摘要：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 ×
1014M⊙ 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, ξ
(ξ =
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 \hbox{$\xi\gtsim\xi_{\rm min}$}) in our sample, under the assumption of a timescale
τm 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 (ξmin,
τm) plane. We find that under the
assumption of τm
~ 2−3 Gy, as constrained by simulations, the observed merger
fraction matches the theoretical value for ξmin ~ 0.1−0.18. This is consistent
with optical and near-infrared (IR) observations of galaxy clusters in the sample that
constrain ξmin ≃
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
ξmin ~
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 different cosmic epochs and for
different cluster masses.
