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 complet enesses of 80 %, 68 %, and 58 % for the bright, intermediate, and fain t flux ranges. Using the newly determined redshift distribution, we ca n derive cosmological limits from the statistics of the six gravitatio nal lenses in the Jodrell Bank-VLA Astrometric Survey sample of 2500 h at-spectrum radio sources brighter than 200 mJy at 5 GHz. For flat cos mological models with a cosmological constant, the limit using only ra dio data is Omega(0) > 0.27 at 2 sigma (0.47 < Omega(0) < 1.38 at 1 si gma). The limits are statistically consistent with those for lensed qu asars, and the combined radio + optical sample requires Omega(0) > 0.3 8 at 2 sigma (0.64 < Omega(0) < 1.66 at 1 sigma) for our most conserva tive redshift completeness model, assuming that there are no quasar le nses produced by spiral galaxies. Our best-fit model improves by appro ximately 1 sigma if extinction in the early-type galaxies makes the le nsed quasars fainter by Delta m = 0.58 +/- 0.45 mag, but we still find a limit of Omega(0) > 0.26 at 2 sigma in flat cosmologies. The increa sing fraction of radio galaxies as compared to quasars at fainter radi o fluxes (rising from similar to 10 % at 1 Jy to similar 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.