A Collision of Subclusters in Abell 754

We present direct evidence of a collision of subclusters in the galaxycluster Abell 754. Our comparison of new optical data and archival ROSAT PSPCX-ray data reveal three collision signatures predicted by n-body/hydrodynamicalsimulations of hierarchical cluster evolution. First, there is strong evidenceof a non-hydrostatic process; neither of the two major clumps in the galaxydistribution lies on the off-center peak of the X-ray emission from theintracluster gas. Second, the peak of the X-ray emission is elongatedperpendicular to the collision axis defined by the centroids of the two galaxyclumps. Third, there is evidence of compression-heated gas; one of A754's twoX-ray temperature components (Henry & Briel 1995) is among the hottest observedin any cluster and hotter than that inferred from the velocity dispersion ofthe associated galaxy clump. These signatures are consistent with thequalitative features of simulations (Evrard 1990a,b) in which two subclustershave collided in the plane of the sky during roughly the last Gyr. Thedetection of such collisions is crucial for understanding both the dynamics ofindividual clusters and the underlying cosmology. First, for systems like A754,estimating the cluster X-ray mass from assumptions of hydrostatic equilibriumand isothermality is incorrect and may produce the discrepancies sometimesfound between X-ray masses and those derived from gravitational lens models(Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in whichsubclusters have collided in the last Gyr is especially sensitive to the meanmass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993;Lacey & Cole 1993). With a large, well-defined cluster sample, it will bepossible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in ApJ Letter