4 Discussion

By using radio and optical multicolour data covering a significant fraction of the sky, we have produced a sample of high-redshift, optically bright radio-loud quasars.

Since this new quasar sample is well-defined, we can use it to estimate the surface density of z>4 quasars. To do this we consider objects with radio flux densities above the brightest limit of the PMN surveys, 72 mJy. The SGC region of our survey has a high completeness of spectroscopic follow up (only two objects, or 1%, were not observed in this region) and also had a high completeness of the NVSS survey (0.955). There are four z>4 QSOs in the SGC region of the complete sample, which implies a surface density of $\rm0.92\pm 0.5\times 10^{-3}~ sq\ deg^{-1}$ for z>4 QSOs with $S\ge 72$ mJy.

If the whole survey is considered, and the completeness for the SGC and NGC are taken as 94% and 61% respectively (as implied by the number of spectroscopically observed candidates in each region combined with the NVSS completeness factors derived in Sect. 2) then the derived surface density is $\rm 1.0\pm 0.4\times 10^{-3} ~sq\ deg^{-1}$. This is fully consistent with the value determined from the SGC alone.

This value is similar to that of 1 per 1600 sq deg ( $6.3\times 10^{-4}$) found by Snellen et al. (2001) using a similar technique although with a slightly different radio and optical flux density limits (R= 20 compared to R=21 for the current survey, $S\ge 30$ mJy compared to S>72 mJy, an upper redshift limit defined by the red optical filter of $z\sim4.5$ rather than $\sim$4.7, and a radio spectral index cut at -0.35 rather than -0.5).

Figure 6: IR spectrum of PMN J1451-1512, obtained at NTT.

There were no quasars found in our survey with z>4.76 despite the fact that bright quasars with $4.9\la z\la 6.3$ should have been detectable on the I-plates. Allowing for the 86% completeness of spectroscopic follow up of the I-band sample, the effective area covered was 2930 sq deg. Therefore we derive an upper limit of $\rm 3.4\times 10^{-4}~ sq\ deg^{-1}$ for the surface density of flat spectrum quasars with I<19.5, $S_{5~\rm GHz}\ge \rm 25~mJy$ and  $4.9\la z\la 6.3$.

Finally, the new sample of quasars presented in this paper represent some of the most luminous objects in the Universe and may also represent extreme peaks in the matter density distribution at high-redshift. They are therefore ideal targets for various follow-up programs such as high-redshift absorption line studies and searches for associated high-redshift clusters.

Acknowledgements

RGM thanks the Royal Society for support. We thank Jason Spyromilio for providing the IR spectrum of PMN J1451-1512.

We also thank Mike Irwin and the staff at the APM facility in Cambridge for producing the scans of UKST plates used in this survey.

This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation.