Tempestuous life of galaxy clusters: X-ray view on the Coma cluster with SRG/eROSITA

January 27, 2021

Galaxy clusters are dynamic systems that grow by continuously accreting large and small chunks of matter. This accretion process should give rise to a rich substructure in the dark matter distribution within the clusters and to shocks and “cold fronts” in the hot baryonic gas. Recent SRG/eROSITA observations provided an unprecedented X-ray view of the Coma cluster, revealing intricate signatures of the merger process, which are predicted by numerical simulations.

Fig 1. X-ray image of the Coma cluster field in the 0.4-2 keV band obtained by eROSITA. The image is ~6 degrees on a side, corresponding to 10 Mpc at the distance of the cluster, with the logarithmic color-code spanning 5 orders of magnitude. The main cluster is in the process of merging with the NGC4839 group (a bright blob to the bottom right from the Coma cluster).

The Coma cluster (or Abell 1656) is a very special cluster of galaxies. It is very massive (containing thousands of galaxies) and nearby (less than 100 Mpc), and it is the very first object where Fritz Zwicky identified the presence of Dark Matter in 1933. In the radio band, it was the first cluster where a radio halo was found in the 1950s. The proximity of Coma makes it an attractive target for studies in all energy bands, although often the cluster’s huge angular size complicates the task. In the X-ray band, the SRG observatory featuring the eROSITA and ART-XC telescopes is specifically designed for wide-field observations, and therefore managed to cover the Coma cluster in its entirety.

The X-ray image (see Fig. 1) accumulated during the first two raster scan observations of the whole sky shows a region (extending ~10 Mpc at the distance of the cluster), which is at least twice the virial radius of the cluster. Apart from a multitude of sources (mostly distant Active Galactic Nuclei), two bright diffuse blobs can be readily spotted, which correspond to the main cluster and the NGC4839 group (to the bottom-right from the center). The cluster and the group are in process of merging. In fact, NGC4839 has already passed through the core of the main cluster once and it is about to start falling back again.

Fig 2. Flattened X-ray image of the Coma cluster field with labels schematically marking some of the features presumably associated with the merger with the NGC4839 group. The blue dashed line is the suggested trajectory of the group, which enters the Coma cluster from the North-East direction, and is currently close to apocenter. The presumed positions of two shocks driven by the NGC4839 group are shown with the red and purple curves. The shock closer to the center is driven by the displaced gas that settles back to hydrostatic equilibrium. This is the most salient feature directly seen in the image as the surface brightness edge. The green line shows the faint X-ray "bridge" connecting NGC4839 and the main cluster, which is a possible trace of the group passage through the Coma cluster. The yellow line shows the interface between cold and hot gas patches with the same pressure (the so-called Contact Discontinuity).

Numerical simulations predict a number of signatures associated with this particular stage of the merger. The bow shock, produced by the NGC4839 group during its first passage, should now be located in the cluster outskirts, while the gas displaced from the core of the main cluster should be falling back, forming a “secondary” shock. The new data obtained with SRG/eROSITA suggest that this structure on the right (Western) side of the core, which extends to a few Mpc is exactly this “secondary” shock (see also Fig.2).

Complementary information can also be obtained based on the Sunyaev-Zeldovich effect: the ratio of the eROSITA X-ray image and the Planck microwave image of the Coma cluster provides a proxy to the gas temperature map (Fig.3). Such temperature measurements do not require any spectral information in the X-ray band, such as emission lines of heavily ionized ions of iron, nickel, etc. or the shape of the continuum spectrum. In reality, this estimate uses only the "negative" surface brightness of the cluster in the microwave band and the X-ray surface brightness in the 0.4 - 2.3 KeV band, where the eROSITA telescope has the highest sensitivity. As expected in the merger scenario, the core of the main cluster is hot, while the less massive NGC4839 group is able to retain some of its cool gas.

Fig 3. Ratio of the microwave and X-ray images of the Coma cluster, converted to the weighted electron temperature. Contours show the X-ray surface brightness. The core of the main cluster is hot, with a temperature of ~100 degrees. The blue region to the bottom right corresponds to cooler gas of the NGC4389 group with three times lower temperature.

Yet another interesting implication of the merger scenario is that the radio halo, which is encompassed by the secondary shock, has passed in fact through two shocks — the first time through the bow shock driven by NGC4839 when it was passing through the Coma core, and more recently through the secondary shock. This process might mitigate the rapid “ageing” of relativistic particles in radio halos observed in many other clusters – work on this problem is in progress.

The SRG spacecraft was designed by Lavochkin Association, Roskosmos corporation and launched on July 13, 2019 with a Proton launcher from Baikonur cosmodrome. The SRG observatory was built with participation of DLR, Germany in the framework of the Russian Federal Space Program by the initiative of the Russian Academy of Sciences represented by its Space Research Institute (IKI). The observatory carries two unique X-ray grazing incidence telescopes: ART-XC (IKI, Russia) and eROSITA (MPE, Germany). The SRG/ eROSITA telescope was built under the leadership of the Max-Planck-Institute for Extraterrestrial Physics (MPE) and DLR. The SRG spacecraft is operated by Lavochkin Association and Deep Space Network Antennae in Bear Lakes, Ussurijsk, and Baikonur funded by Roskosmos.



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