The isotropic radio background and annihilating dark matter

Dan Hooper, Alexander V. Belikov, Tesla E. Jeltema, Tim Linden, Stefano Profumo, and Tracy R. Slatyer

Phys. Rev. D 86, 103003 – Published 5 November 2012

Abstract

Observations by the Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission (ARCADE-2) and other telescopes sensitive to low frequency radiation have revealed the presence of an isotropic radio background with a hard spectral index. The intensity of this observed background is found to exceed the flux predicted from astrophysical sources by a factor of approximately 5–6. In this article, we consider the possibility that annihilating dark matter particles provide the primary contribution to the observed isotropic radio background through the emission of synchrotron radiation from electron and positron annihilation products. For reasonable estimates of the magnetic fields present in clusters and galaxies, we find that dark matter could potentially account for the observed radio excess, but only if it annihilates mostly to electrons and/or muons, and only if it possesses a mass in the range of approximately 5–50 GeV. For such models, the annihilation cross section required to normalize the synchrotron signal to the observed excess is $σ v \approx (0.4 - 30) \times 10^{- 26} {cm}^{3} / s$ , similar to the value predicted for a simple thermal relic ( $σ v \approx 3 \times 10^{- 26} {cm}^{3} / s$ ). We find that in any scenario in which dark matter annihilations are responsible for the observed excess radio emission, a significant fraction of the isotropic gamma-ray background observed by Fermi must result from dark matter as well.

Received 15 August 2012

DOI:https://doi.org/10.1103/PhysRevD.86.103003

Authors & Affiliations

Dan Hooper^1,2, Alexander V. Belikov³, Tesla E. Jeltema⁴, Tim Linden⁴, Stefano Profumo⁴, and Tracy R. Slatyer⁵

¹Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
²Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA
³Institut d’Astrophysique de Paris, Unité Mixte de Recherche 7095, CNRS, Université Pierre et Marie Curie, Paris 06, 98 bis boulevard Arago, 75014 Paris, France
⁴Department of Physics and Santa Cruz Institute for Particle Physics, University of California, 1156 High Street, Santa Cruz, California 95064, USA
⁵School of Natural Sciences, Institute for Advanced Study, Princeton, New Jersey 08540, USA

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 86, Iss. 10 — 15 November 2012

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Measurements of the isotropic radio background, as reported by Roger et al. (0.022 GHz) [3], Guzman et al. (0.045 GHz) [4], Haslam et al. (0.408 GHz) [5], Reich and Reich (1.42 GHz) [6], and by the ARCADE-2 Collaboration (3.20 to 90 GHz) [1] (see also, Ref. 2). In the left frame, the total observed background is shown, including the contribution of the CMB (shown as a dotted line). In the middle frame, the CMB has been subtracted, revealing a power-law spectrum of isotropic emission. In the right frame, an estimate of the contribution from unresolved radio sources has been subtracted as well. See text for more details.Reuse & Permissions
Figure 2
The fraction of energy in electron/positron dark matter annihilation products which is lost to synchrotron emission (left) and the power-averaged magnetic field strength which contributes to that synchrotron emission (right). In the upper and lower frames, we show results for our models for the largest dark matter halos ( $M > 10^{13} M_{⊙}$ ) and somewhat smaller halos ( $10^{10} - 10^{13} M_{⊙}$ ), respectively. In each frame, the solid lines denote the results for our default magnetic field and dark matter distribution models. The dashed lines represent the range of values found using the variety of models described in Sec. 4. In each case shown, we use the Via Lactea substructure model.Reuse & Permissions
Figure 3
The contribution to the isotropic radio background from annihilating dark matter particles with masses of 10 or 100 GeV, and annihilating to $b \bar{b}$ , $τ^{+} τ^{-}$ , $μ^{+} μ^{-}$ , or $e^{+} e^{-}$ . In each case, we have used our default magnetic field models and dark matter distributions, and substructure as favored by the Via Lactea simulation (see Sec. 2). In each case, the annihilation cross section was selected to best fit the observed radio excess. In the case of annihilations to $b \bar{b}$ , we find a very poor fit to the observed spectrum, corresponding to $χ^{2} = 44.2$ and 34.7 for 10 and 100 GeV masses, respectively, over 12 degrees of freedom. We find similarly poor fits for annihilations to other hadronic final states, and for annihilations to gauge or Higgs bosons. Other choices of the magnetic field and substructure models do not significantly alter this conclusion. In contrast, annihilations to leptonic final states can provide a good fit to the spectrum of the observed excess. Annihilations of a 10 GeV dark matter particle to taus, muons, or electrons yield a $χ^{2}$ of 36.0, 24.8, and 12.3, respectively. For a 100 GeV mass, these channels yield a $χ^{2}$ of 14.4, 12.1, and 13.1. The quality of these fits can change somewhat with variations of the magnetic field model, although realistic models universally favor leptonic annihilation channels.Reuse & Permissions
Figure 4
The contribution to the isotropic gamma-ray background from annihilating dark matter, in the same models as shown in Fig. 3. In each case, the upper curve use an extrapolatation to determine the concentration of halos lighter than $10^{5} M_{⊙}$ , whereas the lower curve fixes the concentration below this mass. When these contributions are compared to the isotropic gamma-ray background observed by Fermi, we see that annihilations of heavy ( $m_{DM} ≫ 10 GeV$ ) dark matter particles, or dark matter particles which annihilate primarily to hadronic or $τ^{+} τ^{-}$ channels, are unable to produce the observed radio background without also exceeding the observed gamma-ray spectrum.Reuse & Permissions
Figure 5
The range of dark matter masses and annihilation cross sections which can provide a good fit to the spectrum of the isotropic radio excess, using our default magnetic field and dark matter distribution models (solid line, 95% confidence regions). No models annihilating to quarks or gauge bosons were found to provide a good fit to the observed spectrum. Also shown are gamma-ray constraints derived from the isotropic gamma-ray background (dashed lines) and dwarf spheroidal galaxies (dotted lines). See text for more details.Reuse & Permissions
Figure 6
As in Fig. 5, but over a range of magnetic field and dark matter distribution models. See Sec. 4 for more details.Reuse & Permissions

Physical Review D

covering particles, fields, gravitation, and cosmology