Limits on Cosmological Models from Radio-selected Gravitational Lenses

We are conducting a redshift survey of 177 flat-spectrum radio sources in three samples covering the 5 GHz flux ranges 50-100, 100-200, and 200-250 mJy. So far, we have measured 124 redshifts with completenesses of 80%, 68%, and 58% for the bright, intermediate, and faint flux ranges. Using the newly determined redshift distribution, we can derive cosmological limits from the statistics of the six gravitational lenses in the Jodrell Bank-VLA Astrometric Survey sample of 2500 flat-spectrum radio sources brighter than 200 mJy at 5 GHz. For flat cosmological models with a cosmological constant, the limit using only radio data is Ω0 > 0.27 at 2 σ (0.47 < Ω0 < 1.38 at 1 σ). The limits are statistically consistent with those for lensed quasars, and the combined radio + optical sample requires Ω0 > 0.38 at 2 σ (0.64 < Ω0 < 1.66 at 1 σ) for our most conservative redshift completeness model, assuming that there are no quasar lenses produced by spiral galaxies. Our best-fit model improves by approximately 1 σ if extinction in the early-type galaxies makes the lensed quasars fainter by Δm = 0.58 ± 0.45 mag, but we still find a limit of Ω0 > 0.26 at 2 σ in flat cosmologies. The increasing fraction of radio galaxies as compared to quasars at fainter radio fluxes (rising from ~10% at 1 Jy to ~50% at 0.1 Jy) explains why lensed optical emission is common for radio lenses and partly explains the red color of radio-selected lenses.