The Fermi Gamma-Ray Haze from Dark Matter Annihilations and Anisotropic Diffusion

TL;DR

This paper proposes that anisotropic diffusion along magnetic field lines combined with a prolate dark matter halo can explain the Fermi gamma-ray haze's morphology and spectrum, fitting observational data and cosmic-ray constraints.

Contribution

It introduces a model with anisotropic diffusion and a prolate dark matter halo to account for the gamma-ray haze's characteristics, which standard models struggle to reproduce.

Findings

Anisotropic diffusion explains the haze morphology.

A Sommerfeld enhancement fits the haze spectrum.

Model aligns with cosmic-ray and CMB data.

Abstract

Recent full-sky maps of the Galaxy from the Fermi Gamma-Ray Space Telescope have revealed a diffuse component of emission towards the Galactic center and extending up to roughly +/-50 degrees in latitude. This Fermi "haze" is the inverse Compton emission generated by the same electrons which generate the microwave synchrotron haze at WMAP wavelengths. The gamma-ray haze has two distinct characteristics: the spectrum is significantly harder than emission elsewhere in the Galaxy and the morphology is elongated in latitude with respect to longitude with an axis ratio ~2. If these electrons are generated through annihilations of dark matter particles in the Galactic halo, this morphology is difficult to realize with a standard spherical halo and isotropic cosmic-ray diffusion. However, we show that anisotropic diffusion along ordered magnetic field lines towards the center of the Galaxy coupled with a prolate dark matter halo can easily yield the required morphology without making unrealistic assumptions about diffusion parameters. Furthermore, a Sommerfeld enhancement to the self annihilation cross-section of ~30 yields a good fit to the morphology, amplitude, and spectrum of both the gamma-ray and microwave haze. The model is also consistent with local cosmic-ray measurements as well as CMB constraints.