The halo radio source Coma C and the origin of halo sources

Abstract

We present in this paper the first high-resolution spectral index map of the halo source Coma C, which was obtained combining WSRT data at 326 MHz and DRAO + VLA data at 1.4 GHz. The spectral index distribution shows a central region, of ~8' radius, where the spectral index is almost constant (~0.8), surrounded by a region where the spectrum steepens to α ~ 1.8. In addition, we present new observations of Coma C obtained with the WSRT at 608.5 MHz. The spectral index information and high-resolution maps are used to derive the physical conditions in Coma C and understand the processes which lead to the formation of halo sources in clusters. It is suggested that the origin of relativistic particles of Coma C is the large head-tail radio galaxy NGC 4869, orbiting at the Coma cluster center. The spectral index distribution implies that the electrons left behind from the galaxy's motion are continually reaccelerated in situ, through the weak shocks in the intracluster medium produced by galaxy wakes. According to our suggestion on the origin of radiating electrons in Coma C, we explain the rarity of radio halos as due to the difficulty of finding tailed radio galaxies orbiting at the cluster centers.