Possibility of a Dark Matter Interpretation for the Excess in Isotropic Radio Emission Reported by ARCADE

The ARCADE 2 Collaboration has recently measured an isotropic radio emission which is significantly brighter than the expected contributions from known extra-galactic sources. The simplest explanation of such excess involves a “new” population of unresolved sources which become the most numerous at very low (observationally unreached) brightness. We investigate this scenario in terms of synchrotron radiation induced by weakly interacting massive particle (WIMP) annihilations or decays in extra-galactic halos. Intriguingly, for light-mass WIMPs with a thermal annihilation cross section, the level of expected radio emission matches the ARCADE observations.

Figure 1

Extra-galactic radio background as derived by ARCADE [5], together with three possible interpretations of the low-frequency ($<10\mathrm{GHz}$) excess in terms of WIMP annihilations (blue, green, and orange curves, see text for details). The astrophysical source contribution estimated from number counts (red line), the CMB contribution (black dotted line), and a best-fit power law of the excess (black dashed line) are also reported [6].

Figure 2

X-ray and gamma-ray fluxes for the three benchmark WIMP cases shown in Fig. 1. The CHANDRA [32] bound in the x-ray band and the COMPTEL [33] and FERMI [34] extra-galactic gamma-ray fluxes are shown.

Figure 3

Differential number counts for AGNs (dashed line), star-forming galaxies (dotted line), and the same 10 GeV benchmark DM model (solid lines) shown in Figs. 1, 2. For DM, we consider a case such that all substructures are resolved, and an opposite case where all substructures are unresolved. For data and astrophysical models, see 11 and references therein.