Axion Condensate Dark Matter Constraints from Resonant Enhancement of Background Radiation

Understand

We investigate the possible parametric growth of photon amplitudes in a background of axion-like particle (ALP) dark matter.

• The observed extragalactic background radiation limits the allowed enhancement effect.
• We derive the resulting constraints on the axion-photon coupling constant $g_{a\gamma}$ from Galactic ALP condensates as well as over-densities.
• If ALP condensates of size $R$ exist in our Galaxy, a scan for extremely narrow unresolved spectral lines with frequency $\nu$ can constrain the axion-photon coupling at ALP mass $m_a=4\pi\nu$ to $g_{a\gamma}\lesssim2\times10^{-14}(10\,{\rm kpc}/R)\,{\rm GeV}^{-1}$.