GB1508+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 similar to 300 flat-spectrum radio sources with S-5GHz > 200 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 similar to 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) similar to 19.