The 2QZ BL Lac survey

We have optically identified a sample of 56 featureless continuum objects without significant proper motion from the 2dF quasi-stellar object (QSO) redshift survey (2QZ). The steep number–magnitude relation of the sample, , is similar to that derived for QSOs in the 2QZ and inconsistent with any population of Galactic objects. Follow-up high-resolution, high signal-to-noise spectroscopy of five randomly selected objects confirms the featureless nature of these sources. Assuming the objects in the sample to be largely featureless active galactic nuclei (AGN), and using the QSO evolution model derived for the 2QZ, we predict the median redshift of the sample to be z= 1.1. This model also reproduces the observed number–magnitude relation of the sample using a renormalization of the QSO luminosity function, . Only ∼20 per cent of the objects have a radio flux density of S1.4 > 3 mJy, and further Very Large Array observations at 8.4 GHz place a 5σ limit of S8.4 < 0.2 mJy on the bulk of the sample. We postulate that these objects could form a population of radio-weak AGN with weak or absent emission lines, whose optical spectra are indistinguishable from those of BL Lac objects.