AN X-RAY AND OPTICAL STUDY OF CLUSTERS IN FORMATION

We present an analysis of observations by the ROSAT Position Sensitive Proportional Counter (PSPC) of subclusters in bimodal systems A3395 ( SC 0627-54) and A3528 (SC 1252-28). The best-fit temperatures range fr om 2 to 4 keV and are the first temperature measurements for this clas s of objects. The gas is detected out to 300 kpc (A3395) and 650 kpc ( A3528) from the center of the subclusters, but this is likely not the full extent of the gas. Therefore, we have determined masses for the s ubclusters, assuming the gas is isothermal, of similar to 1.0 x 10(14) h(75)(-1) M(.) within 0.5 Mpc and similar to 2.1 x 10(14) h(75)(-1) M (.) within 1.0 Mpc. The average baryon fraction in gas is similar to 0 .09 h(75)(-3/2) within 0.5 Mpc and similar to 0.14 h(75)(-3/2) within 1.0 Mpc. The average baryon fraction in gas is consistent with the cen tral regions of rich, evolved clusters and suggests that the merger pr ocess does not appreciably increase the gas baryon fraction. The veloc ity dispersion of each subcluster is comparable to that found for rich clusters. The masses we have calculated from the optical data using t hree different estimators are larger than those from the X-ray data be cause of the larger velocity dispersion of the subclusters. Since the velocity dispersions are comparable to the difference in the mean velo cities because of the Hubble flow, we have imposed a radial cutoff; th is reduces, but does not remove, the conflict between the optically an d X-ray-derived masses, suggesting that the galaxy velocity dispersion s are contaminated by the component of radial infall along the line of sight. A dynamical analysis of the A3395 cluster using the X-ray-dete rmined masses indicates that the subclusters will fully merge in 0.3-1 .2 Gyr; this is consistent with an early stage of merger which is impl ied by the elongation of the X-ray emission perpendicular to the merge r axis resulting from the collision of the hot atmospheres. If the mer ger product has a mass similar to the sum of the individual subcluster s, then the merger product will be a low-mass rich cluster, which is s trong support for rich clusters forming from the ''bottom up.'' Extend ing the dynamical analysis to include the cluster SC 0625-53 (A3391), which is similar to 2.8 h(75)(-1) Mpc away in projection, suggests tha t this cluster and the A3395 cluster form a bound, infalling system if they have again as much mass beyond 1 Mpc as within that radius. The improved spectral and spatial resolution of the ROSAT PSPC allows a de termination of the cooling time in the central region of the subcluste rs. The subcluster cooling times are slightly longer than 10 Gyr, the nominal age of the cluster atmosphere, suggesting that cooling flows w ith mass accretion rates similar to nearby clusters are not present du ring this premerger stage or that they have been disrupted. Small cool ing flows on the order of a few solar masses are not ruled out.