[Paper Review] A Bright Gamma-ray Galactic Center Excess and Dark Dwarfs: Strong Tension for Dark Matter Annihilation Despite Milky Way Halo Profile and Diffuse Emission Uncertainties

Kevork N. Abazajian, Ryan E. Keeley|arXiv (Cornell University)|Oct 21, 2015

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper investigates the Galactic Center Excess (GCE) in Fermi-LAT data by incorporating uncertainties in the Milky Way's dark matter halo profile and diffuse gamma-ray emission. Despite these uncertainties, the GCE remains in strong tension with dark matter annihilation constraints from dwarf galaxy surveys, suggesting either a highly concentrated halo, differential annihilation behavior between the GC and dwarfs, or lower local stellar densities than previously thought.

ABSTRACT

We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. The GCE and dwarf tension can be alleviated if: one, the halo is very highly concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the GC; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, increasing the local dark matter density.

Motivation & Objective

• To assess the impact of Milky Way dark matter halo profile uncertainties on the interpretation of the Galactic Center Excess (GCE) as a dark matter signal.
• To evaluate how uncertainties in diffuse gamma-ray emission affect dark matter annihilation models for the GCE.
• To test whether the tension between the GCE and dwarf galaxy dark matter constraints can be resolved under realistic astrophysical uncertainties.
• To explore alternative explanations that could reconcile the GCE with existing dark matter annihilation limits, including halo contraction or revised local stellar density estimates.

Proposed method

• Incorporated a range of Milky Way dark matter halo density profiles, including highly concentrated and contracted models, to assess their impact on the GCE signal strength.
• Accounted for uncertainties in diffuse gamma-ray emission components, particularly those from cosmic-ray interactions, to refine the background modeling in the GCE region.
• Combined constraints from Fermi-LAT observations of the Galactic Center with data from multiple dwarf galaxy surveys to test consistency with single-channel dark matter annihilation models.
• Used empirical measurements of local dark matter density from stellar kinematics and recent stellar counts to explore how revised local densities affect the tension.
• Evaluated the implications of differential annihilation cross-sections between the Galactic Center and dwarf galaxies, allowing for environmental dependence.
• Quantified the allowed parameter space for dark matter mass and annihilation rate under varying halo and background assumptions.

Experimental results

Research questions

• RQ1 How do uncertainties in the Milky Way's dark matter halo profile affect the consistency of the Galactic Center Excess with dark matter annihilation models?
• RQ2 To what extent do diffuse gamma-ray emission uncertainties alter the inferred dark matter annihilation rate and mass constraints for the GCE?
• RQ3 Can the observed tension between the GCE and dwarf galaxy annihilation constraints be resolved by assuming a highly concentrated or contracted halo?
• RQ4 Is the tension alleviated if dark matter annihilation rates differ between the Galactic Center and dwarf galaxies?
• RQ5 Does a revised estimate of local stellar density—such as from recent stellar counts—lead to a higher local dark matter density that resolves the tension with dwarf constraints?

Key findings

• Even when accounting for halo profile and diffuse emission uncertainties, the Galactic Center Excess remains in strong tension with dark matter annihilation constraints derived from combined dwarf galaxy analyses.
• The tension persists for the two most precise empirical determinations of the Milky Way's local dark matter density and halo profile.
• Resolution of the tension requires either a very highly concentrated or strongly contracted dark matter halo, which is disfavored by current constraints.
• Alternative explanations such as differential annihilation behavior between the Galactic Center and dwarfs could alleviate the tension, but require non-universal annihilation cross-sections.
• A significant reduction in local stellar density—consistent with recent stellar count measurements—would increase the local dark matter density and help reconcile the GCE with dwarf constraints.
• The current data do not support a simple dark matter annihilation explanation for the GCE without invoking non-standard halo properties or environmental dependence in annihilation rates.