Detection of Gamma-Ray Halos around Nearby Late-Type Galaxies

Various theoretical models predict the existence of extended γ-ray halos around normal galaxies that could be produced by interactions of cosmic rays with the circumgalactic medium or by annihilation or decay of hypothetical dark matter particles. Observations of M31, the closest massive galaxy, also corroborate this possibility. In this Letter, we describe our search for gamma-ray emission from the galaxies within 15 Mpc at energies higher than 2 GeV and try to assess its spatial extension. We use the latest catalog of local galaxies and apply a simple yet robust method of aperture photometry. By imposing the mass, energy, and spatial cuts, we selected a set of 16 late-type galaxies and found a statistically significant excess above the background level: a p value of 3.7×10^{-7} at E>2 GeV, reaching maximal significance of p-val=2.3×10^{-8} for a subset of front-converted events with E>2 GeV, where the angular resolution is higher. More importantly, our analysis shows that this excess can be ascribed to an extended source with a radius ∼0.3° rather than a pointlike one. This, for D=15 Mpc, corresponds to a physical halo radius of r_{h}=80 kpc. In contrast, six early-type galaxies, which satisfied the same cuts, showed no excess. Our results are supported by the stacking likelihood analysis technique, which significantly (5.6σ) detected an extended excess. The difference between the late- and early-type galaxies and a rather irregular shape of the extended source that we found, indicate that this high-energy emission is more likely caused by the interactions of cosmic rays with the circumgalactic medium, in preference to dark matter annihilation and decay processes.