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

Abstract

Axions of mass 3--8 eV should have a cosmological abundance of about 50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength ${\ensuremath{\lambda}}_{\mathit{a}}$\ensuremath{\sim}3100--8300 \AA{} (width \ensuremath{\Delta}\ensuremath{\lambda}/\ensuremath{\lambda}\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}2}$). 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 ${10}^{\mathrm{\ensuremath{-}}6}$ to ${10}^{\mathrm{\ensuremath{-}}3}$ 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.