Can giant radio halos probe the merging rate of galaxy clusters?

Abstract

Observations of galaxy clusters both in the radio and X-ray bands probe a direct link between cluster mergers and giant radio halos, suggesting that these sources can be used as probes of the cluster merging rate with cosmic time. While all giant radio halos are found in merging clusters, not every merging cluster hosts a giant radio halo. In this paper we carry out an explorative study that combines the observed fractions of merging clusters and radio halos with the merging rate predicted by cosmological simulations, and we attempt to infer constraints on merger properties of clusters that appear disturbed in X-rays and clusters that host radio halos.We used classical morphological parameters to identify merging systems and analysed the largest current (mass-selected M500× 6×1014 M and 0:2 ≤ z ≤ 0:33) sample of galaxy clusters with radio and X-ray data; we extracted this sample from the Planck Sunyaev-Zeldovich cluster catalogue.We found that the fraction of merging clusters in this sample is fm 62-67%, while that of clusters with radio halos is fRH∼ 44-51%.We assume that the morphological disturbance measured in the X-rays is driven by the merger with the largest mass ratio, ϵ(ϵ= Mi=M1< 1, where Mi and M1 are the progenitor masses), which is still ongoing in the cluster at the epoch of observation. Results from theoretical studies allow us to derive the fraction of mergers with mass ratio above a minimum threshold (those with ϵmin) in our sample, under the assumption of a timescale Tm for the duration of merger-induced disturbance. The comparison of the theoretical merger fraction with the observed merger fraction allows us to constrain a region in the (ϵmin,Tm) plane. We find that under the assumption of Tm∼ 2-3 Gy, as constrained by simulations, the observed merger fraction matches the theoretical value for ϵmin∼ 0:1-0:18. This is consistent with optical and near-infrared (IR) observations of galaxy clusters in the sample that constrain ϵmin' 0:14-0:16 through weak lensing analysis or study of the velocity distribution of galaxies in the clusters. The fact that radio halos are only found in a fraction of merging galaxy clusters may suggest that merger events generating radio halos are characterized by larger mass ratios; this seems to be supported by optical/near-IR observations of radio halo clusters in the sample that indeed allow us to constrain ϵmin∼0:2-0:25. Alternatively, radio halos may be generated in all mergers but their lifetime is shorter (by fRH=fm) than the timescale of the merger-induced disturbance. We stress that this is an explorative study, however it suggests that follow-up studies using the forthcoming radio surveys and adequate numerical simulations have the potential to derive quantitative constraints on the link between cluster merging rate and radio halos at di erent cosmic epochs and for different cluster masses.