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We present the results from a weak gravitational lensing study of the mergingcluster A520 based on the analysis of Hubble Space Telescope/Advanced Camerafor Surveys (ACS) data. The excellent data quality allows us to reach a meannumber density of source galaxies of ~109 per sq. arcmin, which improves bothresolution and significance of the mass reconstruction compared to a previousstudy based on Wide Field Planetary Camera 2 (WFPC2) images. We take care inremoving instrumental effects such as the trailing of charge due to radiationdamage of the ACS detector and the position-dependent point spread function(PSF). This new ACS analysis confirms the previous claims that a substantialamount of dark mass is present between two luminous subclusters. We examine thedistribution of cluster galaxies and observe very little light at thislocation. We find that the centroid of the dark peak in the current ACSanalysis is offset to the southwest by ~1 arcmin with respect to the centroidfrom the WFPC2 analysis. Interestingly, this new centroid is in better spatialagreement with the location where the X-ray emission is strongest, and themass-to-light ratio estimated with this centroid is much higher 813+-78M_sun/L_Rsun than the previous value; the aperture mass based on the WFPC2centroid provides a slightly lower, but consistent mass. Although we cannotprovide a definite explanation for the presence of the dark peak, we discuss arevised scenario, wherein dark matter with a more conventional rangesigma_DM/m_DM < 1 cm^2/g of self-interacting cross-section can lead to thedetection of this dark substructure. If supported by detailed numericalsimulations, this hypothesis opens up the possibility that the A520 system canbe used to establish a lower limit of the self-interacting cross-section ofdark matter.

Hubble Space Telescope/Advanced Camera for Surveys Confirmation of the Dark Substructure in A520