<i>MC</i>²: CONSTRAINING THE DARK MATTER DISTRIBUTION OF THE VIOLENT MERGING GALAXY CLUSTER CIZA J2242.8+5301 BY PIERCING THROUGH THE MILKY WAY

The galaxy cluster CIZA J2242.8+5301 at z = 0.19 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 north-south axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and Canada-France-Hawaii Telescope imaging data. Measuring a high signal-to-noise ratio lensing signal from this cluster is potentially a challenging task because of its proximity to the Milky Way plane (|b| 5 ). 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 of CIZA J2242.8+5301 is highly elongated along the north-south 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 Markov Chain Monte Carlo analysis by simultaneously fitting two Navarro-Frenk-White halos without fixing their centroids. The resulting masses of the northern and southern systems are M 200 = 11.0 +3.7 -3.2 10 14 M and 9.8 +3.8 -2.5 10 14 M , 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. After investigating the statistical significance of the offsets by bootstrapping both mass and galaxy centroids, we find that the galaxy luminosity-mass offset for the northern clump is statistically significant at the 2 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 self-interaction of dark matter and discuss caveats.