  X-ray and radio observations of CIZA J2242.8+5301 suggest that it is a major
cluster merger. Despite being well studied in the X-ray, and radio, little has
been presented on the cluster structure and dynamics inferred from its galaxy
population. We carried out a deep (i<25) broad band imaging survey of the
system with Subaru SuprimeCam (g & i bands) and the Canada France Hawaii
Telescope (r band) as well as a comprehensive spectroscopic survey of the
cluster area (505 redshifts) using Keck DEIMOS. We use this data to perform a
comprehensive galaxy/redshift analysis of the system, which is the first step
to a proper understanding the geometry and dynamics of the merger, as well as
using the merger to constrain self-interacting dark matter. We find that the
system is dominated by two subclusters of comparable richness with a projected
separation of 6.9'^{+0.7}_{-0.5} (1.3^{+0.13}_{-0.10} Mpc). We find that the
north and south subclusters have similar redshifts of z=0.188 with a relative
line-of-sight velocity difference of 69+/-190 km/s. We also find that north and
south subclusters have velocity dispersions of 1160^{+100}_{-90} km/s and
1080^{+100}_{-70} km/s, respectively. These correspond to masses of
16.1^{+4.6}_{-3.3}x10^14 M_sun and 13.0^{+4.0}_{-2.5}x10^14 M_sun,
respectively. While velocity dispersion measurements of merging clusters can be
biased we believe the bias in this system to be minor due to the large
projected separation and nearly plane-of-sky merger configuration. CIZA
J2242.8+5301 is a relatively clean dissociative cluster merger with near 1:1
mass ratio, which makes it an ideal merger for studying merger associated
physical phenomena.
