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24th of July 2020
ASKAP detection of cluster halo and relic
by Kim et al.
A galaxy cluster can consist of hundreds to thousands of galaxies, bound into the cluster by gravity. Large-scale diffuse radio emissions provide information about galaxy cluster mergers, and can be classified into two broad categories: halos and relics. Radio halos and relics do not have distinct optical counterparts, and differ primarily in that halos are found in the central regions of merging clusters whereas relics are located in the cluster periphery. Radio relics have been considered a stronger constraint on cluster merger history as they can be used as direct probes of merger shocks. To date, there are ∼60 known radio relics, too small a sample to enable studies allowing overarching principles to be extracted.

The ASKAP EMU survey is a deep wide-field all-sky radio continuum survey. EMU Pilot Survey observations led to the discovery of a radio relic in the merging cluster SPT-CL 2023-5535 at a redshift of 0.23. The deep high-resolution data reveal a ∼2 Mpc-scale radio halo, coincident with the intracluster gas, and a radio relic located at the western edge of the radio halo extending over ∼0.5 Mpc. In combination with multiwavelength data, it appears that the system is composed of at least three subclusters, with the halo and relic arising from the collision between the eastern and middle subclusters.

The radio image above has both diffuse (purple) and compact sources (green) enhanced. The diffuse radio halo near the centre is elongated in the east-west orientation and the radio relic is the white feature to the right of the halo. The yellow arrow points at a "link" feature between the relic and nearby radio galaxy. The discovery illustrates the effectiveness of the ASKAP-EMU survey in detecting diffuse emissions in galaxy clusters. The paper has been submitted to the Astrophysical Journal.

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