5 Conclusions

We performed a VPM QSO survey with a limiting magnitude of $B_{\rm lim} \approx 20$ in a 10 square degrees field at high galactic latitude. The VPM technique proved to be an efficient method for finding QSOs. As the result of the spectroscopic follow-up observations of 198 candidates and the identification of further candidates in the NED, a sample of 175 QSOs/Sey1s with 0.4<z<3 is available. With a stellar contamination of only about 4%, the high-priority QSO subsample from the VPM search is very clean. For the combined sample of high-and-medium-priority objects the fraction of established QSOs/Sey1s among all candidates is still as high as 63%. The completeness of the VPM QSO sample with $B\le 19.7$ is estimated to be 94%. The number-magnitude relation for that sample is in good agreement with the one expected from the relation derived by Wisotzki (1998) from various QSO samples. At brighter magnitudes (B<18.5), we find a somewhat higher QSO surface density.

The optical broad-band colours and the spectra of the VPM-selected QSOs are not significantly different from those of QSOs selected by other optical surveys, in agreement with what we found in the M 92 field (Meusinger & Brunzendorf 2001; Brunzendorf & Meusinger 2002). Such a result can not be a priori expected since the selection criteria of the VPM survey are completely different from those in most other optical surveys. Although there is a large fraction of objects with red colours among the VPM QSO candidates, all candidates with extremely red colours were proved to be stellar contaminants. We estimate that the fraction of QSOs with unusualy red optical colours is at most a few per cent up to the limit of the survey, provided that their variability properties are not significantly different from those of the other QSOs. Some BAL QSOs are known to be considerably redder than the targets of most QSO surveys (e.g., Weymann et al. 1991; Menou et al. 2001; Hall et al. 2002). The fraction of such unusual QSOs in the (incomplete) SDSS Early Data Release is less than 1% (Hall et al. 2002), in good agreement with our result. In this context it is notable that all VPM QSOs with indication for substantial absorption are strongly variabel ( $I_{\rm var} > 1.5$). A VPM search is thus expected to be essentially unbiased against strongly absorbed QSOs, apart from the bias introduced by the band-pass of the search. The general agreement of the properties of the VPM QSO sample with those from more conventional optical surveys suggests that the latter do obviously not ignore a substantial number of red QSOs up to $B \approx 20$. On the other hand, we can conclude that the VPM survey can be combined with colour search criteria in order to reach a very high efficiency without a significant loss of completeness. Of course, we can not exclude the existence of substantial numbers of obscured red QSOs that are fainter than the current survey limit. Such objects can be found by a deeper VPM survey.

Acknowledgements

This research is based on observations made with the 2.2 m telescope of the German-Spanish Astronomical Centre, Calar Alto, Spain. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.