Dark Photon Dark Matter Produced by Axion Oscillations
  Despite growing interest and extensive effort to search for ultralight dark
matter in the form of a hypothetical dark photon, how it fits into a consistent
cosmology is unclear. Several dark photon dark matter production mechanisms
proposed previously are known to have limitations, at least in certain mass
regimes of experimental interest. In this letter, we explore a novel mechanism,
where a coherently oscillating axion-like field can efficiently transfer its
energy density to a dark photon field via a tachyonic instability. The residual
axion relic is subsequently depleted via couplings to the visible sector,
leaving only the dark photon as dark matter. We ensure that the cosmologies of
both the axion and dark photon are consistent with existing constraints. We
find that the mechanism works for a broad range of dark photon masses,
including those of interest for ongoing experiments and proposed detection
techniques.
