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Well-motivated scenarios of thermally-produced dark matter often result in apopulation of electrons and positrons within galaxies produced through darkmatter annihilation -- often in association with high-energy gamma rays. Asthey diffuse through galactic magnetic fields, these $e^\pm$ producesynchrotron radio emission. The intensity and morphology of this signal dependson the properties of the interstellar medium through which the $e^\pm$propagate. Using observations of the Andromeda Galaxy (M31) to construct amodel of the gas, magnetic fields, and starlight, we set constraints on darkmatter annihilation to $b\bar{b}$ using the morphology of 3.6 cm radioemission. As the emission signal at the center of M31 is very sensitive to thediffusion coefficient and dark matter profile, we base our limits on thedifferential flux in the region between $0.9-6.9$ kpc from the center. Weexclude annihilation cross sections $\gtrsim 3 \times 10^{-25}$ cm$^3$/s in themass range $10-500$ GeV, with a maximum sensitivity of $7\times 10^{-26}$cm$^3$/s at $20-40$ GeV. Though these limits are weaker than those found inprevious studies of M31, they are robust to variations of the diffusioncoefficient.

Limits on Dark Matter Annihilation from the Shape of Radio Emission in M31