Abstract
We obtained spectra of 60 red, star-like objects (E < 18.8) identified with FIRST radio sources, S1.4 GHz > 1 mJy. Eight are quasi-stellar objects (QSOs) with redshift z > 3.6. Combined with our earlier pilot search, our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8 per cent of candidates with S1.4 GHz< 10 mJy and 37 per cent of candidates with S1.4 GHz> 10 mJy are QSOs with z > 3.6. The surface density of E < 18.8, S1.4 GHz > 1 mJy, z > 4 QSOs is 0.003 deg−2. This is currently the only well-defined sample of radio-loud QSOs at z≈ 4 selected independently of radio spectral index. The QSOs are highly luminous in the optical (eight have MB < −28, q0= 0.5, H0= 50 km s−1 Mpc−1). The SEDs are as varied as those seen in optical searches for high-redshift QSOs, but the fraction of objects with weak (strongly self-absorbed) Lyα emission is marginally higher (3 out of 18) than for high-redshift QSOs from SDSS (5 out of 96).
Highlights
The evolution with redshift of the space density of quasi-stellar objects (QSOs) places strong constraints on the abundance of massive objects at the earliest cosmological epochs
High-redshift QSOs can be found efficiently by colour selection from very large samples, but space-density measurements based on such selection could be biased by e.g. redshift-dependent dust extinction
Any such bias is reduced when selecting in the radio, but searches for high-redshift radio QSOs have so far concentrated on radiobright objects with flat radio spectra, which yield small samples with relatively low surface density on the sky
Summary
The evolution with redshift of the space density of quasi-stellar objects (QSOs) places strong constraints on the abundance of massive objects at the earliest cosmological epochs. High-redshift QSOs can be found efficiently by colour selection from very large samples (e.g. the Sloan Digital Sky Survey, Anderson et al 2001, see Benn et al 2002, hereafter Paper I, for a summary of previous searches), but space-density measurements based on such selection could be biased by e.g. redshift-dependent dust extinction. Any such bias is reduced when selecting in the radio, but searches for high-redshift radio QSOs have so far concentrated on radiobright objects with flat radio spectra, which yield small samples with relatively low surface density on the sky. This includes the largest sample to date of radio-selected QSOs at z > 4
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