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 \(\tau^+ \tau^-\), and mass of 43 GeV and composition \(b \bar b\) 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. abridged
We study the complex structure of the Bullet cluster radio halo to determine the Dark Matter (DM) contribution to the emission observed in the different subhalos corresponding to the DM and baryonic ...dominated regions. We use different non-thermal models to study the different regions, and we compare our results with the available observations in the radio, X-ray and gamma-ray bands, and the Sunyaev-Zel'dovich (SZ) effect data. We find that the radio emission coming from the main DM subhalo can be produced by secondary electrons produced by DM annihilations. In this scenario there are however some open issues, like the difficulty to explain the observed flux at 8.8 GHz, the high value of the required annihilation cross section, and the lack of observed emission coming from the minor DM subhalo. We also find that part of the radio emission originated by DM annihilation could be associated with a slightly extended radio source present near the main DM subhalo. Regarding the baryonic subhalos, the radio measurements do not allow to discriminate between a primary or secondary origin of the electrons, while the SZ effect data point towards a primary origin for the non-thermal electrons in the Main Subcluster. We conclude that in order to better constrain the properties of the DM subhalos, it is important to perform detailed measurements of the radio emission in the regions where the DM halos have their peaks, and that the separation of the complex radio halo in different subhalos is a promising technique to understand the properties of each specific subhalo.
The fundamental properties of the photon have deep impact on the astrophysical processes that involve it, like the inverse Compton scattering of CMB photon by energetic electrons residing within ...galaxy cluster atmospheres, usually referred to as the Sunyaev-Zel'dovich effect (SZE). We calculate the combined constraints on the photon decay time and mass by studying the impact of the modified CMB spectrum, as recently calculated (Heeck 2013), on the SZE of galaxy clusters. We analyze the modifications of the SZE as produced by photon decay effects. We study in details the frequency regimes where these modifications are large and where the constraints derived from the SZE can be stronger with respect to those already obtained from the CMB spectrum. We show that the SZE can set limits on the photon decay time and mass, or on \(E^* = \frac{t_0}{\tau_\gamma}m_\gamma c^2\), that are stronger than those obtained from the CMB: the main constraints come from the low frequency range \(\nu \approx 10-50\) GHz where the modified SZE \(\Delta I_{mod}\) is larger than the standard one \(\Delta I\), with the difference \(|(\Delta I_{mod} - \Delta I)|\) increasing with the frequency for increasing values of \(E^*\); additional constraints can be set in the range \(120 - 180\) GHz where there is an increase of the frequency position of the minimum of \(\Delta I_{mod}\) with respect to the standard one with increasing values of \(E^*\). We demonstrated that the effect of photon decay can be measured or constrained by the Square Kilometer Array in the optimal range \(\approx 10-30\) GHz setting limits of \(E^* \leq 1.4 \times 10^{-9}\) eV and \(5 \times 10^{-10}\) eV for 30 and 260 hour integration for A2163, respectively. These limits are stronger than those obtained with the COBE-FIRAS spectral measurements of the CMB.
A520 is a hot and luminous galaxy cluster, where gravitational lensing and X-ray measures reveal a different spatial distribution of baryonic and Dark Matter. This cluster hosts a radio halo, whose ...map shows a separation between the North-East and the South-West part of the cluster, similarly to what is observed in gravitational lensing maps. In this paper we study the possibility that the diffuse radio emission in this cluster is produced by Dark Matter annihilation. We find that in the whole cluster the radio emission should be dominated by baryonic phenomena; if a contribution from Dark Matter is present, it should be searched in a region in the NE part of the cluster, where a peak of the radio emission is located close to a Dark Matter sub-halo, in a region where the X-ray emission is not very strong. By estimating the radio spectrum integrated in this region using data from publicly available surveys, we find that this spectrum can be reproduced by a Dark Matter model for a neutralino with mass 43 GeV and annihilation final state \(b \bar b\) for a magnetic field of 5 \(\mu\)G.
