Spherical Cows in Dark Matter Indirect Detection

TL;DR

This study uses the Illustris simulation to analyze the shape and asymmetry of dark matter annihilation and decay signals, revealing that Galactic signals are mostly symmetric while extragalactic signals can be elongated, affecting indirect detection interpretations.

Contribution

It provides a detailed analysis of the asymmetry of dark matter signals considering baryonic effects, using large-scale hydrodynamic simulations to improve modeling accuracy.

Findings

Galactic signals are generally symmetric with axis ratios close to 1.

Extragalactic signals, especially from annihilation, often show elongated profiles due to subhalos and mergers.

Gamma-ray background appears less spherical than expected DM signals, impacting detection strategies.

Abstract

Dark matter (DM) halos have long been known to be triaxial, but in studies of possible annihilation and decay signals they are often treated as approximately spherical. In this work, we examine the asymmetry of potential indirect detection signals of DM annihilation and decay, exploiting the large statistics of the hydrodynamic simulation Illustris. We carefully investigate the effects of the baryons on the sphericity of annihilation and decay signals for both the case where the observer is at 8.5 kpc from the center of the halo (exemplified in the case of Milky Way-like halos), and for an observer situated well outside the halo. In the case of Galactic signals, we find that both annihilation and decay signals are expected to be quite symmetric, with axis ratios very different from 1 occurring rarely. In the case of extragalactic signals, while decay signals are still preferentially spherical, the axis ratio for annihilation signals has a much flatter distribution, with elongated profiles appearing frequently. Many of these elongated profiles are due to large subhalos and/or recent mergers. Comparing to gamma-ray emission from the Milky Way and X-ray maps of clusters, we find that the gamma-ray background appears less spherical/more elongated than the expected DM signal from the large majority of halos, and the Galactic gamma ray excess appears very spherical, while the X- ray data would be difficult to distinguish from a DM signal by elongation/sphericity measurements alone.