GB:1508+5714: a radio-loud quasar with z = 4.30 and the space density of high-redshift radio-loud quasars

We report the discovery of a radio-loud quasar with a redshift of 4.30. This object, which is the first radio-selected quasar with a redshift greater than 4, was discovered during an observational investigation into the evolution of the luminosity function of radio-loud quasars. Here we describe results based on a sample of ∼ 300 flat-spectrum radio sources with $$S_{5GH_z}\gt200$$ mJy. In this study, rather than carry out an indiscriminate redshift campaign on all the radio sources, we have used the APM POSS catalogue to pre-select a subset of stellar-like optical counterparts with red optical colours. Such a subsample is expected to contain a high fraction of high-redshift quasars. 10 of the ∼ 300 sources were selected for follow-up optical spectroscopy, and three of these are identified as quasars with redshifts greater than 3.0. One of the radio sources, GB1508+5714, is a quasar with z = 4.30 and m(R)~19.