Cosmic Ray Mediated Thermal Fronts in the Warm-Hot Circumgalactic Medium

TL;DR

This study explores how cosmic rays influence the thermal transition layers between warm and hot phases in the circumgalactic medium, comparing model predictions with observational data.

Contribution

It demonstrates that cosmic rays primarily control the thermal balance in these transition layers, offering new insights into their role in CGM physics.

Findings

Cosmic rays dominate thermal balance in the transition layers.

Most models do not match observational ion ratios.

Some models successfully reproduce certain observational features.

Abstract

We investigate the 1D plane-parallel front connecting the warm ($10^4$ K) and hot ($10^6$ K) phases of the circumgalactic medium (CGM), focusing on the influence of cosmic rays (CRs) in shaping these transition layers. We find that cosmic rays dictate the thermal balance while other fluxes (thermal conduction, radiative cooling, and gas flow) adjust to compensate. We compute column density ratios for selected transition temperature ions and compare them with observational data. While most of our models fail to reproduce the observations, a few are successful, although we make no claims for their uniqueness. Some of the discrepancies may indicate challenges in capturing the profiles in cooler, photoionized regions, as has been suggested for by previous efforts to model thermal transition layers.