MC${}^2$: Dynamical Analysis of the Merging Galaxy Cluster MACS J1149.5+2223

Abstract

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 between three subclusters occurring $\sim$1 Gyr apart. The primary merger gives rise to elongated X-ray morphology and a radio relic in the southeast. The brightest cluster galaxy is a member of the northern subcluster of the primary merger. This subcluster is very massive (16.7${}_{-\text{1.60}}^{+\text{1.25}}\times {\text{10}}^{\text{14}}$ M${}_{\odot }$). The southern subcluster is also very massive (10.8${}_{-\text{3.54}}^{+\text{3.37}}\times {\text{10}}^{\text{14}}$ M${}_{\odot }$), yet it lacks an associated X-ray surface brightness peak, and it has been unidentified previously despite the detailed study of this \emph{Frontier Field} cluster. A secondary merger is occurring in the north along the line of sight with a third, less massive, subcluster (1.20${}_{-\text{0.34}}^{+\text{0.19}}\times {\text{10}}^{\text{14}}$ M${}_{\odot }$). We perform a Monte Carlo dynamical analysis on the main merger and estimate a collision speed at pericenter of 2770${}_{-\text{310}}^{+\text{610}}$ km s${}^{-\text{1}}$. We show the merger to be returning from apocenter with core passage occurring 1.16${}_{-\text{0.25}}^{+\text{0.50}}$ Gyr before the observed state. We identify the line of sight 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 $\sim$4000 km s${}^{-\text{1}}$.

17 pages, 12 figures

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