Discovery of radio-loud quasars with z=4.72 and z=4.010 |
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Authors: | Isobel M Hook & Richard G McMahon |
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Institution: | University of California at Berkeley, Astronomy Department, Berkeley, CA 94720, USA,;Institute of Astronomy, Madingley Road, Cambridge CB30HA |
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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 P 5 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. |
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Keywords: | quasars: general quasars: individual: GB1428+4217 quasars: individual: GB1713+2148 |
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