The Anisotropy of the Extragalactic Radio Background from Dark Matter Annihilation

TL;DR

This paper investigates whether dark matter annihilation can explain the isotropic extragalactic radio background and its minimal anisotropy, proposing models that reconcile observed excess with dark matter physics.

Contribution

It introduces models of dark matter annihilation that can produce the observed radio excess with minimal anisotropy, considering effects of substructure and reacceleration mechanisms.

Findings

Dark matter annihilation can produce very small anisotropies in the radio background.

Reacceleration of electrons in galaxy clusters relaxes constraints on dark matter models.

The source of the radio excess must be spatially extended beyond typical galaxy scales.

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 Alfv\'en 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, suggesting a novel physics interpretation.