Redshifts of CLASS Radio Sources

Spectroscopic observations of a sample of 42 flat-spectrum radio sources from 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 sample 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 distribution has been studied. The spectra, obtained with the William Herschel Telescope, 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 exhibits little variation over more than 2 orders of magnitude in radio flux density, there is evidence of a decreasing fraction of quasars at weaker flux levels. We present the results of our spectroscopic observations and discuss the implications for constraining cosmological parameters with statistical analyses of CLASS.