Evolution of Shocks and Splashback Boundaries in Cluster Outskirts

Detecting hot gas in cluster outskirts is one of the major goals of our next-generation X-ray and sub-mm/SZ facilities. It is important to understand how accretion and mergers shape clusters near and beyond the virial radius. Even though cosmological simulations have been extensively used for these studies, idealized models have unique advantages of grasping underlying physics behind the complicated assembling history of galaxy clusters. In this talk, I will introduce our novel idealized models designed to explore the evolution of cluster outskirts, including both intracluster medium (ICM) and dark matter (DM) halo.

With our novel models, we built a theoretical framework – “runaway shock” scenario – describing the late evolution of merger shocks driven by infalling substructures (e.g., groups and small clusters) in the ICM. The runaway merger shocks are considered as promising candidates for powering radio relics in the periphery of galaxy clusters. One of the beautiful real examples are shown in the Coma cluster, observed by the up-to-date SRG/eROSITA and LOFAR telescopes.

The runaway shocks eventually overtake the accretion shock at the boundary of the ICM and re-shape gaseous atmospheres in the outer cluster regions. Various shocks and contact discontinuities are formed in this process. Our simulations demonstrated that the radial location of the new ICM boundary heavily depends on the parameter characterizing the smooth mass accretion rate of the system. In contrast, the evolution of the DM halo’s boundary (i.e., splashback radius) depends on the total mass accretion rate contributed by both accretion and mergers. These results suggested that cluster mergers naturally explain the prominent radial offsets between the boundaries of the ICM and DM halos in galaxy clusters long noticed in the cosmological simulations.