We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Durham University

Department of Physics

Staff profile

Publication details for Prof Richard Massey

Bradač, M., Allen, S.W., Treu, T., Ebeling, H., Massey, R., Morris, R.G., von der Linden, A. & Applegate, D. (2008). Revealing the properties of dark matter in the merging cluster MACS J0025.4-1222. Astrophysical journal 687(2): 959-967.

Author(s) from Durham


We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACS J0025.4–1222. As was previously shown by the example of the Bullet Cluster (1E 0657–56), 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 J0025.4–1222 consists of two merging subclusters of similar richness at z = 0.586. We measure the distribution of X-ray-emitting 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 σ significance) from the peak of the hot X-ray-emitting 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 σ/m < 4 cm 2 g−1, reaffirming the results from the Bullet Cluster on the collisionless nature of dark matter.


* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs GO11100 and GO10703. This work is also based on observations collected at the W. M. Keck Observatory.