-driven coagulation

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 (>> c(s)t(cool)), perturbed cold gas clouds develop pulsations which ensure cold gas mass growth over an extended period of time (>> r/c(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.

Automated summary

This study investigates the multiphase structures of astrophysical gases, such as interstellar and circumgalactic mediums. Through hydrodynamical simulations, it is found that pulsations in perturbed cold gas clouds lead to mass growth and coagulation with hot gas and cold droplets. The attractive force between cold gas droplets resembles gravity, where the 'monopole' is in terms of surface area. A simple analytic model is developed to validate the numerical findings.