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 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % \mu \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 23 .
We identify this as an L ^ { \star } ( M _ { B } \simeq - 20.7 ) , disk-dominated ( B / T \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % 0.5 ) galaxy , forming stars at \sim 6 \mathrel { M _ { \odot } 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 ) \geq 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 \leq 500 \mathrel { kpc } , are measured to be \mathrel { M _ { tot } } = ( 6.7 \pm 0.4 ) \times 10 ^ { 14 } \mathrel { M _ { \odot } } and \mathrel { f _ { sub } } = 0.25 \pm 0.12 respectively .
Our model also rules out recent claims of a flat density profile at \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 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 .