The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

TL;DR

This study explores how cosmic rays influence the formation, size, and longevity of cold gas clouds in the circumgalactic medium, revealing their role in reducing galactic accretion and aligning models with observations.

Contribution

It introduces the first detailed magnetohydrodynamic simulations of cosmic ray effects on thermal instability in the CGM, highlighting their impact on cold gas properties and galactic inflow rates.

Findings

Cosmic rays increase cold cloud sizes and lower their densities.

Cosmic rays extend cold gas lifetimes in the CGM.

They significantly reduce galactic accretion rates.

Abstract

Large reservoirs of cold (~ 10^4 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic ray pressure. Cosmic ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic ray transport, by streaming or diffusion, redistributes cosmic ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in-between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.