Properties of BL Lac objects from the 2dF QSO Redshift Survey

This thesis is an analysis of the X-ray, optical, infrared and radio properties of a sample of candidate BL Lac objects, identified from two catalogues of colour-selected point sources, the 2QZ and 6QZ. The importance of the sample lies in the fact that it is the first BL Lac sample in which initial selection has been made from optical spectra, independent of the objects' flux levels at X-ray and radio frequencies. These optically selected candidate BL Lac objects thus provide an unbiased sample (in terms of radio flux density) with which to study the proportion of radio- dominant and X-ray-dominant BL Lac objects in the global population. The observed number counts and redshift distribution of the 2BL are consistent with theoretical predictions based on the QSO evolutionary model. Given the small number statistics we are not, however, able to show that this distribution is significantly different to that of the 2QZ/6QZ white dwarfs. A median redshift of z=1.25 was computed for these candidate BL Lacs, a value supported by redshift information obtained for ~25% of the sample. This median redshift is much higher than redshifts found for X-ray selected BL Lac objects and suggests that in the past high redshift, low luminosity, radio-weak BL Lacs may have been missed in radio- and X-ray-selected samples. From results of the above studies it appears that this sample of optically selected candidate BL Lac objects is different to that of hitherto known radio-loud BL Lacs. We explore mechanisms that might produce such radio-weak/radio-quiet continuum objects, and also demonstrate that such radio-quiet BL Lac objects could exist at low redshift. Failure to include these radio-quiet BL Lacs in X-ray selected samples could explain the negative evolution found for this class of object