Excess cataloged X-ray and radio sources at galaxy-cluster virial shocks

Abstract

We detect a highly significant excess of X-ray (2RXS) and radio (NVSS, GMRT, VLSSr) catalog sources when stacked around MCXC galaxy clusters and groups, narrowly confined within $\lesssim100\mathrm{\,kpc}$ of the $\sim2.4 R_{500}$ virial shock radius (inferred from previous continuum stacking), with similar X-ray ($\sim4\sigma$ for $443$ clusters) and radio ($\sim4\sigma$ for $485$ clusters) characteristics ($>5\sigma$ joint). The excess sources show $10-100$ kpc scales, $L_X(0.1-2.4\mbox{ keV})\simeq10^{42-43}\mathrm{\,erg\,s^{-1}}$ or $\nu L_\nu(\nu=1.4\mbox{ GHz}) \simeq 10^{40-41}\mathrm{\,erg\,s^{-1}}$ luminosities, and a preferentially radial radio-polarization. The narrow localization and properties of the excess identify these sources not as AGN, often invoked speculatively for excess X-ray sources at cluster outskirts, but rather as infalling gaseous clumps interacting with the virial shock, probably galactic halos and possibly outflow remnants. The local excess of such discrete, radio-to-$\gamma$-ray sources around an object can probe its virial shock also at high redshifts and sub-cluster scales.