The extraordinary amount of substructure in the<i>Hubble Frontier Fields</i>cluster Abell 2744

Abstract

We present a joint optical/X-ray analysis of the massive galaxy cluster Abell\n2744 (z=0.308). Our strong- and weak-lensing analysis within the central region\nof the cluster, i.e., at R&lt;1Mpc from the brightest cluster galaxy, reveals\neight substructures, including the main core. All of these dark-matter halos\nare detected with a significance of at least 5sigma and feature masses ranging\nfrom 0.5 to 1.4x10^{14}Msun within R&lt;150kpc. Merten et al. (2011) and\nMedezinski et al. (2016) substructures are also detected by us. We measure a\nslightly higher mass for the main core component than reported previously and\nattribute the discrepancy to the inclusion of our tightly constrained\nstrong-lensing mass model built on Hubble Frontier Fields data. X-ray data\nobtained by XMM-Newton reveal four remnant cores, one of them a new detection,\nand three shocks. Unlike Merten et al. (2011), we find all cores to have both\ndark and luminous counterparts. A comparison with clusters of similar mass in\nthe MXXL simulations yields no objects with as many massive substructures as\nobserved in Abell 2744, confirming that Abell 2744 is an extreme system. We\nstress that these properties still do not constitute a challenge to\n$\\Lambda$CDM, as caveats apply to both the simulation and the observations: for\ninstance, the projected mass measurements from gravitational lensing and the\nlimited resolution of the sub-haloes finders. We discuss implications of Abell\n2744 for the plausibility of different dark-matter candidates and, finally,\nmeasure a new upper limit on the self-interaction cross-section of dark matter\nof sigma_{DM}&lt;1.28cm2/g(68\\% CL), in good agreement with previous results from\nHarvey et al. (2015).\n