Discovery of radio-loud quasars with z = 4.72 and z = 4.01

Abstract

We report the discovery of two radio-loud quasars with redshifts greater than 4: GB1428 + 4217, with z = 4.72, and GB1713 + 2148 with z = 4.01. This doubles the number of published radio-selected quasars with z > 4, bringing the total to 4. GB1428 + 4217 is the third most distant quasar known and the highest redshift radio and X-ray source currently known. It has a radio flux density at 5 GHz of 259 ± 31 mJy and an optical magnitude of R ∼ 20.9. The rest frame absolute UV magnitude, Mv (1450 Å), is - 26.7, similar to that of the archetypal radio-selected quasar 3C273 [z = 0.158; Mv (1450 Å)= -26.4]. GB1428 + 4217 is tentatively detected in ROSAT PSPC observations, which has been confirmed by more recent ROSAT observations described in a companion paper by Fabian et al. Both quasars were discovered during the CCD imaging phase of an investigation into the evolution of the space density of radio-loud quasars at high redshift. Combined with our earlier survey results, these objects give a lower limit on the space density of quasars with radio power P5 GHz > 5.8 × 1026 W Hz-1 sr-1 between z = 4 and z = 5 of 1.4 ± 0.9 × 10-10 Mpc-3. This can be compared to 2.9 ± 0.2 × 10-10 Mpc-3 at z = 2 from Dunlop & Peacock for flat-spectrum sources of the same luminosity.