Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA. II. Shock & Relic

Abstract

This is the second paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained in the course of the calibration and performance verification observations. Here we focus on the region adjacent to the radio source 1253+275 (Radio Relic, RR hereafter). We show that the X-ray surface brightness steepest gradient at ∼ 79 (∼ 2.2 Mpc ≈ R 200c ) is almost co-spatial with the outer edge of RR, with an offset of about ∼ 100−150 kpc between the derived position of the shock front and the brightest part of RR. As in several other relics, the Mach number of the shock derived from the X-ray surface brightness profile (M X ≈ 1.9) appears to be lower than needed to explain the slope of the integrated radio spectrum in the DSA model (M R ≈ 3.5) if the magnetic field is uniform and the radiative losses are fast. However, the shock geometry is plausibly much more complicated than a spherical wedge centered on the cluster, given a non-trivial correlation between radio, X-ray, and SZ images. While the complicated shock geometry alone might bias M X low, we speculate on a few other processes that can affect the M X -M R relation, including a fine structure of the shock that might be modified by the presence of non-thermal filaments stretching across the shock. We also discuss the "history" of the radio galaxy NGC4789 located ahead of the relic in the context of the Coma-NGC4839 merger scenario.

Globular Clusters in NGC 4839 Falling into Coma: Evidence for the Second Infall?

“…Recently from the X-ray images obtained with the SRG/ eROSITA, Churazov et al (2021Churazov et al ( , 2022 found a faint X-ray bridge connecting the NGC 4839 group with the main cluster. This bridge may be a remnant of stripped gas while NGC 4839 moves outward from the main cluster to the current position, showing that it is strong evidence that NGC 4839 already passed the main cluster core (see their Figure 11).…”

“…This bridge may be a remnant of stripped gas while NGC 4839 moves outward from the main cluster to the current position, showing that it is strong evidence that NGC 4839 already passed the main cluster core (see their Figure 11). Churazov et al (2021Churazov et al ( , 2022 also pointed out that the existence of the bow shock at » ¢ R 33 (960 kpc) in the west, and the radio relic at R ≈ 2.1 Mpc in the SW (Bonafede et al 2021) may correspond, respectively, to the secondary shock (produced when crossing the apocenter) and the primary shock (produced when crossing the main cluster core) caused by the merging event with NGC 4839.…”

“…Thus both galaxies are very bright, and the V magnitude of NGC 4816 is only 0.9 mag fainter than that of NGC 4839. While the NGC 4839 group shows a strong X-ray emission, the NGC 4816 group shows little detected X-ray emission even in the recent X-ray images (Lyskova et al 2019;Mirakhor & Walker 2020;Sanders et al 2020;Churazov et al 2021Churazov et al , 2022.…”

“…Each feature can be explained with either the premerger scenario or the postmerger scenario. Recently the postmerger scenario, which can better explain the existence of X-ray-radio substructures (in particular, bridges and streams), appears to be more supported (Lyskova et al 2019;Churazov et al 2021Churazov et al , 2022Bonafede et al 2021). However, even in the recent discussions of both scenarios based on various observations, Healy et al (2021) state that, nevertheless, the question whether the NGC 4839 group is on its first infall or has already passed through the cluster remains open.…”

Globular Clusters in NGC 4839 Falling into Coma: Evidence for the Second Infall?

“…Each feature can be explained with either the pre-merger scenario or the post-merger scenario. Recently the postmerger scenario, which can better explain the existence of X-ray/radio substructures (in particular, bridges and streams), appears to be more supported (Lyskova et al 2019;Churazov et al 2021Churazov et al , 2022Bonafede et al 2021). However, even in the recent discussions of both scenarios based on various observations, Healy et al (2021) state that Nevertheless, the question whether the NGC 4839 group is on its first infall or has already passed through the cluster, remains open.…”