Non-Equilibrium Processes in Relaxed Galaxy Cluster Outskirts

Ka-Wah Wong, Minnesota State University Mankato
Craig L. Sarazin, University of Virginia
Daniel R. Wik, Johns Hopkins University
Li Ji, Massachusetts Institute of Technology
Q. D. Wang, University of Massachusetts
T. M. Tripp, University of Massachusetts


The collisionless accretion shock at the outer boundary of a galaxy cluster should primarily heat the ions instead of electrons since they carry most of the kinetic energy of the infalling gas. Near the accretion shock regions (≳ R_{vir}), the density of the intracluster medium is very low and the Coulomb collisional timescale is longer than the accretion timescale. Electrons and ions may not achieve equipartition in these regions, and the ionization state may also not be in equilibrium. We discuss whether the non-equipartition signature in relaxed galaxy clusters can be detected by future Sunyaev-Zel'dovich (SZ) observations (e.g., with ALMA). We present the impact of the non-equipartition effect on cosmological parameter estimation from SZ surveys. We also discuss briefly the non-equilibrium ionization signatures in relaxed galaxy cluster outskirts.