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

TL;DR

This paper presents an X-ray analysis of the merging galaxy cluster PSZ2 G279.79+39.09, revealing a disturbed, off-axis merger with a weak shock, hot bridge, and evidence of ram-pressure stripping from archival XMM-Newton and Chandra data.

Contribution

The study provides detailed thermodynamic maps and shock analysis of an off-axis merger, highlighting the cluster's disturbed state and merger dynamics.

Findings

Mass ratio of approximately 1:1.7 between subclusters.

Detection of a weak shock with Mach number ~1.48.

Identification of a hot, high-entropy bridge indicating core passage.

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 $\sim 1.35$ 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 $\log(M_{500}/M_\odot)=14.76$ for PSZ-E and 14.54 for PSZ-W, implying a mass ratio of $\sim 1:1.7$. PSZ-E shows X-ray concentration indices of $c_{40}/c_{400}=0.124$ and $c_{100}/c_{500}=0.278$, 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 ICM asymmetries consistent with ram-pressure stripping, and PSZ-W displays an X-ray tail extending nearly to the outskirts of PSZ-E. The orientation and length of this tail support an off-axis merger geometry. Thermodynamic maps reveal a hot ($\sim 7.3$ keV), high-pressure, high-entropy bridge between the cores. From the Rankine-Hugoniot temperature jump, we infer a Mach number $M=1.48^{+0.30}_{-0.28}$, consistent with a weak merger shock propagating at $1695^{+316}_{-300}$ km s$^{-1}$. These results indicate a merger with a non-zero impact parameter, likely observed near core passage ($\lesssim 0.5$ Gyr before or after), with the pre-pericenter scenario slightly preferred based on the projected separation and thermodynamic structure.