Review article to appear in International Journal of Modern Physics D
astro-ph/0410182

For a PDF version of the article, click here.


MAGNETIC FIELDS IN CLUSTERS OF GALAXIES

Federica Govoni


Dipartimento di Astronomia, Universitá di Bologna, Via Ranzani 1,
i-40127 bologna, italy
fgovoni@ira.cnr.it

Luigina Feretti


Istituto di Radioastronomia CNR/INAF, Via Gobetti 101,
i-40129 bologna, italy
lferetti@ira.cnr.it


Abstract. The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the µG level, with values up to tens of µG at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant through the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster radius with a rough dependence on the thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate over a wide range of spatial scales with values from a few kpc up to hundreds kpc. Important information on the cluster field are obtained by comparing the observational results with the prediction from numerical simulations. The origin of cluster magnetic fields is still debated. They might originate in the early Universe, either before or after the recombination, or they could have been deposited in the intracluster medium by normal galaxies, starburst galaxies, or AGN. In either case, magnetic fields undergo significant amplification during the cluster merger processes.


Keywords: Clusters of galaxies; Magnetic field; Intracluster medium; Radio emission; Non-thermal emission


Table of Contents

INTRODUCTION

CLUSTERS OF GALAXIES

THEORETICAL BACKGROUND RELATED TO CLUSTER MAGNETIC FIELDS
Synchrotron radiation
Equipartition magnetic fields derived from the synchrotron emission
Inverse Compton radiation
Magnetic fields derived from IC emission
Faraday Rotation effect

DIFFUSE RADIO EMISSION IN CLUSTERS OF GALAXIES
Radio halos
Relics
Mini-halos
Magnetic fields beyond clusters

HARD X-RAY EMISSION IN CLUSTERS

ROTATION MEASURES

CLUSTER COLD FRONTS

MAGNETIC FIELD PROFILE

RECONCILING MAGNETIC FIELD VALUES

CLUSTER MAGNETIC FIELDS THROUGH A NUMERICAL APPROACH
Simulated rotation measures
Simulated radio halo polarization

ORIGIN AND AMPLIFICATION OF CLUSTER MAGNETIC FIELDS

CONCLUSIONS

REFERENCES

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