Dark matter interpretation of the origin of non-thermal phenomena in galaxy clusters

Abstract

Aims. We studied the multi-frequency predictions of various annihilating dark matter (DM) scenarios in order to explore the possibility to interpret the still unknown origin of non-thermal phenomena in galaxy clusters. Methods. We consider three different DM models with light (9 GeV), intermediate (60 GeV), and high (500 GeV) neutralino mass and study their physical effects in the atmosphere of the Coma cluster. The secondary particles created in the neutralino annihilation processes produce a multi-frequency spectral energy distribution (SED) of non-thermal radiation and also heat the intracluster gas, which we test against the observations available for the Coma cluster from radio to gamma-rays. The various DM-produced SEDs are normalized by the condition to fit the Coma radio halo spectrum, thus obtaining best-fit values of the annihilation cross-section σV and of the central magnetic field B 0 . Results. We find that it is not possible to interpret all the non-thermal phenomena observed in galaxy clusters in terms of DM annihilation. The light-mass DM model with 9 GeV mass produces too little power at all other frequencies, while the high-mass DM model with 500 GeV produces a large excess power at all other frequencies. The intermediate-mass DM model with 60 GeV and τ ± composition is marginally consistent with the HXR and gamma-ray observations, but narrowly fails to reproduce the EUV and soft X-ray observations. The intermediate-mass DM model with 60 GeV and bb composition is, on the other hand, always below the observed fluxes. We note that the radio halo spectrum of Coma is well fitted only in the bb or light-and intermediate-mass DM models. We also find that the heating produced by the DM annihilation in the centre of the Coma cluster is always larger than the intracluster gas cooling rate for an NFW DM density profile and it is substantially smaller than the cooling rate only for a cored DM density profile in light-mass DM model with 9 GeV. Conclusions. The possibility of interpreting the origin of non-thermal phenomena in galaxy clusters with DM annihilation scenarios requires a low neutralino mass and a cored DM density profile. If we then consider the multi-messenger constraints to the neutralino annihilation cross-section, it turns out that this scenario would also be excluded unless we introduce a substantial boost factor that represents DM substructures. If we relax the condition to fit the Coma radio halo and we consider the EUV and HXR detections as upper limits for the non-thermal emission, together with the gamma-ray limit, then the limits on σV are less stringent than those obtained by the multi-messenger analysis.

Is the radio emission in the Bullet cluster due to dark matter annihilation?

“…In this calculation, we assume different masses and compositions for the neutralino by using the DarkSusy package (Gondolo et al 2004). We assume here a boosting factor due to the subhalos DM distribution (Pieri et al 2011) equal to 1, and larger values can be considered eventually simply by multiplying the result by this factor (see, e.g., Colafrancesco et al 2011b).…”

“…Synchrotron emission from electrons either primarily accelerated at shocks (Tribble 1993) or re-accelerated by turbulence (Brunetti et al 2001), or secondarily produced by proton-proton collisions (Blasi & Colafrancesco 1999) or DM annihilation (Colafrancesco & Mele 2001, Colafrancesco et al 2006, 2011b) is expected to be observed in the radio band. The same primary and/or secondary electrons are also expected to emit by inverse Compton Scattering (ICS) off the Cosmic Microwave Background (CMB) photons (and possibly other cosmic backgrounds), and by non-thermal bremsstrahlung off the nuclei of the thermal gas over a wide energy range from ultraviolet to gamma-rays (e.g.…”

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

“…Previous studies of the synchrotron radio emission originating from DMproduced electrons in galaxy clusters [1,2,3,4] have shown that these models can naturally reproduce the observed steepening of the integrated flux spectrum of the Coma cluster at high frequencies (ν ∼ > 1.4 GHz) for realistic values of the magnetic field (i.e., B ∼ 0.1 − 5 µG) in the case of light neutralino DM particles (with mass M χ ∼ 10 − 40 GeV), but they cannot reproduce the observed shape of the radio surface brightness, unless one assumes a magnetic field increasing with radius, that is however excluded by recent observations [5]. Another argument against the simple DM interpretation of the origin of non-thermal electrons in galaxy clusters is given by the recent limits on the DM annihilation cross section obtained with Fermi-LAT through measures in galaxy clusters [6], in dwarf galaxies [7], and in the Galactic center [8], as well as with Planck from the CMB analysis [9]; these results point, in fact, to values of the annihilation cross section upper limits of the order of σv ∼ 10 −27 − 10 −26 cm 3 s −1 , whereas the values required to fit the radio halo flux are of the order of 10 −25 cm 3 s −1 or higher (see, e.g., [3]). This gap is smaller if the results of another interpretation of the gamma-ray excess measured by Fermi-LAT in the Galactic center [10] are considered: in this last work, best fit values of the annihilation cross section of 2.2 × 10 −26 cm 3 s −1 and 7.4 × 10 −26 cm 3 s −1 for neutralino mass of 9 GeV (with τ + τ − composition) and 43 GeV (with bb composition) have been found, respectively.…”

Multi-wavelength emissions from dark matter annihilation processes in galaxy clusters using cosmological simulations

“…We compute the expected flux for synchrotron emission processes from DM annihilation by dividing a cluster of dimension 2 Mpc into cubes of size 10 kpc. We based our studies of radio emission from DM annihilation processes by taking neutralinos to be primarily and dominantly composed of bb species and/or light and intermediate DM mass models as in [10]. We consider neutralino masses M χ of 35 and 60 GeV, and we take the velocity averaged annihilation cross-section, < σV >, of 1.0 × 10 −26 cm 3 s −1 from Fermi upper limit.…”