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 from 2 to 4 keV and are the first temperature measurements for this class 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 subclusters, assuming the gas is isothermal, of ~1. 0 x 10¹4^ h₇5_^-1^ Mₛun_ within 0. 5 Mpc and ~2. 1 x 10¹4^ h₇5_^-1^ Mₛun_ within 1. 0 Mpc. The average baryon fraction in gas is ~0. 09 h₇5_³/2^ within 0. 5 Mpc and ~0. 14 h₇5_³/2^ within 1. 0 Mpc. The average baryon fraction in gas is consistent with the central regions of rich, evolved clusters and suggests that the merger process does not appreciably increase the gas baryon fraction. The velocity dispersion of each subcluster is comparable to that found for rich clusters. The masses we have calculated from the optical data using three different estimators are larger than those from the X-ray data because of the larger velocity dispersion of the subclusters. Since the velocity dispersions are comparable to the difference in the mean velocities because of the Hubble flow, we have imposed a radial cutoff; this reduces, but does not remove, the conflict between the optically and X-ray-derived masses, suggesting that the galaxy velocity dispersions are contaminated by the component of radial infall along the line of sight. A dynamical analysis of the A3395 cluster using the X-ray-determined 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 implied by the elongation of the X-ray emission perpendicular to the merger axis resulting from the collision of the hot atmospheres. If the merger product has a mass similar to the sum of the individual subclusters, then the merger product will be a low-mass rich cluster, which is strong support for rich clusters forming from the "bottom up. " Extending the dynamical analysis to include the cluster SC 0625 - 53 (A3391), which is ~2. 8 h₇5_^-1¹ Mpc away in projection, suggests that 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 determination of the cooling time in the central region of the subclusters. The subcluster cooling times are slightly longer than 10 Gyr, the nominal age of the cluster atmosphere, suggesting that cooling flows with mass accretion rates similar to nearby clusters are not present during this premerger stage or that they have been disrupted. Small cooling flows on the order of a few solar masses are not ruled out.