Much progress has been made recently in the acceleration of ∼104 K clouds to explain absorption line measurements of the circumgalactic medium and the warm, atomic phase of galactic winds. However, the origin of the cold, molecular phase in galactic winds has received relatively little theoretical attention. Studies of the survival of ∼104 K clouds suggest efficient radiative cooling may enable the survival of expelled material from galactic discs. Alternatively, gas colder than 104 K may form within the outflow, including molecules if dust survives the acceleration process. We explore the survival of dusty clouds in a hot wind with three-dimensional hydrodynamic simulations including radiative cooling and dust modelled as tracer particles. We find that cold ∼103 K gas can be destroyed, survive, or transformed entirely to ${\sim}10^4\,$ K gas. We establish analytic criteria distinguishing these three outcomes that compare characteristic cooling times to the system’s ‘cloud crushing’ time. In contrast to typically studied ∼104 K clouds, colder clouds are entrained faster than the drag time as a result of efficient mixing. We find that while dust can in principle survive embedded in the accelerated clouds, the survival fraction depends critically on the time dust spends in the hot phase and on the effective threshold temperature for destruction. We discuss our results in the context of polluting the circumgalactic medium with dust and metals, as well as understanding observations suggesting rapid acceleration of molecular galactic winds and ram-pressure-stripped tails of jellyfish galaxies.
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.
more » « less- Award ID(s):
- 1911198
- NSF-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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