X-SORTER (X-ray Survey Of meRging clusTErs in Redmapper): X-ray and Spectroscopic Characterization of 12 Optically Selected Galaxy Cluster Merger Candidates

Abstract

Merging galaxy clusters offer a unique probe of dark matter (DM) interactions through the spatial offsets between galaxies, the intracluster medium, and the DM halo. Systems that are binary, near the plane of the sky, and observed shortly after first pericenter provide the cleanest constraints on the DM self-interaction cross-section. The X-SORTER (X-ray Survey Of meRging clusTErs in redMaPPer) program aims to systematically identify such mergers using optical indicators of binarity in the redMaPPer cluster catalog and to follow up promising candidates with X-ray and spectroscopic observations. We select clusters where the top redMaPPer brightest cluster galaxy (BCG) probability is below 0.98, the top two BCGs are separated by at least 0.95 arcmin, and the optical richness exceeds lambda = 120. We present XMM and Keck/DEIMOS observations of twelve clusters with no previous XMM-Newton or Chandra archival data meeting these criteria. The X-ray data reveal that most targets are morphologically disturbed, with several clear post-pericenter, dissociative systems exhibiting X-ray peaks between the BCGs. Spectroscopy confirms cluster membership and rules out foreground or background contamination. Together, these results demonstrate that optical BCG properties provide an efficient means of identifying dynamically active clusters suitable for detailed, multi-wavelength studies of dark matter and cluster evolution.

Figures (47)

• Figure 1: Pair plot for the redMaPPer attributes of redshift, richness $\lambda$, separation between the top two BCG candidates, and $P0$ (probability of the top BCG candidate being the actual BCG). This plot includes only clusters with $\lambda>60$ to reduce crowding. Black lines illustrate our cuts, and black points are clusters studied in this paper.
• Figure 2: Decision tree for X-SORTER targets. The green box on the right is the focus of this paper.
• Figure 3: X-ray temperature, with self-similar correction $E(z)^{-2/3}$, versus richness with scaling relation given in \ref{['eq:T']}. The shaded region gives $1\sigma$ uncertainty in the scaling relation by upsdell2023xmm.
• Figure 4: X-ray luminosity in the 0.5--2.0 band, with self-similar correction $E(z)^{-2}$, versus richness with scaling relation given in \ref{['eq:L']}. The shaded region gives $1\sigma$ uncertainty in the scaling relation by upsdell2023xmm.
• Figure 5: Optical image of RMJ0003. Top panel: 6$^\prime$ by 6$^\prime$ (1.86 by 1.86 Mpc) Legacy Survey image. Bottom panel: red sequence density contours (blue), X-ray surface brightness contours (red), and BCG candidates identified by redMaPPer.