HUBBLE SPACE TELESCOPE OBSERVATIONS OF A SPECTACULAR NEW STRONG-LENSING GALAXY CLUSTER: MACS J1149.5+2223 AT z = 0.544

We present Advanced Camera for Surveys observations of MACS?J1149.5+2223, an X-ray luminous galaxy cluster at z = 0.544 discovered by the Massive Cluster Survey. The data reveal at least seven multiply imaged galaxies, three of which we have confirmed spectroscopically. One of these is a spectacular face-on spiral galaxy at z = 1.491, the four images of which are gravitationally magnified by 8 ? 23. We identify this as an L (MB ?20.7), disk-dominated (B/T 0.5) galaxy, forming stars at ~6 M ? yr?1. We use a robust sample of multiply imaged galaxies to constrain a parameterized model of the cluster mass distribution. In addition to the main cluster dark matter halo and the bright cluster galaxies, our best model includes three galaxy-group-sized halos. The relative probability of this model is P(N halo = 4)/P(N halo < 4) ? 1012 where N halo is the number of cluster/group-scale halos. In terms of sheer number of merging cluster/group-scale components, this is the most complex strong-lensing cluster core studied to date. The total cluster mass and fraction of that mass associated with substructures within R ? 500 kpc, are measured to be M tot = (6.7 ? 0.4) ? 1014 M ? and f sub = 0.25 ? 0.12, respectively. Our model also rules out recent claims of a flat density profile at 7? confidence, thus highlighting the critical importance of spectroscopic redshifts of multiply imaged galaxies when modeling strong-lensing clusters. Overall our results attest to the efficiency of X-ray selection in finding the most powerful cluster lenses, including complicated merging systems.