"Characterizing Galaxy Cluster Merger Dynamics With Multi-Probe Observations and Hydrodynamical Simulations via ICM-SHOX"

Abstract:

Galaxy cluster mergers are rich sources of information to test cluster astrophysics and cosmology. However, cluster mergers produce complex signals that are observed in projection and are often difficult to interpret from individual probes. Multi-probe constraints on both the gas and dark matter cluster components are necessary to infer merger parameters that are otherwise degenerate. We have developed ICM-SHOX (Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray), a systematic framework to jointly infer cluster merger parameters quantitatively via a pipeline that directly compares a novel combination of multi-probe observables to mock observables derived from hydrodynamical simulations. In our first application of the ICM-SHOX pipeline to the MACS J0018.5+1626 system, we discovered a velocity-space decoupling of the gas and dark matter distributions, which we attribute to the different collisional properties of the two components. We are now obtaining new and deeper observations of MACS J0018.5+1626 to confirm the possible detection of a rare equatorial shock as a result of the merger, and we have recently demonstrated that the diffuse radio emission in the system, which has nominally been classified as a radio halo, could potentially be described instead by a pair of merger shock-generated radio relics for the geometry constrained with ICM-SHOX. In another system, MACS J2129.4-0741, we find hints of a Bullet Cluster-like morphology in X-rays, which strongly suggests a plane-of-sky merger, along with high line of sight velocities in both the gas and dark matter, which simultaneously suggests a merger in that direction—together implying the possible presence of a uniquely-oriented double merger. In this talk, I will provide an overview of such exciting cluster mergers and the status of our updated ICM-SHOX pipeline.