On Dark Peaks and Missing Mass: A Weak Lensing Mass Reconstruction of
  the Merging Cluster System Abell 520
  Merging clusters of galaxies are unique in their power to directly probe and
place limits on the self-interaction cross-section of dark matter. Detailed
observations of several merging clusters have shown the intracluster gas to be
displaced from the centroids of dark matter and galaxy density by ram pressure,
while the latter components are spatially coincident, consistent with
collisionless dark matter. This has been used to place upper limits on the dark
matter particle self-inteaction cross-section of order 1 cm^2/g. The cluster
Abell 520 has been seen as a possible exception. We revisit A520 presenting new
HST ACS mosaic images and a Magellan image set. We perform a detailed weak
lensing analysis and show that the weak lensing mass measurements and
morphologies of the core galaxy-filled structures are mostly in good agreement
with previous works. There is however one significant difference -- we do not
detect the previously claimed "dark core" that contains excess mass with no
significant galaxy overdensity at the location of the X-ray plasma. This peak
has been suggested to be indicative of a large self-interaction cross-section
for dark matter (at least ~5 sigma larger than the upper limit of 0.7 cm^2/g
determined by observations of the Bullet Cluster). We find no such indication
and instead find that the mass distribution of A520, after subtraction of the
X-ray plasma mass, is in good agreement with the luminosity distribution of the
cluster galaxies. We conclude that A520 shows no evidence to contradict the
collisionless dark matter scenario.
