COSMOLOGICAL MAGNETOHYDRODYNAMIC SIMULATIONS OF GALAXY CLUSTER RADIO RELICS: INSIGHTS AND WARNINGS FOR OBSERVATIONS

Abstract

Non-thermal radio emission from cosmic ray electrons in the vicinity of\nmerging galaxy clusters is an important tracer of cluster merger activity, and\nis the result of complex physical processes that involve magnetic fields,\nparticle acceleration, gas dynamics, and radiation. In particular, objects\nknown as radio relics are thought to be the result of shock-accelerated\nelectrons that, when embedded in a magnetic field, emit synchrotron radiation\nin the radio wavelengths. In order to properly model this emission, we utilize\nthe adaptive mesh refinement simulation of the magnetohydrodynamic evolution of\na galaxy cluster from cosmological initial conditions. We locate shock fronts\nand apply models of cosmic ray electron acceleration that are then input into\nradio emission models. We have determined the thermodynamic properties of this\nradio-emitting plasma and constructed synthetic radio observations to compare\nto observed galaxy clusters. We find a significant dependence of the observed\nmorphology and radio relic properties on the viewing angle of the cluster,\nraising concerns regarding the interpretation of observed radio features in\nclusters. We also find that a given shock should not be characterized by a\nsingle Mach number. We find that the bulk of the radio emission comes from gas\nwith T>5x10^7, \\rho~10^(-28)-10^(-27) g/cm^3, with magnetic field strengths of\n0.1-1.0 \\mu G and shock Mach numbers of M~3-6. We present an analysis of the\nradio spectral index which suggests that the spatial variation of the spectral\nindex can mimic synchrotron aging. Finally, we examine the polarization\nfraction and position angle of the simulated radio features, and compare to\nobservations.\n