Cold Fronts: Probes of Plasma Astrophysics in Galaxy Clusters

TL;DR

Cold fronts in galaxy clusters serve as natural probes to understand the plasma physics of the intracluster medium, revealing insights into magnetic fields, viscosity, and thermal conduction through high-resolution X-ray observations and simulations.

Contribution

This review synthesizes recent simulation studies of cold front formation and evolution, highlighting their use in constraining plasma physics in galaxy clusters.

Findings

Cold fronts are often smooth despite Kelvin-Helmholtz instability expectations.

The interface width is narrower than particle mean free path, indicating suppressed thermal conduction.

Magnetic fields and viscosity significantly influence cold front stability.

Abstract

The most massive baryonic component of galaxy clusters is the "intracluster medium" (ICM), a diffuse, hot, weakly magnetized plasma that is most easily observed in the X-ray band. Despite being observed for decades, the macroscopic transport properties of the ICM are still not well-constrained. A path to determine macroscopic ICM properties opened up with the discovery of "cold fronts". These were observed as sharp discontinuities in surface brightness and temperature in the ICM, with the property that the brighter (and denser) side of the discontinuity is the colder one. The high spatial resolution of the Chandra X-ray Observatory revealed two puzzles about the cold fronts. First, they should be subject to Kelvin-Helmholtz instabilites, yet in many cases they appear relatively smooth and undisturbed. Second, the width of the interface between the two gas phases is typically narrower than the mean free path of the particles in the plasma, indicating negligible thermal conduction. From the time of their discovery, it was realized that these special characteristics of cold fronts may be used to probe the physical properties of the cluster plasma. In this review, we will discuss the recent simulations of cold front formation and evolution in galaxy clusters, with a focus on those which have attempted to use these features to constrain the physics of the ICM. In particular, we will focus on the effects of magnetic fields, viscosity, and thermal conductivity on the stability properties and long-term evolution of cold fronts. We conclude with a discussion on what important questions remain unanswered, and the future role of simulations and the next generation of X-ray observatories.