Redshifts of class radio sources

Spectroscopic observations of a sample of 42 flat-spectrum radio sources fr om the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of ( z) = 1.27 with an rms spread of 0.95, at a completeness level of 64%. The s ample consists of sources with a 5 GHz flux density of 25-50 mJy, making it the faintest flat-spectrum radio sample for which the redshift distributio n has been studied. The spectra, obtained with the William Herschel Telesco pe, consist mainly of broad-line quasars at z > 1 and narrow-line galaxies at z < 0.5. Though the mean redshift of flat-spectrum radio sources exhibit s little variation over more than 2 orders of magnitude in radio flux densi ty, there is evidence of a decreasing fraction of quasars at weaker flux le vels. We present the results of our spectroscopic observations and discuss the implications for constraining cosmological parameters with statistical analyses of CLASS.