[Paper Review] Radio halos in a mass-selected sample of 75 galaxy clusters: II. Statistical analysis

Cuciti, V., Cassano, R.+9 more|arXiv (Cornell University)|Jan 1, 2021

Galaxies: Formation, Evolution, Phenomena67 citations

TL;DR

This study presents the first statistical analysis of radio halos in a mass-selected sample of 75 galaxy clusters from the Planck SZ catalogue, using deep radio and X-ray data. It confirms a strong correlation between radio halo luminosity and cluster mass, shows that over 90% of radio halos occur in merging clusters, and demonstrates that cluster dynamics significantly contribute to the scatter in the radio power–mass relation, with emissivity showing a clear bimodality. The observed drop in radio halo fraction from ~70% in high-mass to ~35% in low-mass clusters aligns with turbulent re-acceleration models.

ABSTRACT

Context. Many galaxy clusters host megaparsec-scale difiuse radio sources called radio halos. Their origin is tightly connected to the processes that lead to the formation of clusters themselves. In order to reveal this connection, statistical studies of the radio properties of clusters combined with their thermal properties are necessary. For this purpose, we selected a sample of galaxy clusters with M500≤ 6 × 1014M and z = 0.08-0.33 from the Planck Sunyaev-Zel'dovich catalogue. In Paper I, we presented the radio and X-ray data analysis that we carried out on the clusters of this sample. Aims. In this paper we exploit the wealth of data presented in Paper I to study the radio properties of the sample, in connection to the mass and dynamical state of clusters. Methods.We used the dynamical information derived from the X-ray data to assess the role of mergers in the origin of radio halos.We studied the distribution of clusters in the radio power-mass diagram, the scaling between the radio luminosity of radio halos and the mass of the host clusters, and the role of dynamics in the radio luminosity and emissivity of radio halos. We measured the occurrence of radio halos as a function of the cluster mass and we compared it with the expectations of models developed in the framework of turbulent acceleration. Results. We find that more than the 90% of radio halos are in merging clusters and that their radio power correlates with the mass of the host clusters. The correlation shows a large dispersion. Interestingly, we show that cluster dynamics contributes significantly to this dispersion, with more disturbed clusters being more radio luminous. Clusters without radio halos are generally relaxed, and the upper limits to their diffiuse emission lie below the correlation. Moreover, we show that the radio emissivity of clusters exhibits an apparent bimodality, with the emissivity of radio halos being at least ~5 times larger than the non-emission associated with more relaxed clusters. We find that the fraction of radio halos drops from ~70% in high-mass clusters to ~35% in the lower mass systems in the sample and we show that this result is in good agreement with the expectations from turbulent re-acceleration models.

Motivation & Objective

• To investigate the statistical connection between radio halo properties and the mass and dynamical state of galaxy clusters.
• To determine how merger activity influences radio halo luminosity and emissivity.
• To measure the occurrence rate of radio halos as a function of cluster mass and compare it with theoretical models.
• To assess the role of cluster dynamics in the scatter of the radio power–mass correlation.
• To test predictions of turbulent re-acceleration models using a complete, mass-selected sample with deep radio observations.

Proposed method

• Selected 75 galaxy clusters from the Planck SZ catalogue with M500 ≥ 6 × 10^14 M⊙ and redshift 0.08 ≤ z ≤ 0.33.
• Combined deep radio observations (610 MHz and 1.4 GHz) with X-ray-derived dynamical information to classify clusters as merging or relaxed.
• Mapped clusters in the radio power–mass (P1.4GHz–M500) and radio emissivity–mass diagrams to study correlations and scatter.
• Used a combined sample including data from Cassano et al. (2013) and Martinez Aviles et al. (2016) to improve constraints on the radio power–mass slope.
• Measured the occurrence of radio halos in two mass bins: M < 8 × 10^14 M⊙ and M ≥ 8 × 10^14 M⊙.
• Compared observed radio halo fractions with predictions from the Cassano & Brunetti (2005) turbulent re-acceleration model.

Experimental results

Research questions

• RQ1 What is the dependence of radio halo luminosity on the mass of the host cluster, and how does the scatter in this relation relate to cluster dynamics?
• RQ2 How does the occurrence rate of radio halos vary with cluster mass, and does it align with predictions from turbulent re-acceleration models?
• RQ3 To what extent do merging events drive the formation of radio halos, and what fraction of merging clusters host such halos?
• RQ4 Is there a bimodal distribution in radio emissivity, and what does it imply about the physical conditions in clusters with and without halos?
• RQ5 Can the observed radio halo properties be explained by the turbulent re-acceleration mechanism, especially in low-mass systems?

Key findings

• More than 90% of radio halos are found in merging galaxy clusters, with only 10% in relaxed systems.
• The radio power of halos correlates with cluster mass, but the correlation exhibits significant scatter, which is strongly influenced by the dynamical state of the cluster.
• Clusters without radio halos have upper limits to their diffuse emission that lie below the radio power–mass correlation, indicating they are not capable of sustaining detectable radio halos.
• Radio emissivity shows a clear bimodal distribution, with radio halos having emissivities at least five times higher than relaxed clusters.
• The fraction of clusters hosting radio halos drops from ~70% in high-mass systems (M ≥ 8 × 10^14 M⊙) to ~35% in low-mass systems (M < 8 × 10^14 M⊙), consistent with turbulent re-acceleration models.
• The observed dependence of radio halo occurrence on mass and merger activity is in good agreement with theoretical expectations from the turbulent re-acceleration model, suggesting that low-mass mergers may not generate sufficient turbulence for GHz-frequency radio emission.