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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.
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Cloudy with a chance of rain: accretion braking of cold clouds
ABSTRACT Understanding the survival, growth, and dynamics of cold gas is fundamental to galaxy formation. While there has been a plethora of work on ‘wind tunnel’ simulations that study such cold gas in winds, the infall of this gas under gravity is at least equally important, and fundamentally different since cold gas can never entrain. Instead, velocity shear increases and remains unrelenting. If these clouds are growing, they can experience a drag force due to the accretion of low-momentum gas, which dominates over ram pressure drag. This leads to subvirial terminal velocities, in line with observations. We develop simple analytic theory and predictions based on turbulent radiative mixing layers. We test these scalings in 3D hydrodynamic simulations, both for an artificial constant background and a more realistic stratified background. We find that the survival criterion for infalling gas is more stringent than in a wind, requiring that clouds grow faster than they are destroyed ($$t_{\rm grow} \lt 4\, t_{\rm cc}$$). This can be translated to a critical pressure, which for Milky Way-like conditions is $$P \sim 3000 \, {k}_\mathrm{ B} \, {\rm K}\, {\rm cm}^{-3}$$. Cold gas that forms via linear thermal instability (tcool/tff < 1) in planar geometry meets the survival threshold. In stratified environments, larger clouds need only survive infall until cooling becomes effective. We discuss applications to high-velocity clouds and filaments in galaxy clusters.
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- Award ID(s):
- 1911198
- PAR ID:
- 10396574
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 520
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 2571-2592
- Size(s):
- p. 2571-2592
- Sponsoring Org:
- National Science Foundation
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