The role of Dark Matter sub-halos in the non-thermal emission of galaxy clusters

Abstract

Abstract. Annihilation of Dark Matter (DM) particles has been recognized as one of the possible mechanisms for the production of non-thermal particles and radiation in galaxy clusters. Previous studies have shown that, while DM models can reproduce the spectral properties of the radio halo in the Coma cluster, they fail in reproducing the shape of the radio halo surface brightness because they produce a shape that is too concentrated towards the center of the cluster with respect to the observed one. However, in previous studies the DM distribution was modeled as a single spherically symmetric halo, while the DM distribution in Coma is found to have a complex and elongated shape. In this work we calculate a range of non-thermal emissions in the Coma cluster by using the observed distribution of DM sub-halos. We find that, by including the observed sub-halos in the DM model, we obtain a radio surface brightness with a shape similar to the observed one, and that the sub-halos boost the radio emission by a factor between 5 and 20%, thus allowing to reduce the gap between the annihilation cross section required to reproduce the radio halo flux and the upper limits derived from other observations, and that this gap can be explained by realistic values of the boosting factor due to smaller substructures. Models with neutralino mass of 9 GeV and composition τ + τ − , and mass of 43 GeV and composition bb can fit the radio halo spectrum using the observed properties of the magnetic field in Coma, and do not predict a gamma-ray emission in excess compared to the recent Fermi-LAT upper limits. These findings make these DM models viable candidate to explain the origin of radio halos in galaxy clusters, avoiding the problems connected to the excessive gamma-ray emission expected from proton acceleration in most of the currently proposed models, where the acceleration of particles is directly or indirectly connected to events related to clusters merging. Therefore, DM models deserve to be better studied both from the theoretical and observational sides; the best spectral bands where it is possible to obtain better information are the radio and the gamma-ray bands, while we do not expect a strong emission in the X-ray band.

Studying the dark matter annihilation in galaxy clusters through radio and gravitational lensing measures

“…The required values of the normalization of the DM model for the 9 and 43 GeV cases are B × σ v = 6 × 10 −25 cm 3 s −1 and 4 × 10 −24 cm 3 s −1 respectively. Comparing these numbers with the best fit values for the annihilation cross section found in the Galactic center [27], we found that the required value of the substructures boosting factor is of the order of 30-50 for the two models with 9 and 43 GeV mass, that is reasonable for galaxy clusters [3,29,7]. If instead the values of the cross section are fixed to the upper limits found in Dwarf Galaxies studies with Fermi-LAT [30] or in CMB studies with Planck [31], we found that the boosting factor should take values of the order of 150-300, that are more difficult but not impossible to have in galaxy clusters [32,7].…”

“…where i is the index referring to the output product (i.e. electrons/positrons or photons), σ v is the thermally-averaged WIMP annihilation cross-section, the index f labels annihilation final states with branching ratios B f , the production spectra (dN f i )/(dE) can be calculated using the DarkSusy package [11], and B is a multiplicative boosting factor produced by the effect of smaller DM substructures [12,7]. In the following we will use the neutralino as a candidate WIMP, but the results can be valid for a generic WIMP having similar properties.…”

“…In this paper, we report the results obtained in the cases of the Bullet cluster [6] and the Coma cluster [7], discuss and compare the results obtained in the two clusters, and discuss new possible applications of this technique.…”

Fitting dark matter mass with the radio continuum spectral data of the Ophiuchus cluster

“…In the above analysis, we did not include any substructure contributions. Simulations show that some substructures exist in dark matter profile which can boost the annihilation signals [35,36,37]. However, no observational evidence of these substructures is found.…”

Ruling out dark matter interpretation of the galactic GeV excess by gamma-ray data of galaxy clusters

“…Therefore, the parametrization can be applied and the corresponding boost factors are 35 and 30 respectively. As discussed in 32 , the boost factor of the order are standard values for galaxy clusters. By taking these boost factors, we can calculate the direct emission flux for each galaxy cluster by , where D is the distance of the galaxy cluster.…”