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We constrain the physical nature of dark matter using the newly identifiedmassive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by theexample of the Bullet Cluster (1E0657-56), such systems are ideal laboratoriesfor detecting isolated dark matter, and distinguishing between cold dark matter(CDM) and other scenarios (e.g. self-interacting dark matter, alternativegravity theories). MACSJ0025.4-1222 consists of two merging subclusters ofsimilar richness at z=0.586. We measure the distribution of X-ray emitting gasfrom Chandra X-ray data and find it to be clearly displaced from thedistribution of galaxies. A strong (information from highly distorted arcs) andweak (using weakly distorted background galaxies) gravitational lensinganalysis based on Hubble Space Telescope observations and Keck arc spectroscopyconfirms that the subclusters have near-equal mass. The total mass distributionin each of the subclusters is clearly offset (at >4sigma significance) from thepeak of the hot X-ray emitting gas (the main baryonic component), but alignedwith 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 withthose of typical clusters, finding that dark matter is the dominant contributorto the gravitational field. Under the assumption that the subclustersexperienced a head-on collision in the plane of the sky, we obtain anorder-of-magnitude estimate of the dark matter self-interaction cross-sectionof sigma/m < 4cm^2/g, re-affirming the results from the Bullet Cluster on thecollisionless nature of dark matter.

Revealing the properties of dark matter in the merging cluster MACSJ0025.4-1222