On the origin of radio halos in galaxy clusters

Previously, it has been recognized that radio halos in galaxy clusters are preferentially associated with merging systems, as indicated by substructur e in the X-ray images and temperature maps. Since, however, many clusters w ithout radio halos also possess substructure, the role of mergers in the fo rmation of radio halos has remained unclear. By using power ratios to relat e gravitational potential fluctuations to substructure in X-ray images, we provide the first quantitative comparison of the dynamical states of cluste rs possessing radio halos. A correlation between the 1.4 GHz power (P-1.4) of the radio halo (or relic) and the magnitude of the dipole power ratio (P -1/P-0) is discovered such that approximately P-1.4 proportional to P-1/P-0 ; i.e., the strongest radio halos appear only in those clusters currently e xperiencing the largest departures from a virialized state. From the additi onal consideration of a small number of highly disturbed clusters without r adio halos detected at 1.4 GHz and recalling that radio halos are more comm on in clusters with high X-ray luminosity (Giovannini, Tordi, & Feretti), w e argue that radio halos form preferentially in massive (L-x greater than o r similar to 0.5 x 10(45) ergs s(-1)) clusters experiencing violent mergers (P-1/P-0 greater than or similar to 0.5 x 10(-4)) that have seriously disr upted the cluster core. The association of radio halos with massive, large- P-1/P-0, core-disrupted clusters can account for both the vital role of mer gers in accelerating the relativistic particles responsible for the radio e mission as well as the rare occurrence of radio halos in cluster samples.