title: Glow in the dark matter: observing galactic halos with scattered light.
abstract: We consider the observation of diffuse halos of light around the discs of spiral galaxies, as a probe of the interaction cross section between dark matter (DM) and photons. Using the galaxy M101 as an example, we show that for a scattering cross section at the level of 10(-23)(m/GeV)  cm(2) or greater dark matter in the halo will scatter light out from the more luminous center of the disc to larger radii, contributing to an effective increased surface brightness at the edges of the observed area on the sky. This allows us to set an upper limit on the DM-photon cross section using data from the Dragonfly instrument. We then show how to improve this constraint, and the potential for discovery, by combining the radial profile of DM-photon scattering with measurements at multiple wavelengths. Observation of diffuse light presents a new and potentially powerful way to probe the interactions of dark matter with photons, a way that is complementary to existing searches.
authors.affiliation: Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France and Institute for Particle Physics Phenomenology, Durham University, Durham DH1 3LE, United Kingdom.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Davis
authors.affiliation: Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France and Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA and Beecroft Institute of Particle Astrophysics and Cosmology, Department of Physics, University of Oxford, Denys Wilkinson Building, 1 Keble Road, Oxford OX1 3RH, United Kingdom.
authors.forename: Joseph
authors.initials: J
authors.lastname: Silk