Hubble Space Telescope/Advanced Camera for Surveys Confirmation of the
  Dark Substructure in A520
  We present the results from a weak gravitational lensing study of the merging
cluster A520 based on the analysis of Hubble Space Telescope/Advanced Camera
for Surveys (ACS) data. The excellent data quality allows us to reach a mean
number density of source galaxies of ~109 per sq. arcmin, which improves both
resolution and significance of the mass reconstruction compared to a previous
study based on Wide Field Planetary Camera 2 (WFPC2) images. We take care in
removing instrumental effects such as the trailing of charge due to radiation
damage of the ACS detector and the position-dependent point spread function
(PSF). This new ACS analysis confirms the previous claims that a substantial
amount of dark mass is present between two luminous subclusters. We examine the
distribution of cluster galaxies and observe very little light at this
location. We find that the centroid of the dark peak in the current ACS
analysis is offset to the southwest by ~1 arcmin with respect to the centroid
from the WFPC2 analysis. Interestingly, this new centroid is in better spatial
agreement with the location where the X-ray emission is strongest, and the
mass-to-light ratio estimated with this centroid is much higher 813+-78
M_sun/L_Rsun than the previous value; the aperture mass based on the WFPC2
centroid provides a slightly lower, but consistent mass. Although we cannot
provide a definite explanation for the presence of the dark peak, we discuss a
revised scenario, wherein dark matter with a more conventional range
sigma_DM/m_DM < 1 cm^2/g of self-interacting cross-section can lead to the
detection of this dark substructure. If supported by detailed numerical
simulations, this hypothesis opens up the possibility that the A520 system can
be used to establish a lower limit of the self-interacting cross-section of
dark matter.
