2016ApJ...831..110G

We present an analysis of the merging cluster MACS J1149.5+2223 using archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from Keck/DEIMOS and Gemini/GMOS. We employ two- and three-dimensional substructure tests and determine that MACS J1149.5+2223 is composed of two separate mergers among three subclusters occurring ∼1 Gyr apart. The primary merger gives rise to elongated X-ray morphology and a radio relic in the southeast. The brightest cluster galaxy is a member of the northern subcluster of the primary merger. This subcluster is very massive (16.7_–1.60^+1.25×10¹⁴ M_☉). The southern subcluster is also very massive (10.8_–3.54^+3.37×10¹⁴ M_☉), yet it lacks an associated X-ray surface brightness peak, and it has been unidentified previously despite the detailed study of this Frontier Field cluster. A secondary merger is occurring in the north along the line of sight (LOS) with a third, less massive subcluster (1.20_–0.34^+0.19×10¹⁴ M_☉). We perform a Monte Carlo dynamical analysis on the main merger and estimate a collision speed at pericenter of 2770_–310⁺⁶¹⁰ km s^–1. We show the merger to be returning from apocenter with core passage occurring 1.16_–0.25^+0.50 Gyr before the observed state. We identify the LOS merging subcluster in a strong lensing analysis in the literature and show that it is likely bound to MACS J1149 despite having reached an extreme collision velocity of ∼4000 km s^–1.