Effects of Cloud Geometry and Metallicity on Shattering and Coagulation of Cold Gas, and Implications for Cold Streams Penetrating Virial Shocks

TL;DR

This study uses simulations to explore how cloud shape, metallicity, and UV background influence the fragmentation and coagulation of cold gas clouds in the circumgalactic medium, impacting galaxy evolution.

Contribution

It introduces a detailed analysis of cloud geometry effects on shattering and coagulation, revealing new regimes and size distributions of cold gas fragments.

Findings

Shattering occurs via cloud implosion and shock reflection, not characteristic size.

Sheets coagulate faster than spheres and streams, with maximum overdensity for rapid coagulation.

Fragmentation continues down to the grid scale, with a power-law mass distribution.

Abstract

Theory and observations reveal that the circumgalactic medium (CGM) and the cosmic web at high redshifts are multiphase, with small clouds of cold gas embedded in a hot, diffuse medium. A proposed mechanism is `shattering' of large, thermally unstable clouds into tiny cloudlets of size lshatter~min(cs*tcool). We study these processes using idealized numerical simulations of thermally unstable gas clouds. We expand upon previous works by exploring the effects of cloud geometry (spheres, streams, and sheets), metallicity, and the inclusion of an ionizing UV background. We find that `shattering' is triggered by clouds losing sonic contact and rapidly imploding, leading to a reflected shock which causes the cloud to re-expand and induces Richtmyer-Meshkov instabilities at its interface. After fragmentation the cloudlets experience a drag force from the surrounding hot gas, leading to recoagulation into larger clouds. We distinguish between `fast' and `slow' coagulation regimes, showing that sheets are always in the `fast' coagulation regime while streams and spheres have a maximum overdensity for rapid coagulation. The critical overdensity for spheres is smaller than for streams, such that the coagulation efficiency increases from spheres to streams to sheets. Surprisingly, lshatter does not appear to be a characteristic clump size even if it is well resolved. Rather, fragmentation continues until the grid scale with a mass distribution of N(>m)~m^{-1}. We apply our results to the case of cold streams feeding massive (Mv>10^{12}Msun) high-z (z>2) galaxies from the cosmic web, finding that streams are likely to shatter upon entering the CGM through the virial shock. This could explain the large clumping factors and covering fractions of cold gas in the CGM around such galaxies, and may be related to galaxy quenching by preventing cold streams from reaching the central galaxy. [abridged]