A dark core in abell 520

The rich cluster Abell 520 (z = 0.201) exhibits truly extreme and puzzling multiwavelength characteristics. It may best be described as a cosmic train wreck. It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the Bullet Cluster, 1E 0657- 558). However, the most striking feature is a massive dark core (721 h(70) M-circle dot/L circle dot B) in our weak- lensing mass reconstruction. The core coincides with the central X- ray emission peak, but is largely devoid of galaxies. An unusually low mass- to- light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X- ray gas and the galaxy/ dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (approximate to 0.15), mass- to- light ratio (220 h(70) M-circle dot/L circle dot B), and position on the X- ray luminosity-temperature and mass- temperature relations are unremarkable. Thus, gross properties and scaling relations are not always useful indicators of the dynamical state of clusters.