We present deep total intensity and polarization observations of the Coma cluster at 1.4 and 6.6 GHz performed with the Sardinia Radio Telescope. By combining the single-dish 1.4 GHz data with ...archival Very Large Array observations we obtain new images of the central radio halo and of the peripheral radio relic where we properly recover the brightness from the large scale structures. At 6.6 GHz we detect both the relic and the central part of the halo in total intensity and polarization. These are the highest frequency images available to date for these radio sources in this galaxy cluster. In the halo, we find a localized spot of polarized signal, with fractional polarization of about 45%. The polarized emission possibly extends along the north-east side of the diffuse emission. The relic is highly polarized, up to 55%, as usually found for these sources. We confirm the halo spectrum is curved, in agreement with previous single-dish results. The spectral index is alpha=1.48 +/- 0.07 at a reference frequency of 1 GHz and varies from alpha ~1.1, at 0.1 GHz, up to alpha ~ 1.8, at 10 GHz. We compare the Coma radio halo surface brightness profile at 1.4 GHz (central brightness and e-folding radius) with the same properties of the other halos, and we find that it has one of the lowest emissivities observed so far. Reanalyzing the relic's spectrum in the light of the new data, we obtain a refined radio Mach number of M=2.9 +/- 0.1.
We discuss a new technique to constrain models for the origin of radio relics in galaxy clusters using the correlation between the shock Mach number and the radio power of relics. This analysis is ...carried out using a sample of relics with information on both the Mach numbers derived from X-ray observation, \(\mathcal{M}_X\), and using spectral information from radio observations of the peak and the average values of the spectral index along the relic, \(\mathcal{M}_R\). We find that there is a lack of correlation between \(\mathcal{M}_X\) and \(\mathcal{M}_R\); this result is an indication that the spectral index of the relic is likely not due to the acceleration of particles operated by the shock but it is related to the properties of a fossil electrons population. We also find that the available data on the correlation between the radio power \(P_{1.4}\) and Mach numbers (\(\mathcal{M}_R\) and \(\mathcal{M}_X\)) in relics indicate that neither the DSA nor the adiabatic compression can simply reproduce the observed \(P_{1.4}-\mathcal{M}\) correlations. Furthermore, we find that the radio power is not correlated with \(\mathcal{M}_X\), whereas it is not possible to exclude a correlation with \(\mathcal{M}_R\). This also indicates that the relic power is mainly determined by the properties of a fossil electron population rather than by the properties of the shock. Our results require either to consider models of shock (re)acceleration that go beyond the proposed scenarios of DSA and adiabatic compression at shocks, or to reconsider the origin of radio relics in terms of other physical scenarios.
Giant radio galaxy (GRG) lobes are excellent laboratories to study the evolution of the particle and B-field energetics. However, these results are based on assumptions of the shape and extension of ...the GRG lobe electron spectrum. We re-examine the energetics of GRG lobes as derived by inverse Compton scattering of CMB photons (ICS-CMB) by relativistic electrons in RG lobes to assess the physical conditions of RG lobes, their energetics and their radiation regime. We consider the GRG DA 240 recently observed by Suzaku as a reference case and we also discuss other RG lobes observed with Chandra and XMM. We model the spectral energy distribution of the DA 240 East lobe to get constraint on the shape and the extension of the electron spectrum in the lobe by using multi-frequency information from radio to gamma-rays. We use radio and X-ray data to constrain the shape and normalization of the electron spectrum and we then calculate the SZ effect expected in GRG lobes that is sensitive to the total electron energy density. We show that the electron energy density U_e derived form X-ray observations yields only a rough lower limit to its actual value and that most of the estimates of U_e based on X-ray measurements have to be increased even by a large factor by considering realistic estimates of the lower electron momentum p_1. This brings RG lobes away from the equipartition condition towards a particle-dominated and Compton power dominance regime. We use the distribution of RG lobes in the U_e/U_B vs. U_e/U_CMB plane as a further divide between different physical regimes of particle and field dominance, and radiation mechanism dominance in RG lobes. We conclude that the SZ effect produced by ICS-CMB mechanism observable in RG lobes provides reliable estimate of p_1 and U_e and is the best tool to determine the total energy density of RG lobes and to assess their physical regime.
