Statistical Properties of Radio Halos in Galaxy Clusters and Origin of Seed Electrons for Reacceleration

TL;DR

This study compares primary and secondary seed electron origins for radio halos in galaxy clusters, analyzing their statistical properties, lifetimes, and merger conditions to understand their formation and predict detection rates.

Contribution

It introduces a combined method analyzing electron evolution and cluster merger history to distinguish seed electron origins and predicts radio halo counts for upcoming surveys.

Findings

Secondary seed electrons can produce long-lived radio halos from major mergers.

Primary seed electrons require frequent minor mergers for short-lived halos.

Predicted ~1000 radio halos detectable by ASKAP survey.

Abstract

One of the most promising mechanisms for producing radio halos (RHs) in galaxy clusters is the reacceleration of cosmic-ray electrons by turbulences. However, the origin of the seed electrons for the reacceleration is still poorly constrained. In the secondary scenario, most of the seed electrons are injected via collision of proton cosmic-rays, while non-thermal electrons are directly injected in the primary scenario. In this paper, we examine the two scenarios for the seed electrons with the observed statistical properties of RHs, combining two methods: following the temporal evolutions of the electron energy and radial distributions in a cluster, and the merger history of clusters. We find that the RH lifetime largely depends on the seed origin, as it could be longer than the cosmological timescale in the secondary scenario. We study the condition for the onset of RHs with the observed RH fraction and the RH lifetime we obtained, and find that long-lived RHs in the secondary scenario should be originated from major mergers with a mass ratio of $\xi\sim0.1$, while the short lifetime in the primary scenario requires more frequent onsets by minor mergers with $\xi\sim0.01$. Our simple model of the turbulence acceleration can reproduce the observed radio luminosity-mass relation. The RH luminosity functions we obtained suggest that the expected RH number count with the ASKAP survey will detect $\approx10^3$ RHs in both the scenarios.