Surface brightness discontinuities in radio halos. Insights from the MeerKAT Galaxy Cluster Legacy Survey

TL;DR

This study uses MeerKAT radio observations to identify surface brightness discontinuities in galaxy cluster radio halos, revealing their connection with thermal gas features and the dynamics of the intracluster medium.

Contribution

It introduces a gradient-filtering edge-detection method to identify radio surface brightness edges, demonstrating their frequent occurrence and correlation with X-ray gas features in galaxy clusters.

Findings

Half of the studied radio halos show at least one brightness discontinuity.

Radio discontinuities often coincide with X-ray edges, indicating thermal and nonthermal interplay.

The method enhances detection of radio edges, aiding future ICM shock and cold front studies.

Abstract

Dynamical motions in the ICM can imprint distinctive features on X-ray images that map the thermal emission from galaxy clusters, such as sharp surface brightness discontinuities due to shocks and cold fronts. The gas dynamics during cluster mergers may also drive large-scale turbulence in the ICM, which in turn generates extended synchrontron sources known as radio halos. Surface brightness edges have been found numerous times in the thermal gas of clusters based on X-ray observations. In contrast, edges in radio halos have only been observed in a handful of cases. Our goal is to search for new radio surface brightness discontinuities in the ICM. We inspected the images of the Bullet Cluster and the other 25 radio halos reported in the MeerKAT Galaxy Cluster Legacy Survey. To aid the identification of surface brightness discontinuities, we applied a gradient-filtering edge-detection method to the radio images. We find that the adopted filtering technique is helpful in identifying surface brightness edges in radio images, allowing us to identify at least one gradient in half of the radio halos studied. For the Bullet Cluster, we find excellent agreement between the locations of the 4 radio discontinuities detected and the X-ray edges. This similarity informs us that there is substantial interplay between thermal and nonthermal components in galaxy clusters. This interplay is likely due to the frozen-in ICM magnetic field, which mediates the advection of cosmic rays while being dragged by thermal gas flows. We conclude that radio halos are shaped by dynamical motions in the ICM and that they often display surface brightness discontinuities, which appear to be co-located with edges in the thermal gas emission. Our results demonstrate that new and future generations of radio telescopes will provide a complementary approach to X-rays to efficiently detect shocks and cold fronts in the ICM.