A Textbook Example of a Bow Shock in the Merging Galaxy Cluster 1E 0657-56

Abstract

The Chandra image of the merging, hot galaxy cluster 1E 0657-56 reveals a bow shock propagating in front of a bullet-like gas cloud just exiting the disrupted cluster core. This is the first clear example of a shock front in a cluster. From the jumps in the gas density and temperature at the shock, the Mach number of the bullet-like cloud is 2-3. This corresponds to a velocity of 3000-4000 km s-1 relative to the main cluster, which means that the cloud traversed the core just 0.1-0.2 Gyr ago. The 6-7 keV "bullet" appears to be a remnant of a dense cooling flow region once located at the center of a merging subcluster whose outer gas has been stripped by ram pressure. The bullet's shape indicates that it is near the final stage of being destroyed by ram pressure and gasdynamic instabilities, as the subcluster galaxies move well ahead of the cool gas. The unique simplicity of the shock front and bullet geometry in 1E 0657-56 may allow a number of interesting future measurements. The cluster's average temperature is 14-15 keV but shows large spatial variations. The hottest gas (T > 20 keV) lies in the region of the radio halo enhancement and extensive merging activity involving subclusters other than the bullet.