Dark Photons in the Radio Sky: I. Resonant Conversions in Halos

Mixing between dark photons and visible photons leads to substantial anisotropies in the cosmic microwave background due to resonant conversions of visible photons into dark photons in baryonic matter found in dark matter halos. In this work, we forecast the sensitivity of the Square Kilometre Array (SKA) to this signal. We find that SKA could be the first experiment to discover dark photons with a mass between 10-13 and 5 times 10-12 eV and kinetic mixing parameter ε as small as about 10-8 by cross-correlating their data with a low-redshift galaxy survey, potentially improving on the sensitivity from a similar analysis using Planck data by a factor of 4 in ε. This improvement is largely due to an enhancement of the signal at low frequencies and the unique experimental advantages of radio telescopes such as small beam sizes.