Telescope search for a 3-eV to 8-eV axion

Axions of mass 3–8 eV should have a cosmological abundance of about 50 ${\mathrm{cm}}^{\mathrm{-}3}$ and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength ${\lambda }_{\mathit{a}}$∼3100–8300 Å (width Δλ/λ∼${10}^{\mathrm{-}2}$). We have searched unsuccessfully for such a feature in the intergalactic light of three rich clusters, closing this ‘‘window,’’ and leaving open only the window from ${10}^{\mathrm{-}6}$ to ${10}^{\mathrm{-}3}$ eV. This implies that if the axion exists, it likely comprises the dark matter. Our flux limits are of relevance to other relics whose decays produce monoenergetic photons.