2020ApJ...900..127H

The ASKAP-EMU survey is a deep wide-field radio continuum survey designed to cover the entire southern sky and a significant fraction of the northern sky up to +30°. Here, we report a discovery of a radio relic in the merging cluster SPT-CL J2023-5535 at z = 0.23 from the ASKAP-EMU pilot 300 square degree survey (800-1088 MHz). The deep high-resolution data reveal a ∼2 Mpc scale radio halo elongated in the east-west direction, coincident with the intracluster gas. The radio relic is located at the western edge of this radio halo stretched ∼0.5 Mpc in the north-south orientation. The integrated spectral index of the radio relic within the narrow bandwidth is α_800MHz^1088MHz=-0.76±0.06. Our weak-lensing analysis shows that the system is massive (M₂₀₀=1.04±0.36×10¹⁵M_☉) and composed of at least three subclusters. We suggest a scenario, wherein the radio features arise from the collision between the eastern and middle subclusters. Our discovery illustrates the effectiveness of the ASKAP-EMU survey in detecting diffuse emissions in galaxy clusters and when completed, the survey will greatly increase the number of merging cluster detections with diffuse radio emissions.