Leiden University Student Repository

Callenbach, Job (2025)

Exploring new radio galaxy cluster frontiers with LOFAR and ACT: Finding and characterising radio haloes and relics at low mass, low frequency, and high redshift

Bachelor thesis | Natuurkunde (BSc)

Galaxy clusters have been shown to host relativistic cosmic rays and µG magnetic fields by emitting diffuse synchrotron radiation in the form of radio haloes and radio relics. These Megaparsec-scale sources commonly trace active merging clusters, with a broad consensus that the formation is linked to turbulence and shocks in the intra-cluster medium. Notably, the origin of the magnetic fields required to host this emission is not well understood, and this calls for the discovery of more clusters exhibiting this diffuse emission. Due to inverse Compton losses and redshift dimming, there is only handful of confirmed radio haloes above z > 1. The study of these distant clusters is needed to understand the introduction of large magnetic fields in the early universe. In this thesis, we aim to find new examples of diffuse emission in previously unexplored parameter spaces. We apply the mosaic images from the complete LoTSS survey to classify radio emission in 1378 clusters, selected from...Show moreGalaxy clusters have been shown to host relativistic cosmic rays and µG magnetic fields by emitting diffuse synchrotron radiation in the form of radio haloes and radio relics. These Megaparsec-scale sources commonly trace active merging clusters, with a broad consensus that the formation is linked to turbulence and shocks in the intra-cluster medium. Notably, the origin of the magnetic fields required to host this emission is not well understood, and this calls for the discovery of more clusters exhibiting this diffuse emission. Due to inverse Compton losses and redshift dimming, there is only handful of confirmed radio haloes above z > 1. The study of these distant clusters is needed to understand the introduction of large magnetic fields in the early universe. In this thesis, we aim to find new examples of diffuse emission in previously unexplored parameter spaces. We apply the mosaic images from the complete LoTSS survey to classify radio emission in 1378 clusters, selected from the ACT DR6 catalogue. We uncover three samples of clusters, including high-redshift (z > 0.75) and low-mass (M500c < 3·10^14 M⊙) samples, and clusters with expected radio powers close to the detection limit of the LoTSS survey. We present a list of 177 galaxy clusters exhibiting signs of diffuse radio emission, from which we further analyse 19 clusters by producing LOFAR images at different resolutions and overlays with optical legacy survey images. This work includes six radio haloes at high redshift, including the most distant confirmed radio halo at z=1.31. We further report four cases of unknown radio emission, as well as a radio halo and relic from the low-mass sample. The 150 MHz radio power of these high-z clusters deviates from the established mass-power correlation as found at lower redshift. Notably, the scatter in this correlation decreases when the halo power is plotted against E(z)·M500c, reducing σraw from 0.41 to 0.25. This reduction suggests that the E(z) term, effectively correcting for variations in critical Universe density, offers a more accurate description, and previous assumptions that attribute the scatter solely to cluster dynamical states are insufficient. LOFAR and ACT overlap at lower declination (δ < 20◦), making this thesis a pioneer in the discovery of this new LoTSS area. We highlight the limitations and challenges of the LoTSS survey at lower declination.Show less

Author

Callenbach, Job

Faculty

Faculty of Science

Specialisation

Astronomy

Supervisors

Marrewijk, J. van
Weeren, R.J. van

ECTS Credits

24

Language

en