Spherical harmonics analysis of Fermi gamma-ray data and the Galactic dark matter halo

Abstract

We argue that the decomposition of gamma-ray maps in spherical harmonics is a sensitive tool to study dark matter (DM) annihilation or decay in the main Galactic halo of the Milky Way. Using the spherical harmonic decomposition in a window excluding the Galactic plane, we show for one year of Fermi data that adding a spherical template (such as a line-of-sight DM annihilation profile) to an astrophysical background significantly reduces chi^2 of the fit to the data. In some energy bins the significance of this DM-like fraction is above three sigma. This can be viewed as a hint of DM annihilation signal, although astrophysical sources cannot be ruled out at this moment. We use the derived DM fraction as a conservative upper limit on DM annihilation signal. In the case of bb-bar annihilation channel the limits are about a factor of two less constraining than the limits from dwarf galaxies. The uncertainty of our method is dominated by systematics related to modeling the astrophysical background. We show that with one year of Fermi data the statistical sensitivity would be sufficient to detect DM annihilation with thermal freeze out cross section for masses below 100 GeV.Comment: 16 pages, 10 figures, 1 table; v2: minor corrections, v3: major revision of the presentation, results unchange