Abell 754: A non-head-on collision of subclusters

We have analyzed spatially resolved spectra of the A754 cluster of galaxies obtained with ASCA. Through earlier observations with HEAO 1, Einstein, and ROSAT as well as optical studies, A754 has been established as the prototype system for a merger in progress. The combination of spectral and spatial resolution over a broad energy band provided by ASCA has set unprecedented constraints on the hydrodynamical effects of a cluster merger. In general agreement with ROSAT (Henry & Briel 1995), we find significant gas temperature variations over the cluster face, indicating shock heating of the atmosphere during the merger. The hottest region, greater than 12 keV (90% confidence), is located in the region of the northwest Galaxy clump though the entire region along the cluster axis appears to be hotter than the mean cluster temperature (similar to 9 keV). The cool, less than or equal to 5 keV, gas originally found with the HEAO 1 A-2 experiment, resides in the exterior of the cluster atmosphere and in a plume of gas we identify with a stripped cool atmosphere of the infalling subcluster. We have also attempted to reconstruct an iron abundance map of this merging system. Though poorly constrained, no significant deviations of abundance from the mean value are apparent in the individual regions. A754 is the only cluster so far which shows the significant temperature pattern expected in a subcluster merger, in both the ROSAT and ASCA data, providing the first possibility to compare it with theoretical predictions. The cluster does not feature a hot peak accompanied by two hot lobes perpendicular to the cluster axis, predicted by hydrodynamic simulations of a head-on merger. The observed temperature and surface brightness maps suggest that the two colliding subunits have missed each other by about 1 Mpc and are now moving perpendicular to the cluster axis in the image plane (as, e.g., in the simulations by Evrard, Metzler, & Navarro 1996).