MeerKAT discovery of a double radio relic and odd radio circle: connecting cluster and galaxy merger shocks

Abstract: We present the serendipitous discovery of (1) a large double radio relic associated with the galaxy cluster PSZ2 G277.93+12.34 and (2) a new odd radio circle, ORC J1027-4422, both found in the same deep MeerKAT 1.3 GHz wide-band radio continuum image. The angular separation of the two arc-shaped cluster relics is ~16 arcmin or ~2.6 Mpc for a cluster redshift of z ~ 0.158. The thin southern relic, which shows several ridges/shocks including one possibly moving inwards, has a linear extent of ~1.64 Mpc. In contrast, the northern relic is about twice as wide, twice as bright, but only has a largest linear size of ~0.66 Mpc. Complementary SRG/eROSITA X-ray images reveal extended emission from hot intracluster gas between the two relics and around the narrow-angle tail (NAT) radio galaxy PMN J1033-4335 (z ~ 0.153) located just east of the northern relic. The radio morphologies of the NAT galaxy and the northern relic, which are also detected with the Australian Square Kilometer Array Pathfinder (ASKAP) at 888 MHz, suggest both are moving in the same outward direction. The discovery of ORC J1027-4422 in a different part of the same MeerKAT image makes it the 4th known single ORC. It has a diameter of ~90 arcsec corresponding to 400 kpc at a tentative redshift of z ~ 0.3 and remains undetected in X-ray emission. Supported by simulations, we discuss similarities between outward moving galaxy and cluster merger shocks as the formation mechanisms for ORCs and radio relics, respectively.

• The paper demonstrates MeerKAT's role in detecting a double radio relic in PSZ2 G277.93+12.34, revealing signs of merger-induced cluster shocks.
• It identifies ORC J1027-4422, a notable odd radio circle with a 400 kpc diameter, and discusses its non-thermal origins.
• Multi-wavelength analyses combining radio and X-ray data provide insights into shock propagation and the dynamics of galaxy mergers.

Insights into Cluster Merger Shocks and Odd Radio Circles

The paper by Koribalski et al. presents a serendipitous discovery using the MeerKAT radio telescope of both a double radio relic associated with the galaxy cluster PSZ2 G277.93+12.34 and a new odd radio circle (ORC), namely ORC J1027--4422. The study investigates these phenomena within the context of astrophysical shocks related to both galaxy and cluster mergers, providing insights that are valuable for understanding the dynamic processes in large-scale structures in the universe.

Discovery and Characterization of the Double Radio Relic

The first remarkable aspect of this study is the identification of a double radio relic associated with the PSZ2 G277.93+12.34 galaxy cluster, observed at a redshift of approximately 0.158. The angular separation of the relics is about 2.6 Mpc, which suggests substantial cluster-wide shock phenomena typically interpreted as the aftermath of a major merger event when such relics appear on diametrically opposite sides of a galaxy cluster. The southern relic is particularly intriguing due to its thinness and length of about 1.64 Mpc, containing multiple substructures or ridges, whereas the northern relic appears broader and shorter. The contrast in appearance and location of these relics offers critical insights into the geometry and dynamics of the merger process.

Complementary X-ray data from SRG/eROSITA help confirm the merging status of this galaxy cluster, displaying significant emission from hot gas between the relics. This data combination in both radio and X-ray spectra enables a deep analysis of intracluster medium conditions and merger shock characteristics.

Odd Radio Circle ORC J1027--4422

The study further reports on the discovery of ORC J1027--4422, the fourth known single ORC, which adds another layer of intrigue to the field of radio astronomy. ORCs, characterized by their circular radio emission patterns, have recently been presented in literature with ORC J1027--4422 distinguished by its relatively large diameter of about 400 kpc at a putative redshift of 0.3. The formation mechanism for ORCs remains an open question, yet the authors propose galaxy mergers as a viable scenario. ORC J1027--4422's non-detection in X-rays places constraints on models predicting thermal emissions from associated hot gas, which in turn aids speculation on possible non-thermal origins and the nature of constituent galaxy dynamics.

Model Implications and Further Research Directions

This research underscores both practical and theoretical implications. From a practical standpoint, the discovery demonstrates MeerKAT's capabilities in identifying and characterizing faint, diffuse radio features. Theoretically, it suggests the efficacy of using multi-wavelength observations, particularly combining deep radio continuity with X-ray mappings, in understanding the interrelated processes occurring during major cosmological structure formation events.

The contrasting morphology and conditions of PSZ2 G277.93+12.34 and ORC J1027--4422 suggest areas for future development, particularly in high-resolution X-ray follow-up observations to refine the understanding of shock propagation and merger events. Simulations simulating the dynamics of both galaxy and cluster mergers could provide quantitative insights that correlate with the empirically determined features of relics and ORCs.

Finally, the work beckons for additional surveys targeting more ORCs and double relic clusters, which could help establish a more definitive theoretical framework surrounding these phenomena. Understanding the characteristics of these rare but telling cosmic features could significantly contribute to the larger puzzle of galaxy and cluster formation and evolution in our universe.