Hubble space telescope observations of a spectacular new strong-lensing galaxy cluster: MACSJ1149.5+2223 at z = 0.544

We present Advanced Camera for Surveys observations of MACSJ1149.5+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 ≲ μ ≲ 23. We identify this as an L ^* (M_B ≃ -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 ≳7σ 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.