Radio Loud Quasars at High Redshift

Abstract

We present results from an observational investigation into the evolution of the luminosity function of flat spectrum radio loud quasars based on a new survey for radio loud quasars with z>3. Rather than carry out an indiscriminate redshift campaign on all the radio sources in our sample, we use optical colours to preselect red stellar identifications prior to optical spectroscopy. The resultant sample of 12 quasars with z>3, identified on POSS-I and including one with z>4, is used to make the first explicit derivation of a luminosity function for a radio source population at z>3. An extension of the survey to fainter optical flux limits has resulted in the discovery of two more z>4 quasars which have been used to further constrain the evolution at high redshift. We find that the space density of the strongest radio sources decreases by a factor of two between z∼2 and z∼4. One of the quasars GB1428+4217, with z=4.72, is the highest red-shift radio source known. More importantly, it has been serendipitously detected in an archival ROSAT PSPC image, and subsequently confirmed with a ROSAT HRI images with a flux of 10^-12erg cm^-2s^-1, in the (observed) 0.1-2.4 keV band, making it the brightest X-ray source known above a redshift of four.