Chandra observation of two shock fronts in the merging galaxy cluster Abell 2146

TL;DR

This paper reports Chandra X-ray observations of galaxy cluster Abell 2146, revealing two shock fronts from a recent merger, with detailed measurements of shock properties and implications for the cluster's merger history.

Contribution

The study provides detailed measurements of shock fronts and the merger dynamics in Abell 2146, including shock Mach numbers and the timing of the core passage, enhancing understanding of cluster mergers.

Findings

Detected a bow shock with Mach number 2.2±0.8

Observed a slower upstream shock with Mach number 1.7±0.3

Estimated the merger occurred 0.1-0.3 Gyr ago

Abstract

We present a new Chandra observation of the galaxy cluster Abell 2146 which has revealed a complex merging system with a gas structure that is remarkably similar to the Bullet cluster (eg. Markevitch et al. 2002). The X-ray image and temperature map show a cool 2-3 keV subcluster with a ram pressure stripped tail of gas just exiting the disrupted 6-7 keV primary cluster. From the sharp jump in the temperature and density of the gas, we determine that the subcluster is preceded by a bow shock with a Mach number M=2.2+/-0.8, corresponding to a velocity v=2200^{+1000}_{-900} km/s relative to the main cluster. We estimate that the subcluster passed through the primary core only 0.1-0.3 Gyr ago. In addition, we observe a slower upstream shock propagating through the outer region of the primary cluster and calculate a Mach number M=1.7+/-0.3. Based on the measured shock Mach numbers M~2 and the strength of the upstream shock, we argue that the mass ratio between the two merging clusters is between 3 and 4 to one. By comparing the Chandra observation with an archival HST observation, we find that a group of galaxies is located in front of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak.