A quasar with z = 3.71 and limits on the number of more distant objects

On the redshift cut-off for flat-spectrum radio sources

“…Investigations into the comoving space density ρ of flat‐spectrum radio sources selected at 2.7 GHz (Peacock 1985, hereafter P85; Dunlop & Peacock 1990, hereafter DP90), have found evidence of a large increase in ρ out to redshift and seemingly strong evidence for a decline in ρ at higher redshifts. This high‐redshift decline, regardless of its magnitude, has come to be known as the ‘redshift cut‐off’, a term which was first introduced by Sandage (1972).…”

“…Dunlop 1998). With these studies in mind, the second motivation of this paper was to determine whether the cosmic evolution of the high‐redshift flat‐spectrum population provides indirect evidence for a cut‐off which, in our opinion, has yet to be established beyond doubt in the parent radio‐luminous steep‐spectrum population (but see DP90, Dunlop 1998).…”

“…As emphasized by P85, DP90, SH96 and others, the crucial advantage of any radio‐based work is that with sufficient optical follow‐up, it can be made free of optical selection effects, such as increasing dust obscuration at high redshift. It is chiefly for this reason that the SH96 work is often highlighted as the most convincing evidence to date for the existence of any redshift cut‐off for the active galaxy population.…”

The statistics of radio galaxies & quasars at high redshift

“…The remaining sources are almost all galaxies, for which photometric redshift estimation should be possible. The most spectacular result of the spectroscopy programme was the spectrum of 1351-018 (Dunlop et al 1985).…”

“…Dunlop et al (1985) show that objects with weak lines like 1351-018 will tend to have stellar UJF colours at ζ 2.5, and so be missed. We must await confirmation of Koo's result from CCD grism surveys sensitive to Lyct equivalent widths W < 100Â.…”