2015ApJ...805..143D

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) broadband imaging survey of the system with Subaru SuprimeCam (g and 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 DEep Imaging Multi-Object Spectrograph. We use these data to perform a comprehensive galaxy/redshift analysis of the system, which is the first step to a proper understanding of 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_–0.5^+0.7 (1.3_–0.10^+0.13Mpc). We find that the north and south subclusters have similar redshifts of z≈0.188 with a relative line-of-sight (LOS) velocity difference of 69±190 km/s. We also find that north and south subclusters have velocity dispersions of 1160_–90⁺¹⁰⁰ and 1080_–70⁺¹⁰⁰km/s, respectively. These correspond to masses of 16.1_–3.3^+4.6x10¹⁴ and 13.0_–2.5^+4.0 x10¹⁴M_☉, 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. We also find that the cDs of the north and south subclusters are very near their subcluster centers, in both projection (55 and 85 kpc, respectively) and normalized LOS velocity (|Δv|σv = 0.43 ± 0.13 and 0.21±0.12 for the north and south, respectively). 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.