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We present Advanced Camera for Surveys observations of MACSJ1149.5+2223, anX-ray luminous galaxy cluster at z=0.544 discovered by the Massive ClusterSurvey. The data reveal at least seven multiply-imaged galaxies, three of whichwe have confirmed spectroscopically. One of these is a spectacular face-onspiral galaxy at z=1.491, the four images of which are gravitationallymagnified by ~8<mu<~23. We identify this as an L* (M_B=-20.7), disk-dominated(B/T<~0.5) galaxy, forming stars at ~6Msol/yr. We use a robust sample ofmultiply-imaged galaxies to constrain a parameterized model of the cluster massdistribution. In addition to the main cluster dark matter halo and the brightcluster galaxies, our best model includes three galaxy-group-sized halos. Therelative probability of this model is P(N_halo=4)/P(N_halo<4)>=10^12 whereN_halo is the number of cluster/group-scale halos. In terms of sheer number ofmerging cluster/group-scale components, this is the most complex strong-lensingcluster core studied to date. The total cluster mass and fraction of that massassociated with substructures within R<=500kpc, are measured to beM_tot=(6.7+/-0.4)x10^14Msol and f_sub=0.25+/-0.12 respectively. Our model alsorules out recent claims of a flat density profile at >~7sigma confidence, thushighlighting the critical importance of spectroscopic redshifts ofmultiply-imaged galaxies when modeling strong lensing clusters. Overall ourresults attest to the efficiency of X-ray selection in finding the mostpowerful cluster lenses, including complicated merging systems.

Hubble Space Telescope Observations of a Spectacular New Strong-lensing Galaxy Cluster - MACSJ1149.5+2223 at z=0.544