Is Multiphase Gas Cloudy or Misty?

TL;DR

This study uses 3D hydrodynamic simulations to explore how cold gas clouds fragment or reassemble in hot media, revealing conditions that lead to cloud shattering into droplets or their survival as larger structures.

Contribution

The paper provides new insights into the fragmentation process of cold gas clouds, highlighting the role of perturbations and overdensity in determining their fate in hot environments.

Findings

Cloud shattering occurs with large perturbations and high overdensity.

Droplets can coagulate due to turbulence and cooling.

Cloud size has a weak effect on the fragmentation outcome.

Abstract

Cold $T \sim 10^{4}$K gas morphology could span a spectrum ranging from large discrete clouds to a fine `mist' in a hot medium. This has myriad implications, including dynamics and survival, radiative transfer, and resolution requirements for cosmological simulations. Here, we use 3D hydrodynamic simulations to study the pressure-driven fragmentation of cooling gas. This is a complex, multi-stage process, with an initial Rayleigh-Taylor unstable contraction phase which seeds perturbations, followed by a rapid, violent expansion leading to the dispersion of small cold gas `droplets' in the vicinity of the gas cloud. Finally, due to turbulent motions, and cooling, these droplets may coagulate. Our results show that a gas cloud `shatters' if it is sufficiently perturbed out of pressure balance ($\delta P/P\sim 1$), and has a large final overdensity $\chi_{\mathrm{f}}\gtrsim 300$, with only a weak dependence on the cloud size. Otherwise, the droplets reassemble back into larger pieces. We discuss our results in the context of thermal instability, and clouds embedded in a shock heated environment.