Constraints on dark matter annihilation by radio observations of M31

TL;DR

This study uses radio observations of M31 to set constraints on dark matter particle properties, specifically mass and annihilation cross section, by comparing predicted synchrotron emission with observed radio data.

Contribution

It provides new constraints on dark matter particle mass and annihilation cross section using radio data of M31, considering uncertainties in magnetic field and density profiles.

Findings

Dark matter annihilation cross section > 3×10^{-26} cm^3/s is only allowed for WIMP masses > 100 GeV (bb channel) and > 55 GeV (tau channel).

Uncertainties in magnetic field and density profiles can lower the mass limits to 23 GeV.

Results surpass previous gamma-ray constraints from Fermi observations.

Abstract

We used radio observations of the neighbour galaxy M31 in order to put constraints on 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: \chi\chi -> bb* and \chi\chi -> \tau^+\tau^-. 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 NFW dark matter density profile and a conservative magnetic field distribution inside the Andromeda galaxy, we find that the thermal relic annihilation cross section <\sigma v> = 3*10^{-26} cm^3/s or higher are only allowed for WIMP masses greater than 100 GeV and 55 GeV for annihilation into bb* and \tau^+\tau^- respectively. Taking into account potential uncertainties in the distributions of DM density and magnetic field, the mentioned WIMP limiting masses can be as low as 23 GeV for both channels, and as high as 280 and 130 GeV for annihilation into bb* and \tau^+\tau^- respectively. These mass values exceed the best up-to-day known constraints from Fermi gamma observations: 40 GeV 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 DM density profile of M31 will be able to reduce the uncertainties of our exclusion limits.