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THE ATACAMA COSMOLOGY TELESCOPE: ACT-CL J0102−4915 “EL GORDO,” A MASSIVE MERGING CLUSTER AT REDSHIFT 0.87

We present a detailed analysis from new multi-wavelength observations of the exceptional galaxy cluster ACT-CL J0102−4915, likely the most massive, hottest, most X-ray luminous and brightest Sunyaev–Zel'dovich (SZ) effect cluster known at redshifts greater than 0.6. The Atacama Cosmology Telescope (ACT) collaboration discovered ACT-CL J0102−4915 as the most significant SZ decrement in a sky survey area of 755 deg2. Our Very Large Telescope (VLT)/FORS2 spectra of 89 member galaxies yield a cluster redshift, z = 0.870, and velocity dispersion, σgal = 1321 ± 106 km s−1. Our Chandra observations reveal a hot and X-ray luminous system with an integrated temperature of TX = 14.5 ± 0.1 keV and 0.5–2.0 keV band luminosity of LX = (2.19 ± 0.11) × 1045 h−270 erg s−1. We obtain several statistically consistent cluster mass estimates; using empirical mass scaling relations with velocity dispersion, X-ray YX, and integrated SZ distortion, we estimate a cluster mass of M200a = (2.16 ± 0.32) × 1015 h−170 M☉. We constrain the stellar content of the cluster to be less than 1% of the total mass, using Spitzer IRAC and optical imaging. The Chandra and VLT/FORS2 optical data also reveal that ACT-CL J0102−4915 is undergoing a major merger between components with a mass ratio of approximately 2 to 1. The X-ray data show significant temperature variations from a low of 6.6 ± 0.7 keV at the merging low-entropy, high-metallicity, cool core to a high of 22 ± 6 keV. We also see a wake in the X-ray surface brightness and deprojected gas density caused by the passage of one cluster through the other. Archival radio data at 843 MHz reveal diffuse radio emission that, if associated with the cluster, indicates the presence of an intense double radio relic, hosted by the highest redshift cluster yet. ACT-CL J0102−4915 is possibly a high-redshift analog of the famous Bullet cluster. Such a massive cluster at this redshift is rare, although consistent with the standard ΛCDM cosmology in the lower part of its allowed mass range. Massive, high-redshift mergers like ACT-CL J0102−4915 are unlikely to be reproduced in the current generation of numerical N-body cosmological simulations.