Cooling Flows and Cluster Evolution

The X-ray emission from bimodal clusters is analyzed to determine whether or not there is evidence for cooling in the center of the clusters. All of the 14 clusters have long cooling times. When compared with the frequency of clusters with short cooling times in nearby rich clusters, this suggests a significant lack of cooling in the bimodal clusters. Tidal forces between the components of the bimodal clusters are insufficient to disrupt an existing cooling flow; apparently one has not yet formed. The clusters in the bimodal systems individually have luminosities comparable to nearby clusters and have relatively short merge times so that they represent an intermediate evolutionary stage in the formation of a luminous, rich cluster in a hierarchical clustering scenario. These data suggest that the formation of a cooling flow may occur relatively late in the formation of a rich cluster since the merging of nearly equal subclusters occurs at between 40%-70% of the collapse time of the cluster in n-body simulations. We find that cooling flows are as likely to occur in clusters with optical substructure (but without X-ray substructure) as without optical substructure. This suggests that cooling flows form after the merger of the separate cluster atmospheres but before the galaxies have relaxed to a smooth distribution. Since the luminosities of the bimodal clusters indicate that the cluster temperatures are near the minimum in the cooling function, the increase in gas density resulting from the merger may substantially increase the cooling rate and drop the temperature enough to start the cooling flow. Since cooling flows appear in clusters which have optical substructure, the time scale for the galaxies to reach a smooth equilibrium distribution is an upper limit to time scale for the formation of a cooling flow.