Effects of cloud geometry and metallicity on shattering and coagulation of cold gas, and implications for cold streams penetrating virial shocks

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. We study the ‘shattering’ of large, thermally unstable clouds into tiny cloudlets of size $\ell _{\rm shatter}\sim {\rm min}(c_{\rm s}t_{\rm cool})$ using idealized numerical simulations. We expand upon previous works by exploring the effects of cloud geometry (spheres, streams, and sheets), metallicity, and an ionizing ultraviolet background. We find that ‘shattering’ is mainly triggered by clouds losing sonic contact and rapidly imploding, leading to a reflected shock that causes the cloud to re-expand and induces Richtmyer–Meshkov instabilities at its interface. The fragmented cloudlets experience a drag force from the surrounding hot gas, leading to recoagulation into larger clouds. We distinguish between ‘fast’ and ‘slow’ coagulation regimes. Sheets are always in the ‘fast’ coagulation regime, while streams and spheres transition to ‘slow’ coagulation above a critical overdensity, which is smallest for spheres. Surprisingly, $\ell _\mathrm{shatter}$ 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 $N(\gt m)\propto m^{-1}$. We apply our results to cold streams feeding massive ($M_{\rm v}\lower.5ex\rm{\,\, \buildrel\gt \over \sim \,\,}10^{12}\, {\rm M}_\odot$) galaxies at $z\lower.5ex\rm{\,\, \buildrel\gt \over \sim \,\,}2$ from the cosmic web, finding that streams likely shatter upon entering the hot CGM through the virial shock. This could explain the large clumping factors and covering fractions of cold gas around such galaxies, and may be related to galaxy quenching by preventing cold streams from reaching the central galaxy.