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

Abstract

One of the most promising mechanisms for producing radio halos (RHs) in galaxy clusters is the reacceleration of cosmic-ray electrons by turbulence. However, the origin of the seed electrons for reacceleration is still poorly constrained. In the secondary scenario, most of the seed electrons are injected via collision of proton cosmic-rays, while nonthermal electrons are directly injected in the primary scenario. In this paper, we examine the two scenarios for seed electrons with the observed statistical properties of RHs by combining two methods: by following the temporal evolutions of the electron energy and the radial distributions in a cluster, as well as 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 originate from major mergers with a mass ratio of ξ ∼ 0.1, while the short lifetime in the primary scenario requires more frequent onsets from minor mergers with ξ ∼ 0.01. 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 ≈103 RHs in both scenarios.