An observational constraint on gravitational lensing by objects of mass 109.5-1010.9 M⊙

A radio-based search for strong gravitational lensing with image separations in the range 160-300 milliarcsec (mas) has yielded a null result for a sample of 1665 sources the mean redshift of which is estimated to be similar to1.3. The lensing rate for this previously unexplored separation range, <1:555 at the 95 per cent confidence level, is less than on arcsecond-scales - as expected from models of lensing galaxy populations. Lensing on 160-300 mas scales is expected to arise predominantly from spiral galaxies at a rate dependent on the disk-halo mass ratio and the evolving number density of the population with redshift. While the present sample is too small for there to be a high probability of finding spiral galaxy lenses, our work is a pilot survey for a much larger search based on the full CLASS data base, which would provide useful information on galactic structure at z similar to0.5. We examine other possible lens populations relevant to our present search, in particular dwarf galaxies and supermassive black holes in galactic nuclei, and conclude that none of them are likely to be detected. Our null result enables us formally to rule out a cosmologically significant population of uniformly distributed compact objects: Omega (CO)<0.1 (95 per cent confidence) in the mass range 10(9.5)-10(10.9) M-..