On The Origin Of The Gamma Rays From The Galactic Center

The region surrounding the center of the Milky Way is both astrophysically rich and complex, and is predicted to contain very high densities of dark matter. Utilizing three years of data from the Fermi Gamma Ray Space Telescope (and the recently available Pass 7 ultraclean event class), we study the morphology and spectrum of the gamma ray emission from this region and find evidence of a spatially extended component which peaks at energies between 300 MeV and 10 GeV. We compare our results to those reported by other groups and find good agreement. The extended emission could potentially originate from either the annihilations of dark matter particles in the inner galaxy, or from the collisions of high energy protons that are accelerated by the Milky Way’s supermassive black hole with gas. If interpreted as dark matter annihilation products, the emission spectrum favors dark matter particles with a mass in the range of 7-12 GeV (if annihilating dominantly to leptons) or 25-45 GeV (if annihilating dominantly to hadronic final states). The intensity of the emission corresponds to a dark matter annihilation cross section consistent with that required to generate the observed cosmological abundance in the early universe (sigma v ~ 3 x 10^-26 cm^3/s). We also present conservative limits on the dark matter annihilation cross section which are at least as stringent as those derived from other observations.

💡 Research Summary

This paper presents a detailed analysis of the gamma‑ray emission from the inner few degrees of the Milky Way’s Galactic Center (GC) using three years of Fermi‑LAT data (Pass 7 ultraclean class). The authors first construct smoothed (0.5°) count maps in five energy bands from 100 MeV to 100 GeV, selecting only front‑converting events to benefit from the superior point‑spread function. Known point sources from the 2FGL catalog are modeled with their reported positions and intensities, and a template for the diffuse Galactic disk emission is built from a gas distribution model (ρ_gas ∝ e^{‑|z|/z_sc(R)} with an exponential radial fall‑off beyond 7 kpc). After subtracting both the point‑source and gas templates, the residual maps reveal a bright, roughly spherically symmetric component centered on the GC, together with a weaker elongated feature along the plane that likely corresponds to the “Galactic Ridge” seen by HESS.

The residual spectrum, extracted from a 5° radius around the GC, peaks between 300 MeV and 10 GeV and falls by roughly an order of magnitude above ~10 GeV. Comparison with previous independent analyses (Boyarsky et al., Chernyakova et al., Hooper & Goodenough) shows that below ~300 MeV the emission can be accounted for by a central point source, but at higher energies the residual exceeds the point‑source contribution by factors of 2–3 (0.3–3 GeV) and ~5 (>3 GeV). This indicates a genuine extended component.

Interpreting the extended emission as dark‑matter (DM) annihilation, the authors find two viable mass ranges: 7–12 GeV for dominantly leptonic final states (e.g., τ⁺τ⁻) and 25–45 GeV for hadronic final states (e.g., b b̄). The required annihilation cross section is ⟨σv⟩ ≈ 3 × 10⁻²⁶ cm³ s⁻¹, consistent with the canonical thermal relic value that yields the observed cosmological DM abundance. The morphology is compatible with a cusped halo profile (ρ_DM ∝ r⁻¹·³⁴). The authors also derive conservative limits on ⟨σv⟩ from the same data, finding them at least as stringent as those obtained from dwarf spheroidal galaxies, galaxy clusters, the isotropic gamma‑ray background, and nearby subhalos.

Alternative astrophysical explanations are examined. One possibility is that the supermassive black hole (Sgr A*) accelerates cosmic‑ray protons, which then diffuse and collide with ambient gas, producing neutral pions that decay into gamma rays. While this mechanism can generate a spatially extended component, the required proton injection history, diffusion parameters, and gas density distribution are highly uncertain, making quantitative predictions difficult. Moreover, models where Sgr A* accelerates electrons to produce TeV gamma rays via inverse Compton scattering predict far less GeV emission than observed, disfavoring that scenario. A population of unresolved millisecond pulsars could also contribute, but the observed spectrum and spatial extent exceed what is expected from a simple pulsar population model.

Overall, the paper concludes that the extended GeV excess is most naturally explained by DM annihilation, though a definitive attribution requires further data. The analysis demonstrates that the GC remains a uniquely powerful laboratory for indirect DM searches, and that the derived constraints on ⟨σv⟩ are competitive with, or superior to, those from other astrophysical targets. Future observations with improved angular resolution and multi‑wavelength studies will be essential to disentangle the possible contributions from dark matter, black‑hole‑driven cosmic rays, and compact object populations.