A search for high-redshift quasars among GB/FIRST flat-spectrum radio sources

Abstract

We present the method and first results of a survey for high-redshift (z > 3) radio-loud quasars, which is based on optical identifications of 2902 flat-spectrum radio sources with S5GHz ≥ 25 mJy. The radio sample was defined over a 1600 deg2 region using the 5-GHz Green Bank survey and the 1.4-GHz VLA FIRST survey. 560 sources were identified to a limit of E= 19.5 on APM scans of POSS-I plates, and 337 of these optical counterparts are unresolved. From these a complete sample of 73 sources for spectroscopic follow-up was defined based on criteria of red (O − E ≥ 1.2) optical colour. We have obtained spectra for 36 of these, and an additional 14 have redshifts in the literature, thus 70 per cent of the spectroscopic sample is completed. Six objects in the sample were found to be radio-loud quasars with z > 3, of which two were previously known. The efficiency of the spectroscopic phase of the survey is therefore about 1 in 9, whereas without the colour selection criterion the efficiency would have been 1 in sim; 40. The six z > 3 quasars were found in an effective area of 1100 deg2, implying a surface density of one flat-spectrum z > 3 radio-loud quasar per 190 deg2 to limits of E= 19.5 and S5GHz ≥ 25 mJy. This survey has also produced the first known radio-loud BAL quasar, 1556+3517 with z = 1.48, which has been reported in an earlier paper. This object has a redder optical colour (O − E = 2.56) than all the z > 3 quasars found in this survey to date. In addition, we have obtained spectra of 22 GB/FIRST sources that are not part of the complete sample. We give positions, E (red) magnitudes, O − E colours, radio fluxes, radio spectral indices and redshifts where possible for objects for which we have obtained spectra. We give spectra and finding charts for the z > 3 quasars.