The Gamma-Ray-Flux Probability Distribution Function from Galactic Halo Substructure

TL;DR

This paper proposes using the gamma-ray-flux probability distribution function (PDF) as a statistical tool to detect and analyze dark matter substructure in the Galactic halo, potentially revealing features undetectable by individual observations.

Contribution

It introduces the gamma-ray-flux PDF as a novel method to probe dark matter substructure, including microhalos, and assesses its detectability with Fermi data under various models.

Findings

PDF analysis can statistically detect substructure even if individual halos are not observable.

Detection of substructure is challenging with current Fermi data for neutralino WIMPs.

The method can potentially measure the substructure mass function.

Abstract

One of the targets of the recently launched Fermi Gamma-ray Space Telescope is a diffuse gamma-ray background from dark-matter annihilation or decay in the Galactic halo. N-body simulations and theoretical arguments suggest that the dark matter in the Galactic halo may be clumped into substructure, rather than smoothly distributed. Here we propose the gamma-ray-flux probability distribution function (PDF) as a probe of substructure in the Galactic halo. We calculate this PDF for a phenomenological model of halo substructure and determine the regions of the substructure parameter space in which the PDF may be distinguished from the PDF for a smooth distribution of dark matter. In principle, the PDF allows a statistical detection of substructure, even if individual halos cannot be detected. It may also allow detection of substructure on the smallest microhalo mass scales, $\sim M_\oplus$, for weakly-interacting massive particles (WIMPs). Furthermore, it may also provide a method to measure the substructure mass function. However, an analysis that assumes a typical halo substructure model and a conservative estimate of the diffuse background suggests that the substructure PDF may not be detectable in the lifespan of Fermi in the specific case that the WIMP is a neutralino. Nevertheless, for a large range of substructure, WIMP annihilation, and diffuse background models, PDF analysis may provide a clear signature of substructure.