QSOs from the variability and proper motion survey in the M 3 field

We present results of the spectroscopical follow-up observations of QSO candidates from a combined variability and proper motion (VPM) survey in a ∼ 10 square degrees region centered on the globular cluster M 3. The search is based on a large number of digitised Schmidt plates with a time-baseline of three decades. This paper reviews the candidate selection, the follow-up spectroscopy, and general properties of the resulting QSO sample. In total, 175 QSOs and Sey1s were identified among the objects from the VPM survey, with 114 QSOs and 10 Sey1s up to the pre-estimated 90% completeness limit of the survey at Blim 19.7. The redshift range of the QSOs is 0.4 < z < 3. Among the 80 QSO candidates of highest priority we confirm 75 QSOs/Sey1s and 2 NELGs. We present magnitudes, colours, redshifts, and variability indices for all 181 identified QSOs/Sey1s/NELGs and spectra for the 77 QSOs/Sey1s/NELGs from our spectroscopic follow-up observations. The VPM survey uses selection criteria that are not directly relying on the spectral energy distribution of QSOs. It is therefore remarkable that the properties of the VPM QSOs do not significantly differ from those of samples from colour selection or slitless spectroscopy. In particular, we do not detect a substantial number of unusually red QSOs. The total surface density of the brighter QSOs (17 &le B &le 18.5) in our search field is found to be by a factor of ∼ 1.8 larger than that derived from previous optical surveys.