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

Abstract

Spectroscopy of objects identified with radio sources from the Parkes Selected Regions has shown the compact, flat-spectrum source 1351–018 to be a quasar with redshift 3.71, the second most distant discovered to date. Compared with known quasars at such high redshifts, the object is optically very faint (B = 20.9), red in colour (B-R = 1.6) and is distinguishable photometrically from faint galaxies only by its relatively blue optical-infrared colour (R-K =2.1). In common with other high-redshift, radio-loud quasars, 1351–018 has weak emission lines when compared with objects selected optically through objective-prism or grism techniques. Moreover, weak-lined objects such as this may also be missed even by surveys using broad-band optical colours, which may help to explain why such searches have failed to find faint, high-redshift quasars. Because it is a member of a complete sample, 1351–018, in fact, confirms the recent suggestion1 that the quasar radio luminosity function declines for redshifts z>2. Extrapolation of the existing data predicts that the main Parkes Survey may contain about eight 8 flat-spectrum quasars with z>4.