The Redshift Distribution of Flat-Spectrum Radio Sources at Low Flux-Densities

Gravitational lens statistics can be used to constrain the cosmological parameters providing one has a statistically complete sample. The relative size and completeness of CLASS coupled with its stringent search criteria make it ideal for performing a lens statistics analysis. A major uncertainty in the analysis is the poor statistical information known about the unlensed parent population at low flux-densities. These sources could potentially be lensed into the CLASS sample due to the lensing amplification. We conducted a pseudo-survey to find sources at low flux-densities (5 to 12 mJy). These sources were selected in exactly the same manner as the CLASS, S >= 30 mJy, sample. Objects in our final sample of 36 flat-spectrum radio sources were observed photometrically (in 6 colours) and spectroscopically depending on their optical brightness. The aim of these observations was to obtain redshifts which would fill the gaps in our knowledge of the redshift distribution at low flux-densities. These results, as well as forming a vital part of the CLASS lens statistics analysis, will also contribute to our understanding of the flat-spectrum radio source population at low flux-densities.