AN IMAGING STUDY OF THE ENVIRONMENTS OF RADIO-SELECTED BL LAC OBJECTS

We present direct imaging data of the fields of BL Lac objects drawn from the complete 1 Jy sample. An increase in galaxy surface density towards the BL Lac objects in subsamples with mean redshifts < z > = 0.28 and < z > = 0.65 is found. The covariance amplitudes A(gBL)(< Z > = 0.28) = 215 and A(gBL)(< z > = 0.65) = 515 correspond roughly to galaxy clusters of richness classes 0 and 1, respectively. Similar covariance amplitudes have been found for radio galaxies; our result therefore can be naturally understood if the BL Lac phenomenon is due to relativistic beaming, i.e. the BL Lac objects are FRI radio galaxies with the jet pointing towards us. In contrast, a constant galaxy surface density is found for the sub-sample with mean redshift < z > = 0.97. This would also be expected under the beaming hypothesis, since cluster galaxies at z almost-equal-to 1 would be well below our detection limit. However, since at least in one case we find strong evidence for gravitational lensing, we suggest that the high redshift BL Lac objects are not a homogeneous class of objects, but rather contain both beamed and lensed objects.