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Spectroscopic observations of a sample of 42 flat-spectrum radio sources fromthe Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of =1. 27 with an RMS spread of 0. 95, at a completeness level of 64The sampleconsists of sources with a 5-GHz flux density of 25-50 mJy, making it thefaintest flat-spectrum radio sample for which the redshift distribution hasbeen studied. The spectra, obtained with the Willam Herschel Telescope (WHT), consist mainly of broad-line quasars at z>1 and narrow-line galaxies atz<0. 5. Though the mean redshift of flat-spectrum radio sources exhibitslittle variation over more than two orders of magnitude in radio flux density, there is evidence for a decreasing fraction of quasars at weaker flux levels. In this paper we present the results of our spectroscopic observations, anddiscuss the implications for constraining cosmological parameters withstatistical analyses of the CLASS survey.
Marlow et al. (Thu,) studied this question.
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