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Abstract The galaxy cluster A746 (z= 0.214), featuring a double radio relic system, two isolated radio relics, a possible radio halo, disturbed V-shaped X-ray emission, and intricate galaxy distributions, is a unique and complex merging system. We present a weak-lensing analysis of A746 based on wide-field imaging data from Subaru/Hyper Suprime-Cam observations. The mass distribution is characterized by a main peak, which coincides with the center of the X-ray emission. At this main peak, we detect two extensions toward the north and west tracing the cluster galaxy and X-ray distributions. Despite the ongoing merger, our estimate of the A746 global massM₅₀₀= 4.4 ± 1.0 × 10¹⁴M_⊙is consistent with the previous results from Sunyaev-Zel'dovich and X-ray observations. We conclude that reconciling the distributions of mass, galaxies, and intracluster medium with the double radio relic system and other radio features remains challenging. 

Abstract Studies of star formation in various galaxy cluster mergers have reached apparently contradictory conclusions regarding whether mergers stimulate star formation, quench it, or have no effect. Because the mergers studied span a range of time since pericenter (TSP), it is possible that the apparent effect on star formation is a function of the TSP. We use a sample of 12 bimodal mergers to assess the star formation as a function of TSP. We measure the equivalent width of the Hαemission line in ∼100 member galaxies in each merger, classify galaxies as emitters or nonemitters, and then classify emitters as star-forming galaxies (SFGs) or active galactic nucleus (AGN) based on the [Nii]λ6583 line. We quantify the distribution of SFG and AGN relative to nonemitters along the spatial axis defined by the subcluster separation. The SFG and AGN fractions vary from merger to merger but show no trend with TSP. The spatial distribution of SFG is consistent with that of nonemitters in eight mergers, but show significant avoidance of the system center in the remaining four mergers, including the three with the lowest TSP. If there is a connection between star formation activity and TSP, probing it further will require more precise TSP estimates and more mergers with TSP in the range of 0–400 Myr. 

Abstract Galaxy cluster mergers that exhibit clear dissociation between their dark matter, intracluster gas, and stellar components are great laboratories for probing dark matter properties. Mergers that are binary and in the plane of the sky have the additional advantage of being simpler to model, allowing for a better understanding of the merger dynamics. We report the discovery of a galaxy cluster merger with all these characteristics and present a multiwavelength analysis of the system, which was found via a search in the redMaPPer optical cluster catalog. We perform a galaxy redshift survey to confirm the two subclusters are at the same redshift (0.541, with 368 ± 519 km s⁻¹line-of-sight velocity difference between them). The X-ray morphology shows two surface brightness peaks between the brightest cluster galaxies (BCGs). We construct weak-lensing mass maps that reveal a mass peak associated with each subcluster. Fitting Navarro–Frenk–White profiles to the lensing data, we find masses ofM_200c= 36 ± 11 × 10¹³and 38 ± 11 × 10¹³M_⊙h⁻¹for the southern and northern subclusters, respectively. From the mass maps, we infer that the two mass peaks are separated by ${520}_{-125}^{+162}$kpc along the merger axis, whereas the two BCGs are separated by 697 kpc. We also present deep GMRT 650 MHz data to search for a radio relic or halo and find none. Using the observed merger parameters, we find analog systems in cosmologicaln-body simulations and infer that this system is observed between 96 and 236 Myr after pericenter, with the merger axis within 28° of the plane of the sky. 

We present a multiwavelength analysis of 29 merging galaxy clusters that exhibit radio relics. For each merging system, we perform a weak-lensing analysis on Subaru optical imaging. We generate high-resolution mass maps of the dark matter distributions, which are critical for discerning the merging constituents. Combining the weak-lensing detections with X-ray emission, radio emission, and galaxy redshifts, we discuss the formation of radio relics from the past collision. For each cluster, we obtain mass estimates by fitting a multicomponent Navarro–Frenk–White model with and without a concentration–mass relation. We compare the mass estimates of each subcluster to their velocity dispersion measurements and find that they preferentially lie below the expected velocity dispersion scaling relation, especially at the low-mass end (∼10¹⁴M_⊙). We show that the majority of the clusters that exhibit radio relics are in major mergers with a mass ratio below 1:4. We investigate the position of the mass peak relative to the galaxy luminosity peak, number density peak, and brightest cluster galaxy (BCG) locations and find that the BCG tends to better trace the mass peak position. Finally, we update a golden sample of eight galaxy clusters that have the simplest geometries and can provide the cleanest picture of the past merger, which we recommend for further investigation to constrain the nature of dark matter and the acceleration process that leads to radio relics.