X-ray-optical classification of cluster mergers and the evolution of the cluster merger fraction

We present the results of a simple but robust morphological classification of a statis-tically complete sample of 108 of the most X-ray luminous clusters at 0.15 6 z 6 0.7 observed with Chandra. Our aims are to (a) identify the most disturbed massive clus-ters to be used as gravitational lenses for studies of the distant universe and as probes of particle acceleration mechanisms resulting in non-thermal radio emission, (b) find cluster mergers featuring subcluster trajectories that make them suitable for quanti-tative analyses of cluster collisions, and (c) constrain the evolution with redshift of the cluster merger fraction. Finally, (d) this paper represents the third public release of clusters from the MACS sample, adding 24 clusters to the 46 published previously. To classify clusters by degree of relaxation, we use the projected offset of the brightest cluster galaxy from the peak (or the global centroid) of the X-ray emission as a mea-sure of the segregation between the intracluster gas and dark matter, and also perform a visual assessment of the optical and X-ray morphology on all scales. Regarding (a), we identify ten complex systems likely to have undergone multiple merger events in