Anisotropy of the extragalactic radio background from dark matter annihilation

Abstract

Observations of the extragalactic radio background have uncovered a significant isotropic emission across multiple frequencies spanning from 22 MHz to 10 GHz. The intensity of this non-thermal emission component significantly exceeds the expected contribution from known astrophysical sources. Interestingly, models have indicated that the annihilation of dark matter particles may reproduce both the flux and spectrum of the excess. However, the lack of a measurable anisotropy in the residual emission remains challenging for both dark matter and standard astrophysical interpretations of the ARCADE-2 data. We calculate the expected synchrotron anisotropy from dark matter annihilation and show that these models can produce very small anisotropies, though this requires galaxy clusters to have large substructure contributions and strong magnetic fields. We show that this constraint can be significantly relaxed, however, in scenarios where electrons produced via dark matter annihilation can be efficiently reaccelerated by Alfvén waves in the intra-Cluster medium. Our analysis indicates that any source capable of explaining the intensity and isotropy of the extragalactic radio excess must have a spatial extension far larger than typical for baryons in galaxies, hinting at a novel physics interpretation.

Cluster mergers and the origin of the ARCADE-2 excess

“…Dark matter models are well motivated for explanations of ARCADE-2 since they naturally produce large fluxes of relativistic e + e − while producing negligible thermal emission. This allows dark matter induced emission to simultaneously fit the intensity of the ARCADE-2 excess while remaining consistent with infrared constraints, and several similar models have been formulated to date [11][12][13][14].…”

“…where η ∼ 3/8 [14], and B 0 is the central magnetic field strength of clusters. We leave B 0 as our first adjustable parameter with a default value B 0 = 10µG.…”

“…An alternative model, first noted in [14], concerns the re-acceleration of non-thermal electrons through hydrodynamic shocks produced in cluster mergers. In the context of the dark matter models originally considered by [14], these shocks were employed to re-accelerate electrons originally produced via dark matter annihilation, significantly amplifying the total synchrotron flux produced via a dark matter annihilation event. This mechanism is capable of smearing out the resulting synchrotron signal via re-acceleration, decreasing the total anisotropy of the synchrotron emission.…”

The Radio Synchrotron Background: Conference Summary and Report

“…A flatter spectrum can be obtained by hardening the electron injection in some energy range, such as having electron secondaries produced through hadronic interactions A possible solution would be a model where local magnetic field enhancements trace cluster dark matter substructure, even far away from cluster centers. As investigated in Fang & Linden (2015), in such regions of locally enhanced magnetic field, the magnetic field turbulence can give rise to Alfvén waves, which re-accelerate the electrons, leading to a locally enhanced synchtrotron emissivity. Since the synchrotron emission would be happening only in these localized regions of the energetic electrons and enhanced magnetic fields, the overall inverse-Compton emission level for a given amount of synchrotron could be suppressed.…”

The Radio Synchrotron Background -- A Cosmic Conundrum

“…A number of potential particle-related explanations for RSB emission mechanisms have been suggested, including intergalactic dark matter decays and annihilations in halos and filaments (e.g. [24,25,26]), ultracompact halos [27], or in the relatively early Universe (e.g. [28]), "dark stars" in the early universe [29], supernovae of massive population III (first generation) stars [30], and dense nuggets of quarks [31].…”