A FIRST–APM–SDSS survey for high‐redshift radio QSOs

We selected from the VLA FIRST survey a sample of 94 objects with star‐like counterparts in the Sloan Digital Sky Survey (SDSS), and with APM POSS‐I colour O − E ≥ 2 , i.e. consistent with their being high‐redshift quasars. 78 of the 94 candidates can be classified spectroscopically on the basis of either published data (mainly SDSS) or the observations presented here. The fractions of QSOs (51 out of 78) and redshift z > 3 QSOs (23 out of 78, 29 per cent) are comparable to those found in other photometric searches for high‐redshift QSOs. We confirm that selecting colour O − E ≥ 2 ensures inclusion of all QSOs with 3.7 ≤ z ≤ 4.4 . The fraction of 2 ≤ z ≤ 4.4 QSOs with broad absorption lines (BALs) is 27 ± 10 per cent (7/26) and the estimated BAL fraction for radio‐loud QSOs is at least as high as for optically selected QSOs (∼13 per cent). Both the high BAL fraction and the high fraction of low‐ionization BALs among BALs (four to five out of seven) in our sample, compared to previous work, are likely due to the red colour selection O − E ≥ 2 . The space density of radio‐loud QSOs in the range with M AB (1450) ≤−26.6 and P ≥ 10 25.7  W Hz −1 is 1.7 ± 0.6 Gpc −3 . Adopting a radio‐loud fraction of 13.4 ± 3 per cent, this corresponds to ρ= 12.5 ± 5.6 Gpc −3 , in substantial agreement with the cumulative luminosity function of SDSS QSOs in Fan et al. We note the unusual flat‐spectrum radio‐luminous QSO FIRST 1413+4505 ( z = 3.11) , which shows strong associated Lyα absorption (rest‐frame equivalent width ∼40 Å) and an extreme observed luminosity, L ∼ 2 × 10 15  L ⊙ .