The halo and magnetic field of the Coma cluster of galaxies

New observations of the radio emission from the Coma cluster of galaxies have revealed the most detailed image of an intergalactic halo yet seen in a cluster of galaxies. The centroid of the halo emission is significantly displaced from that of the X-ray emission. The Faraday rotation of polarized emission from background radio sources has been measured. There is a significant contribution to the rotation measure (RM) of sources seen through the cluster. The excess RM, when combined with the X-ray data, has allowed direct measurement of the magnetic-field strength, at about 2 microG, providing an independent check of the widely used assumption of equipartition between the energy in relativistic particles and that in the magnetic field. The observations also allow an estimate of the degree of tangling of the cluster magnetic field. The magnetic field reversals seem to occur on a scale size of 13-40 kpc. The extent of the halo emission and the degree of field tangling indicate that the radio halo source must be sustained by particle reacceleration in the intracluster medium. 50 refs.