Evidence for a pressure discontinuity at the position of the Coma relic from Planck Sunyaev-Zel'dovich effect data

Abstract

Radio relics are Mpc-scale diffuse synchrotron sources found in galaxy cluster outskirts. They are believed to be associated with large-scale shocks propagating through the intracluster medium, although the connection between radio relics and the cluster merger shocks is not yet proven conclusively. We present a first tentative detection of a pressure jump in the well-known relic of the Coma cluster through Sunyaev-Zel'dovich effect (SZE) imaging. The SZE data are extracted from the first public all-sky data release of Planck and we use high-frequency radio data at 2.3 GHz to constrain the shock-front geometry. The SZE data provide evidence for a pressure discontinuity, consistent with the relic position, without requiring any additional prior on the shock-front location. The derived Mach number M = 2.9^{+0.8}_{-0.6} is consistent with X-ray and radio results. A high-pressure `filament' without any pressure discontinuity is disfavoured by X-ray measurements and a `sub-cluster' model based on the infalling group NGC 4839 can be ruled out considering the published mass estimates for this group. These results signify a first attempt towards directly measuring the pressure discontinuity for a radio relic and the first SZE-detected shock feature observed near the virial radius of a galaxy cluster.