ABELL-754 - A NON-HEAD-ON COLLISION OF SUBCLUSTERS

We have analyzed spatially resolved spectra of the A754 cluster of gal axies obtained with ASCA. Through earlier observations with HEAO 1, Ei nstein, and ROSAT as well as optical studies, A754 has been establishe d 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 o f a cluster merger. In general agreement with ROSAT (Henry & Briel 199 5), we find significant gas temperature variations over the cluster fa ce, indicating shock heating of the atmosphere during the merger. The hottest region, greater than 12 keV (90% confidence), is located in th e region of the northwest Galaxy clump though the entire region along the cluster axis appears to be hotter than the mean cluster temperatur e (similar to 9 keV). The cool, less than or equal to 5 keV, gas origi nally 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 strip ped cool atmosphere of the infalling subcluster. We have also attempte d to reconstruct an iron abundance map of this merging system. Though poorly constrained, no significant deviations of abundance from the me an value are apparent in the individual regions. A754 is the only clus ter so far which shows the significant temperature pattern expected in a subcluster merger, in both the ROSAT and ASCA data, providing the f irst possibility to compare it with theoretical predictions. The clust er does not feature a hot peak accompanied by two hot lobes perpendicu lar to the cluster axis, predicted by hydrodynamic simulations of a he ad-on merger. The observed temperature and surface brightness maps sug gest 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 199 6).