  A search for optical line emission from the two-photon decay of relic axions
was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from
the VIMOS (Visible Multi-Object Spectrograph) integral field unit at the Very
Large Telescope. New upper limits to the two-photon coupling of the axion are
derived, and are at least a factor of 3 more stringent than previous upper
limits in this mass window. The improvement follows from larger collecting
area, integration time, and spatial resolution, as well as from improvements in
signal to noise and sky subtraction made possible by strong-lensing mass models
of these clusters. The new limits either require that the two-photon coupling
of the axion be extremely weak or that the axion mass window between 4.5 eV and
7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed
briefly also.
