Neutralinos and the Origin of Radio Halos in Clusters of Galaxies

Abstract

We assume that the supersymmetric lightest neutralino is a good candidate for the cold dark matter in the galaxy halo and explore the possibility to produce extended di †use radio emission from high-energy electrons arising from the neutralino annihilation in galaxy clusters whose intracluster medium is Ðlled with a large-scale magnetic Ðeld. We show that these electrons Ðt the population of seed relativistic electrons that is postulated in many models for the origin of cluster radio halos. For a uniform magnetic Ðeld of B1È3 kG the population of seed relativistic electrons from neutralino annihilation can Ðt the radio halo spectra of two well-studied clusters : Coma and 1E 0657[56. In the case of a magnetic Ðeld that is radially decreasing from the cluster center, central values B8 kG (for Coma) and B50 kG (for 1E 0657[56) are required to Ðt the data. The radio halo data strongly favor a centrally peaked dark matter density proÐle (like a Navarro, Frenk, & White [NFW97] density proÐle). The shape and the frequency extension of the radio halo spectra are connected with the mass and physical composition of the neutral-ino. A pure gaugino neutralino with mass GeV can reasonably Ðt the radio halo spectra of M s º 80 both Coma and 1E 0657[56. The model we present here provides a number of extra predictions that make it deÐnitely testable. On the one hand, it agrees quite well with the observations that (1) the radio halo is centered on the cluster dynamical center, usually coincident with the center of its X-ray emission ; (2) the radio halo surface brightness is similar to the X-ray one ; and (3) the monochromatic radio lumi-nosity at 1.4 GHz correlates strongly with the intracluster (IC) gas temperature. On the other hand, the same model predicts that radio halos should be present in every cluster, which is not presently observed, although the predicted radio halo luminosities can change (decrease) by factors of up to D102È106, depending on the amplitude and the structure of the IC magnetic Ðeld. In addition, neutral pions arising from neutralino annihilation should give rise to substantial amounts of di †use gamma-ray emission, up to energies of order that could be tested by the next-generation gamma-ray experiments.