On the Survival of Cool Clouds in the Circum-Galactic Medium

TL;DR

This study investigates the survival mechanisms of cool clouds in the circum-galactic medium through simulations that include various physical processes, revealing four distinct regimes of cloud longevity influenced mainly by radiative cooling and conduction.

Contribution

It provides a comprehensive simulation-based analysis of cool cloud survival, identifying key physical regimes and developing analytic models to predict cloud lifetimes under different conditions.

Findings

Realistic magnetic fields and turbulence have weaker effects on cloud survival.

Four regimes of cloud longevity are identified based on physical parameters.

Cloud lifetime can exceed the cloud-crushing time due to conduction effects.

Abstract

We explore the survival of cool clouds in multi-phase circum-galactic media. We revisit the "cloud crushing problem" in a large survey of simulations including radiative cooling, self-shielding, self-gravity, magnetic fields, and anisotropic Braginskii conduction and viscosity (with saturation). We explore a wide range of parameters including cloud size, velocity, ambient temperature and density, as well as a variety of magnetic field configurations and cloud turbulence. We find that realistic magnetic fields and turbulence have weaker effects on cloud survival; the most important physics is radiative cooling and conduction. Self-gravity and self-shielding are important for clouds which are initially Jeans-unstable, but largely irrelevant otherwise. Non-self-gravitating, realistically magnetized clouds separate into four regimes: (1) At low column densities, clouds evaporate rapidly via conduction. (2) A "failed pressure confinement" regime, where the ambient hot gas cools too rapidly to provide pressure confinement for the cloud. (3) An "infinitely long-lived" regime, in which the cloud lifetime becomes longer than the cooling time of gas swept up in the leading bow shock, so the cloud begins to accrete and grow. (4) A "classical cloud destruction" regime, where clouds are eventually destroyed by instabilities. In the final regime, the cloud lifetime can exceed the naive cloud-crushing time owing to conduction-induced compression. However, small and/or slow-moving clouds can also evaporate more rapidly than the cloud-crushing time. We develop simple analytic models that explain the simulated cloud destruction times in this regime.