A search for distant radio-loud quasars in the CLASS survey: three new radio-selected quasars at z>4

We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S5GHz > 30 mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e < 19.0 and red o−e > 2.0 colours, in an effective area of ∼ 6400 deg. Four sources were found to be quasars with z > 4, of which one was previously known. This sample bridges the gap between the strong radio surveys with S5GHz > 200 mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars. In addition, 4 new quasars at z > 3 have been found. The selection criteria result in a success-rate of ∼1:7 for radio-loud quasars at z > 4, which is a significant improvement over previous studies. This search yields a surface density of 1 per 1600 deg, which is about a factor of ∼15 lower than that found in a similar search for radio-quiet quasars at z > 4. The study presented here is strongly biased against quasars beyond z > 4.5, since the e-passband of the POSS-I only samples the spectra shortward of 1200Å at these redshifts.