Can giant radio halos probe the merging rate of galaxy clusters?
Jun 23, 2016
11 pages
Published in:
- Astron.Astrophys. 593 (2016) A81
- Published: Sep 26, 2016
e-Print:
- 1606.07293 [astro-ph.CO]
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Abstract: (EDP Sciences)
Observations of galaxy clusters both in the radio and X-ray bands probe a direct linkbetween cluster mergers and giant radio halos, suggesting that these sources can be usedas probes of the cluster merging rate with cosmic time. While all giant radio halos arefound in merging clusters, not every merging cluster hosts a giant radio halo. In thispaper we carry out an explorative study that combines the observed fractions of mergingclusters and radio halos with the merging rate predicted by cosmological simulations, andwe attempt to infer constraints on merger properties of clusters that appear disturbed inX-rays and clusters that host radio halos. We used classical morphological parameters toidentify merging systems and analysed the largest current (mass-selected M_500 ≳ 6 x10^14M_⊙ and 0.2 ) sample ofgalaxy clusters with radio and X-ray data; we extracted this sample from thePlanck Sunyaev-Zeldovich cluster catalogue. We found that the fractionof merging clusters in this sample is f_m ~ 62-67%, while that of clusterswith radio halos is f_RH ~ 44-51%. We assume that themorphological disturbance measured in the X-rays is driven by the merger with the largestmass ratio, xi(xi =M_i/M_1, where M_i andM_1 are the progenitor masses), which isstill ongoing in the cluster at the epoch of observation. Results from theoretical studiesallow us to derive the fraction of mergers with mass ratio above a minimum threshold(those with [??] ) in our sample, under the assumption of a timescaletau_m for the duration of merger-induceddisturbance. The comparison of the theoretical merger fraction with the observed mergerfraction allows us to constrain a region in the (xi_min,tau_m) plane. We find that under theassumption of tau_m~ 2-3 Gy, as constrained by simulations, the observed mergerfraction matches the theoretical value for xi_min ~ 0.1-0.18. This is consistentwith optical and near-infrared (IR) observations of galaxy clusters in the sample thatconstrain xi_min ≃0.14-0.16 through weak lensing analysis or study of the velocitydistribution of galaxies in the clusters. The fact that radio halos are only found in afraction of merging galaxy clusters may suggest that merger events generating radio halosare characterized by larger mass ratios; this seems to be supported by optical/near-IRobservations of radio halo clusters in the sample that indeed allow us to constrainxi_min ~0.2-0.25. Alternatively, radio halos may be generated in all mergersbut their lifetime is shorter (by ~f_RH/f_m) thanthe timescale of the merger-induced disturbance. We stress that this is an explorativestudy, however it suggests that follow-up studies using the forthcoming radio surveys andadequate numerical simulations have the potential to derive quantitative constraints onthe link between cluster merging rate and radio halos at different cosmic epochs and fordifferent cluster masses.Note:
- 10 pages, 3 figures, accepted for publication in A&A
- galaxies: clusters: intracluster medium
- cosmology: theory
- radio continuum: general
- X-rays: galaxies: clusters
References(91)
Figures(3)