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 h₇₀ M_☉/L_☉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 (≈0.15), mass-to-light ratio (220 h₇₀ M_☉/L_☉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.