Radially polarized synchrotron from galaxy-cluster virial shocks

Abstract

Radio-to-$\gamma$-ray signals, recently found narrowly confined near the characteristic $2.4R_{500}$ scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude GMIMS radio data around MCXC galaxy clusters, we identify ($3\sigma$--$4\sigma$) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar $\sim2.4R_{500}$ minimal radii.