Cloudy with A Chance of Rain: Accretion Braking of Cold Clouds

TL;DR

This paper develops a theoretical framework and simulations to understand how cold gas clouds survive and grow during infall in galaxy environments, emphasizing accretion braking effects and critical conditions for survival.

Contribution

It introduces a new analytic theory for accretion braking of cold clouds and validates it with 3D hydrodynamic simulations, highlighting the importance of growth rates for cloud survival.

Findings

Cold gas clouds can experience sub-virial velocities due to accretion drag.

Survival requires clouds to grow faster than they are destroyed, with a specific timescale criterion.

A critical pressure threshold for cloud survival in Milky Way-like conditions is identified.

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 sub-virial 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, as well as 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} < 4\,t_{\rm cc} $). This can be translated to a critical pressure, which for Milky Way like conditions is $P \sim 3000 {\rm k}_B {\rm K}\,{\rm cm}^{-3}$ . Cold gas which forms via linear thermal instability ($t_{\rm cool}/t_{\rm ff} < 1$) 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.