Toward Solving the Puzzle: Dissecting the Complex Merger A521 with
  Multi-wavelength Data
  A521 has been a subject of extensive panchromatic studies from X-ray to
radio. The cluster possesses a number of remarkable features including a bright
radio relic with a steep spectrum, more than three distinct galaxy groups
forming a filament, and two disturbed X-ray peaks at odds with the distant
position and tilted orientation of the radio relic. These several lines of
evidence indicate a complex merger. In this paper, we present a
multi-wavelength study of A521 based on Subaru optical, Hubble Space Telescope
infrared, Chandra X-ray, GMRT radio, and MMT optical spectroscopic
observations. Our weak-lensing (WL) analysis with improved systematics control
reveals that A521 is mainly composed of three substructures aligned in the
northwest to southeast orientation. These WL mass substructures are remarkably
well-aligned with the cluster optical luminosity distribution constructed from
our new enhanced cluster member catalog. These individual substructure masses
are determined by simultaneously fitting three NFW profiles. We find that the
total mass of A521 modeled by the superposition of the three halos is
$13.0_{-1.3}^{+1.0} \times 10^{14}M_{\odot}$, a factor of two higher than the
previous WL measurement. With these WL mass constraints combined with X-ray and
radio features, we consider two merging scenarios, carry out the corresponding
numerical simulations, and discuss strengths and weaknesses of each case.
