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3rd of February 2021
SPT-CL J2106-5844, a massive galaxy cluster merger at z>1
by Di Mascolo et al.
SPT-CL J2106-5844 is among the most massive galaxy clusters at z>1 yet discovered. Recent studies indicate SPT-CL J2106-5844 is undergoing a major merger, and efforts are underway to better characterise the system. Di Mascolo et al. have used ALMA, ACA and Chandra data to study the intracluster medium in this system. These measurements are coupled with radio observations from the ASKAP EMU pilot survey and the ATCA to search for diffuse non-thermal emission. The ASKAP and ATCA data are processed and imaged to specifically highlight any potential diffuse radio emission. The EMU radio observations reveal a diffuse radio structure ~400 kpc in projected extent along the north-west/south-east direction, indicative of strong activity from the active galactic nucleus within the brightest cluster galaxy. The image above shows a composite Hubble Space Telescope image with contours from the EMU (dashed yellow) and 2.1 GHz ATCA (solid white) images. The corresponding synthesized beams are shown in the bottom right corner following the same color convention. The outskirts of the extended structure observed in the EMU image are found to be contaminated by point-like sources. Among the most prominent features is the southernmost component in the EMU map, though, with no redshift identification, we cannot rule out the possibility that it is not associated with the cluster itself. This feature (labelled C1) coincides with a compact emission in the ATCA data, and has an optical counterpart in the Hubble Space Telescope (HST) image. A second source (C2) is identified in the ATCA image and found to be surrounded by a combination of diffuse emission and a blend of multiple compact sources. If associated with the central radio galaxy, these may correspond to knot-like features in one arm of the radio jet. On the other hand, the radio emission from C2 may be related to a separate radio galaxy, although with the current resolution it is difficult to determine its optical counterpart. The southern diffuse emission (C3) detected midway between the BCG and the compact source C1 appears cospatial with a collection of elliptical galaxies identified in the HST image and it is not yet possible to determine whether the radio emission is generated in its entirety by the dominant AGN or by a complex of radio-loud sources. More details are give in the preprint of the paper.

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