Observation of a very massive galaxy cluster at z=0.76 in SRG/eROSITA all-sky survey

Abstract

The results of multiwavelength observations of the very massive galaxy cluster SRGe CL2305.2−2248 detected in X-rays during the first SRG/eROSITA all-sky survey are discussed. This galaxy cluster was also detected earlier in microwave band through the observations of Sunyaev-Zeldovich effect in South Pole Telescope (SPT-CL J2305−2248), and in Atacama Cosmological Telescope (ACT-CL J2305.1−2248) surveys. Spectroscopic redshift measurement, z = 0.7573, was measured at the Russian 6-m BTA telescope of the SAO RAS, in good agreement with its photometric estimates, including a very accurate one obtained using machine learning methods. In addition, deep photometric measurements were made at the Russian-Turkish 1.5-m telescope (RTT150), which allows to study cluster galaxies red sequence and projected galaxies distribution. Joint analysis of the data from X-ray and microwave observations show that this cluster can be identified as a very massive and distant one using the measurements of its X-ray flux and integral comptonization parameter only. The mass of the cluster estimated according to the eROSITA data is M500 = (9.0 ± 2.6) • 10 14 M . We show that this cluster is found among of only several dozen of the most massive clusters in the observable Universe and among of only a few the most massive clusters of galaxies at z > 0.6.