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 ∼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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">500</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>14.76</mml:mn> </mml:math> for PSZ-E and 14.54 for PSZ-W, implying a mass ratio of ∼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 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 (∼7.3 keV), high-pressure, high-entropy bridge between the cores. From the Rankine–Hugoniot temperature jump, we infer a Mach number <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mn>1.48</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.28</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.30</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , consistent with a weak merger shock propagating at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mn>1695</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>300</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>316</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> [km s −1 . These results indicate a merger with a nonzero impact parameter, likely observed near core passage (≲0.5 Gyr before or after), with the prepericenter scenario slightly preferred based on the projected separation and thermodynamic structure.