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 less than or similar to mu less than or similar to 23. We identify this as an L* (M-B similar or equal to -20.7), disk-dominated (B/T less than or similar to 0.5) galaxy, forming stars at similar to 6 M-circle dot 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) >= 10(12) 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) x 10(14) M-circle dot and f(sub) = 0.25 +/- 0.12, respectively. Our model also rules out recent claims of a flat density profile at greater than or similar to 7 sigma 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.