A COLLISION OF SUBCLUSTERS IN ABELL-754

We present direct evidence of a collision of subclusters in the galaxy cluster Abell 754. Our comparison of new optical data and archival RO SAT PSPC X-ray data reveal three collision signatures predicted by n-b ody/ hydrodynamical simulations of hierarchical cluster evolution. Fir st, there is strong evidence of a nonhydrostatic process; neither of t he two major clumps in the galaxy distribution lies on the off-center peak of the X-ray emission from the intracluster gas. Second, the peak of the X-ray emission is elongated perpendicular to the collision axi s defined by connecting the centroids of the two galaxy clumps. Third, there is evidence of compression-heated gas; one of A754's two X-ray temperature components (Henry and Briel 1995) is among the hottest obs erved in any cluster and hotter than that inferred from the velocity d ispersion of the associated galaxy clump. These signatures are consist ent with the qualitative features of simulations (Evrard 1990a, b) in which two subclusters have collided in the plane of the sky during the last similar to 1 Gyr. The detection of such collisions is crucial fo r understanding both the dynamics of individual clusters and the under lying cosmology. First, for systems like A754, estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium and isothermali ty is incorrect and may produce the discrepancies sometimes found betw een X-ray masses and those derived from gravitational lens models (Bab ul and Miralda-Escude 1994). Second, the fraction of nearby clusters i n which subclusters have collided in the last similar to 1 Gyr is espe cially sensitive to the mean mass density parameter Ohm(0) (see Richst one, Loeb, and Turner 1992; Evrard et al. 1993; Lacey and Cole 1993). As we show for A754, we now have the means to identify recent collisio ns. With a large, well-defined cluster sample, it will be possible to place a new and powerful constraint on cosmological models.