Radio spectral studies of Sunyaev-Zel’dovich-detected galaxy clusters

Abstract

Galaxy clusters are the largest gravitationally bound structures in the universe, thought to form through hierarchical mergers and the accretion of smaller systems. When these massive entities collide, immense amounts of energy are released that heat the intracluster medium (ICM) and generate turbulence. This turbulence leads to non-thermal synchrotron radiation from hot ICM regions. Giant radio halos, found near cluster centers, arise from synchrotron radiation by re-accelerated electrons in the cluster’s magnetic field due to merger-driven turbulence. On the other hand, radio relics, typically located in cluster outskirts, trace merger shock waves. These emissions are crucial indicators of cluster mergers, offering valuable insights into their dynamic processes. In this thesis, the radio environments of three Sunyaev-Zel’dovich detected galaxy clusters (ACT-CL J0034+0225, ACT-CL J0137-0827, and ACT-CL J0330-5228) from the MeerKAT Exploration of Relics, Giant Halos, and Extragalactic Radio Sources survey were investigated, with particular focus on diffuse radio emission. We utilised data from the MeerKAT (1.2 GHz and 0.8 GHz) and JVLA (1.5 GHz) telescopes, calibrated using the CARACal and VLA Calibration and Imaging pipelines, respectively. The final self calibrated images have noise values ranging from 8−24 μJy/beam for the MeerKAT data and 10 − 30 μJy/beam for the JVLA. Four methods of compact source subtraction were tested on the calibrated MeerKAT images, revealing that image-plane subtraction methods yield satisfactory results when compared to visibility-plane subtraction procedures. From the source subtracted images, we measured integrated flux densities for the halos in ACT-CL J0034+0225 and ACTCL J0137-0827 to be 2.38 }0.63 mJ/beam and 0.85 }0.70 mJy/beam, respectively. The integrated spectral indices of these halos were measured using complementary GMRT images and were found to be α1.16 GHz 0.32 GHz = −1.34 } 0.14 and α1.16 GHz 0.32 GHz = −1.33 } 0.10 respectively which are in line with those reported in the literature. The emission in ACTCL J0330-5228 was found to have an integrated flux density of 3.31 } 1.14 mJy/beam. The emission in ACT-CL J0330-5228 was proposed to be as a result of multiple sources, not solely a radio halo. Archival optical and X-ray data were used to investigate the dynamic state of the clusters and search for cluster members, revealing ongoing activity indicative of merger events in recent or early stages. Additionally, we investigated serendipitous diffuse radio emission in the ACT-CL J0330-5228 field using archival multiwavelength data along with newly observed MeerKAT data. The integrated spectral indices of the emission were measured as α944 MHz 802 MHz = −1.4 }0.2 for the eastern emission and α944 MHz 802 MHz = −1.1 } 0.1 for the western emission. The wideband spectral index maps indicate a spectral gradient in the eastern structure, typical of a cluster radio relic, while results for the western structure were more in line with (reaccelerated) active galactic nuclei (AGN) driven emission. We identified potential optical sources of the emission and proposed that the western emission arises from the interaction between a nearby disturbed cluster and AGN emission. The eastern structure lacks an optical counterpart at its peak and was propsed to be a radio relic viewed along its length or AGN emission shaped by turbulent interactions with a nearby cluster.