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We present deep Chandra observations of A3411-12, a remarkable merging cluster that hosts the most compelling evidence for electron reacceleration at cluster shocks to date. Using the YX-M scaling relation, we find r 500 ∼ 1.3 Mpc, M500 =(7.1 ± 0.7) × 1014 M⊙, kT =6.5 ± 0.1 keV, and a gas mass of Mg,500 =(9.7 ± 0.1) × 1013 M⊙. The gas mass fraction within r 500 is fg = 0.14 ± 0.01. We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small (M ≤ 1.15-0.09+0.14). We also present density, temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map, we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411-12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster.

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found 1,2. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster-cluster merger events 3. A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. revealed unexpected overdensities of candidate Hα emitters near the ~1-Mpc-wide shock fronts of the massive (~2 × 1015 M⊙) 'Sausage' merging cluster, CIZA J2242.8+5301. We used the Keck/Deep Imaging Multi-Object Spectrograph and the William Herschel Telescope/AutoFib2+WYFFOS to confirm 83 Hα emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster subcores (away from the shock fronts) show high [S II]6716/[S II]6761 and high [S II] 6716/Hα, implying very low electron densities ( 2.5 Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (~1015 M⊙) clusters can provide the conditions for significant star formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100 Myr.

CIZA J2242.8+5301 ('Sausage') and 1RXS J0603.3+4213 ('Toothbrush') are two lowredshift (z ~ 0.2), massive (~2 × 1015M·), post-core passage merging clusters, which host-shock waves traced by diffuse radio emission. To study their star formation properties, we uniformly survey the 'Sausage' and 'Toothbrush' clusters in broad- and narrow-band filters and select a sample of 201 and 463 line emitters, down to a rest-frame equivalent width (13 Å). We robustly separate between Hα and higher redshift emitters using a combination of optical multiband (B, g, V, r, i, z) and spectroscopic data. We build Hα luminosity functions for the entire cluster region, near the shock fronts, and away from the shock fronts and find striking differences between the two clusters. In the dynamically younger, 1 Gyr old 'Sausage' cluster we find numerous (59) Hα emitters above a star formation rate (SFR) of 0.17 M· yr-1 surprisingly located in close proximity to the shock fronts, embedded in very hot intracluster medium plasma. The SFR density for the cluster population is at least at the level of typical galaxies at z ~2. Down to the same SFR, the possibly dynamically more evolved 'Toothbrush' cluster has only nine Hα galaxies. The cluster Hα galaxies fall on the SFR-stellarmass relation z ~ 0.2 for the field. However, the 'Sausage' cluster has an Hαa emitter density >20 times that of blank fields. If the shock passes through gas-rich cluster galaxies, the compressed gas could collapse into dense clouds and excite star formation for a few 100 Myr. This process ultimately leads to a rapid consumption of the molecular gas, accelerating the transformation of gas-rich field spirals into cluster S0s or ellipticals.

X-ray and radio observations of CIZA J2242.8+5301 suggest that it is a majorcluster merger. Despite being well studied in the X-ray, and radio, little hasbeen presented on the cluster structure and dynamics inferred from its galaxypopulation. We carried out a deep (i

The galaxy cluster CIZA J2242.8+5301 at z = 0.19 is a merging system with a prominent (2 Mpc long) radio relic, which together with the morphology of the X-ray emission provides strong evidence for a violent collision along the north-south axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and Canada-France-Hawaii Telescope imaging data. Measuring a high signal-to-noise ratio lensing signal from this cluster is potentially a challenging task because of its proximity to the Milky Way plane (|b| 5°). We overcome this challenge with careful observation planning and systematics control, which enables us to successfully map the dark matter distribution of the cluster with high fidelity. The resulting mass map shows that the mass distribution of CIZA J2242.8+5301 is highly elongated along the north-south merger axis inferred from the orientation of the radio relics. Based on our mass reconstruction, we identify two sub-clusters, which coincide with the cluster galaxy distributions. We determine their masses using Markov Chain Monte Carlo analysis by simultaneously fitting two Navarro-Frenk-White halos without fixing their centroids. The resulting masses of the northern and southern systems are and , respectively, indicating that we are witnessing a post-collision of two giant systems of nearly equal mass. When the mass and galaxy centroids are compared in detail, we detect 1′ (190 kpc) offsets in both northern and southern sub-clusters. After investigating the statistical significance of the offsets by bootstrapping both mass and galaxy centroids, we find that the galaxy luminosity-mass offset for the northern clump is statistically significant at the ≳ 2σ level whereas the detection is only marginal for the southern sub-cluster in part because of a relatively large mass centroid error. We conclude that it is yet premature to uniquely attribute the galaxy-mass misalignment to self-interaction of dark matter and discuss caveats.