MC^2: Constraining the Dark Matter Distribution of the Violent Merging
  Galaxy Cluster CIZA J2242.8+5301 by Piercing through the Milky Way
  CIZA J2242.8+5301 is a merging system with a prominent (~2 Mpc long) radio
relic, which together with the morphology of the X-ray emission provides strong
evidence for a violent collision along the N-S axis. We present our constraints
on the dark matter distribution of this unusual system using Subaru and CFHT
imaging data. Measuring a high S/N lensing signal from this cluster is
potentially a challenging task because of its proximity to the Milky Way plane
(|b|~5 deg). We overcome this challenge with careful observation planning and
systematics control, which enables us to successfully map the dark matter
distribution of the cluster with high fidelity. The resulting mass map shows
that the mass distribution is highly elongated along the N-S merger axis
inferred from the orientation of the radio relics. Based on our mass
reconstruction, we identify two sub-clusters, which coincide with the cluster
galaxy distributions. We determine their masses using MCMC analysis by
simultaneously fitting two NFW halos without fixing their centroids. The
resulting masses of the northern and southern systems are
$M_{200}=11.0_{-3.2}^{+3.7}\times10^{14} M_{\sun}$ and
$9.8_{-2.5}^{+3.8}\times10^{14} M_{\sun}$, respectively, indicating that we are
witnessing a post-collision of two giant systems of nearly equal mass. When the
mass and galaxy centroids are compared in detail, we detect ~ 1' (~190 kpc)
offsets in both northern and southern sub-clusters. We find that the galaxy
luminosity-mass offset for the northern clump is statistically significant at
the ~2 sigma level whereas the detection is only marginal for the southern
sub-cluster in part because of a relatively large mass centroid error. We
conclude that it is yet premature to uniquely attribute the galaxy-mass
misalignment to SIDM and discuss caveats.
