Constraints on dark matter annihilation by radio observations of M31

We used radio observations of the neighbor galaxy M31 in order to put constraints on the dark matter particle mass and annihilation cross section. Dark matter annihilation in M31 halo produces highly energetic leptons, which emit synchrotron radiation on radio frequencies in the galactic magnetic field. We predicted expected radio fluxes for the two annihilation channels: χχ→bb̄ and χχ→τ⁺τ^-. We then compared them with available data on the central radio emission of M31 as observed by four radio surveys: VLSS (74 MHz), WENSS (325 MHz), NVSS (1400 MHz), and GB6 (4850 MHz). Assuming a standard Navarro-Frenk-White dark matter density profile and a conservative magnetic field distribution inside the Andromeda galaxy, we find that the thermal relic annihilation cross section or higher ⟨σv⟩≥3×10^-26 cm³/s are only allowed for weakly interacting massive particle masses greater than ≈100 and ≈55 GeV for annihilation into bb̄ and τ⁺τ^-, respectively. Taking into account potential uncertainties in the distributions of dark matter density and the magnetic field, the mentioned weakly interacting massive particle limiting masses can be as low as 23 GeV for both channels, and as high as 280 and 130 GeV for annihilation into bb̄ and τ⁺τ^-, respectively. These mass values exceed the best up-to-date known constraints from Fermi gamma observations: 40 and 19 GeV, respectively [A. Geringer-Sameth and S. M. Koushiappas, Phys. Rev. Lett. 107, 241303 (2011)]. Precise measurements of the magnetic field in the relevant region and better reconstruction of the dark matter density profile of M31 will be able to reduce the uncertainties of our exclusion limits.