Revealing the Properties of Dark Matter in the Merging Cluster MACS J0025.4−1222

We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACS J0025.41222. As was previously shown by the example of the Bullet Cluster (1E 065756), such systems are ideal laboratories for detecting isolated dark matter and distinguishing between cold dark matter (CDM) and other scenarios (e.g., self-interacting dark matter, alternative gravity theories). MACS J0025.41222 consists of two merging subclusters of similar richness at z 0:586. We measure the distribution of X-rayYemitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass. The total mass distribution in each of the subclusters is clearly offset (at >4 significance) from the peak of the hot X-rayYemitting gas (the main baryonic component) but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas (0:09 0:07 0:03 ) and stars (0:010 0:007 0:004 ), consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field. Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross section of /m < 4 cm 2 g 1 , reaffirming the results from the Bullet Cluster on the collisionless nature of dark matter.