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 and Ryan E. Keeley

Phys. Rev. D 93, 083514 – Published 15 April 2016

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.

Received 2 November 2015

DOI:https://doi.org/10.1103/PhysRevD.93.083514

Physics Subject Headings (PhySH)

Dark matter

Neutron stars & pulsars

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Kevork N. Abazajian^* and Ryan E. Keeley^†

Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697, USA

^*kevork@uci.edu
^†rkeeley@uci.edu

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Vol. 93, Iss. 8 — 15 April 2016

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Images

Figure 1
Plotted is the scaled likelihood for the Galactic Center’s J-factor for our ROI given relaxation of the constraints on the Milky Way dark matter halo, as described in the text.
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Figure 2
In (a) and (b), we plot contours of the $Δ$ log-likelihood that correspond to 68%, 95% and 99.7% credible regions for the full IC, noIC, and noB data sets, when marginalizing over Milky Way halo uncertainties, which demonstrate the systematic errors involved in the inclusion of diffuse sources in the GC; (a) is for the $b / \bar{b}$ -quark channel and (b) is for the $τ^{\pm}$ channel. The full IC model is shown in blue, noIC is in orange, and noB is in green. We also show, in red contours, a nonstandard high-concentration/contraction Milky Way halo model that would escape dwarf galaxy limits, but would be in conflict with local density and Milky Way halo simulations. We also show the 95% limits from dwarf galaxy searches by Ackermann et al. [19]. In the (c) and (d), for the $b / \bar{b}$ -quark and $τ^{\pm}$ channels respectively, we plot contours of the $Δ$ log-likelihood that correspond to 68%, 95% and 99.7% for different numbers of low-energy bins excluded, demonstrating GCE spectrum determination systematic uncertainties in our method. The red contours are those derived from excluding data below 2.03 GeV, blue from excluding data below 1.24 GeV, and purple with a 0.764 GeV cut. The blue contours are for our optimal GCE spectrum determination, as described in the text.
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Figure 3
Plotted in filled green are contours the $Δ$ log-likelihood that correspond to 68%, 95%, and 99.7% and 99.99997% credible regions (corresponding approximately to 1, 2, 3 and $5 σ$ ) when marginalizing over Milky Way halo uncertainties, in our best estimates for background uncertainties, with the local dark matter density determination by Zhang et al. [27]. Counter to the expectation that a symmetric error becomes asymmetric in a logarithmic plot, with larger extent downward, the error regions are asymmetrically oriented upward due to the anticorrelation of the J-factor with the annihilation rate $⟨ σ v ⟩$ . We also show, in light red, the respective approximate error contours from the inferred approximate dark matter density in the low stellar density star count measures of McKee et al. [29]. In purple, we show the error contours derived from the local dark matter density of Pato et al. [32]. We also show the 95% limits from the dwarf galaxy annihilation search by Ackermann et al. [19], and the signal regions as presented in Refs. [10, 12, 13]. As seen here, both the Zhang et al. and Pato et al. local dark matter density determinations leave single-channel dark matter annihilation interpretations of the GCE in strong tension with dwarf limits. The $b$ -quark annihilation channel is on the left and the $τ$ -lepton annihilation channel is on the right.
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