Radio synchrotron emission is expected as a natural by-product of the self-annihilation of super-symmetric dark matter particles. In this work we discuss the general properties of the radio emission ...expected in a wide range of dark matter halos, from local dwarf spheroidal galaxies to large and distant galaxy clusters with the aim to determine the neutralino dark matter detection prospects of the Square Kilometre Array (SKA). The analysis of the SKA detection of dark matter(DM)-induced radio emission is presented for structures spanning a wide range of masses and redshifts, and we also analyze the limits that the SKA can set on the thermally averaged neutralino annihilation cross-section in the event of non-detection. To this aim, we construct a model of the redshift evolution of the radio emissions of dark matter halos and apply it to generate predicted fluxes from a range of neutralino masses and annihilation channels for the dark matter halos surrounding dwarf galaxies, galaxies and galaxy clusters. Using the available SKA performance predictions and its ability to determine an independent measure of the magnetic field in cosmic structures, we explore both the detailed detection prospects and the upper-bounds that might be placed on the neutralino annihilation cross-section in the event of non-detection. We find that the SKA can access a neutralino parameter space far larger than that of any preceding indirect-detection experiment, also improving on the realistic CTA detection prospects, with the possibility of setting cross-section upper-bounds up to four orders of magnitude below the thermal relic density bound. Additionally, we find that neutralino radio emissions carry redshift-independent signatures of the dominant annihilation channel and of neutralino mass, offering therefore a means to identify such non-thermal emissions within the observing frequency range of the SKA.
We study here an alternative technique to probe the Dark Ages (DA) and the Epoch of Reonization (EoR) that makes use of the Comptonization of the CMB spectrum modified by physical effects occurring ...during this epoch related to the emergence of the 21-cm radiation background. Inverse Compton scattering of 21-cm photon background by thermal and non-thermal electrons residing in the atmospheres of cosmic structures like galaxy clusters, radiogalaxy lobes and galaxy halos, produces a specific form of Sunyaev-Zel'dovich effect (SZE) that we refer to as SZE-21cm. We derive the SZE-21cm in a general relativistic approach which is required to describe the correct spectral features of this astrophysical effect. We calculate the spectral features of the thermal and non-thermal SZE-21cm in galaxy clusters and in radiogalaxy lobes, and their dependence on the history of physical mechanisms occurring during the DA and EoR. We study how the spectral shape of the SZE-21cm can be used to establish the global features in the mean 21-cm spectrum generated during and prior to the EoR, and how it depends on the properties of the (thermal and non-thermal) plasma in cosmic structures. We find that the thermal and non-thermal SZE-21cm have peculiar spectral shapes that allow to investigate the physics and history of the EoR and DA. Its spectrum depends on the gas temperature (for the thermal SZE-21cm) and on the electrons minimum momentum (for the non-thermal SZE-21cm). The global SZE-21cm signal can be detected (in \(\sim 1000\) hrs) by SKA1-low in the frequency range \(\nu \simgt 75-90\) MHz, for clusters in the temperature range 5 to 20 keV, and the difference between the SZE-21cm and the standard SZE can be detected by SKA1 or SKA2 at frequencies depending on the background model and the cluster temperature. abridged
Abridged Inverse Compton scattering of CMB fluctuations off cosmic electron plasma generates a polarization of the associated Sunyaev-Zel'dovich (SZ) effect. This signal has been studied so far ...mostly in the non-relativistic regime and for a thermal electron population and, as such, has limited astrophysical applications. Partial attempts to extend this calculation for a thermal electron plasma in the relativistic regime have been done but cannot be applied to a general relativistic electron distribution. Here we derive a general form of the SZ effect polarization valid in the full relativistic approach for both thermal and non-thermal electron plasmas, as well as for a generic combination of various electron population co-spatially distributed in the environments of galaxy clusters or radiogalaxy lobes. We derive the spectral shape of the Stokes parameters induced by the IC scattering of every CMB multipole, focusing on the CMB quadrupole and octupole that provide the largest detectable signals in galaxy clusters. We found that the CMB quadrupole induced Stoke parameter Q is always positive with a maximum amplitude at 216 GHz which increases slightly with increasing cluster temperature. The CMB octupole induced Q spectrum shows, instead, a cross-over frequency which depends on the cluster electron temperature, or on the minimum momentum p_1 as well as on the power-law spectral index of a non-thermal electron population. We discuss some possibilities to disentangle the quadrupole-induced Q spectrum from the octupole-induced one which allow to measure these quantities through the SZ effect polarization. We finally apply our model to the realistic case of the Bullet cluster and derive the visibility windows of the total, quandrupole-induced and octupole-induced Stoke parameter Q in the frequency ranges accessible to SKA, ALMA, MILLIMETRON and CORE++ experiments.
