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

We present a new Chandraobservation 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 exit ing 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 −1 relative to the main cluster. We estimate that the subcluste r 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 fro nt of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak.