X-Ray Analysis of an Off-axis Merger Stage Binary Galaxy Cluster: PSZ2 G279.79+39.09

Abstract

We present an X-ray analysis of the merging galaxy cluster system PSZ2 G279.79+39.09 (z = 0.29) using archival XMM-Newton and Chandra observations. The surface brightness image is bimodal, elongated east-west with a projected core separation of similar to 1.35 Mpc. We measure gas temperatures of 5.36 keV for the eastern subcluster (PSZ-E) and 5.44 keV for the western component (PSZ-W). Assuming isothermal intracluster gas, the hydrostatic masses are log(M500/M circle dot)=14.76 for PSZ-E and 14.54 for PSZ-W, implying a mass ratio of similar to 1:1.7. PSZ-E shows X-ray concentration indices of c40/c400 = 0.124 and c100/c500 = 0.278, together with a centroid shift of w = 0.016, indicating a disturbed halo that still hosts a compact cool core; PSZ-W is comparably disturbed even in its core. Both subclusters exhibit intracluster medium asymmetries consistent with ram-pressure stripping, and PSZ-W displays an X-ray tail extending nearly to PSZ-E's outskirts. The orientation and length of this tail support an off-axis merger geometry. Thermodynamic maps reveal a hot (similar to 7.3 keV), high-pressure, high-entropy bridge between the cores. From the Rankine-Hugoniot temperature jump, we infer a Mach number M=1.48-0.28+0.30 , consistent with a weak merger shock propagating at 1695-300+316 [km s-1. These results indicate a merger with a nonzero impact parameter, likely observed near core passage (less than or similar to 0.5 Gyr before or after), with the prepericenter scenario slightly preferred based on the projected separation and thermodynamic structure.