ChandraandXMM‐NewtonObservations of the Double Cluster A1758

Abstract

A1758 was classified by Abell as a single rich cluster of galaxies, but a ROSAT observation showed that this system consists of two distinct clusters (A1758N and A1758S) separated by approximately 8' (a projected separation of 2 Mpc in the rest frame of the clusters). Only a few galaxy redshifts have been published for these two clusters, but the redshift of the Fe lines in the Chandra and XMM-Newton spectra shows that the recessional velocities of A1758N and A1758S are within 2100 km s-1. Thus, these two clusters most likely form a gravitationally bound system, but our imaging and spectroscopic analyses of the X-ray data do not reveal any sign of interaction between the two clusters. The Chandra and XMM-Newton observations show that A1758N and A1758S are both undergoing major mergers. A1758N is in the late stages of a large impact parameter merger between two 7 keV clusters. The two remnant cores have a projected separation of 800 kpc. Based on the measured pressure jumps preceding the two cores, they are receding from one another at less than 1600 km s-1. The two cores are surrounded by hotter gas (kT = 9-12 keV) that was probably shock heated during the early stages of the merger. The gas entropy in the two remnant cores is comparable with the central entropy observed in dynamically relaxed clusters, indicating that the merger-induced shocks stalled as they tried to penetrate the high-pressure cores of the two merging systems. Each core also has a wake of low-entropy gas, indicating that this gas was ram pressure stripped without being strongly shocked. A1758S is undergoing a more symmetric (lower impact parameter) merger between two 5 keV clusters. The two remnant cores are nearly coincident as seen in projection on the sky. The two cores are surrounded by hotter gas (9-11 keV), which was probably shock heated during the merger. Based on the pressure jumps preceding the two cores, they must have a relative velocity of less than 1400 km s-1. Unlike A1758N, there is no evidence of wakes of low-entropy gas.