Published in Journal of Plasma Physics, Volume 82, Issue 3, article id. 535820301, 48 pp, 2016.
https://arxiv.org/abs/1603.08882

For a PDF version of the article, click here.

COLD FRONTS: PROBES OF PLASMA ASTROPHYSICS IN GALAXY CLUSTERS

J. A. ZuHone 1,2 and E. Roediger 3


1 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
2 Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138, USA
3 E.A. Milne Centre for Astrophysics, Department of Physics and Mathematics, University of Hull, Hull, HU6 7RX, United Kingdom


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.


Table of Contents

INTRODUCTION
Classes of Cold Fronts
Cold Fronts and ICM Plasma Physics

OBSERVATIONS OF COLD FRONTS
NGC 1404/Fornax
A3667
A2142
Virgo
NGC 7618/UGC 12491

SIMULATIONS OF COLD FRONTS
Basic Physics
Results from Simulations of the Formation and Global Evolution of Cold Fronts
Magnetic Fields
Cold Fronts and Viscosity
Cold Fronts and Thermal Conductivity

DISCUSSION
Open Questions
The Role of Plasma Instabilities
Distinguishing Between the Effects of Magnetic Fields and Viscosity
How Efficient is Thermal Conduction in the ICM?
Future Observations

SUMMARY

APPENDIX A

REFERENCES

Next