The environments of BL lacertae objects

This thesis reports the results of an environmental study of seventeen BL Lacertae objects (BL Lacs). We performed a detailed, multicolor analysis of the fields around the BL Lacs, and obtained spectra of selected galaxies near a subgroup of seven objects. We find that BL Lac objects are often located in regions of higher-thanaverage galaxy density. The host galaxies of BL Lacs are probably the dominant members of small groups or clusters of galaxies, rarely richer than Abell richness class 0. The richest environment of the sample is around the X-ray selected object, H 0414+009, also the first BL Lac to be found in such a cluster. Spectroscopy gives direct proof of the physical association between the BL Lac and at least one other galaxy in the field in six of seven cases observed. For five sources we note the BL Lac has a very close (<~ 5 arcsec) companion galaxy, which is, perhaps, in the process of interacting. Spectra confirm the association of a galaxy, with a projected distance of 11 kpc from the bright BL Lac, PKS 2155-304, and of galaxies within 90 kpc of three objects. The imaging results for BL Lacs are compared to those (taken from tne literature) for F-R I and II radio galaxies, the proposed BL Lac parent population. While the scarcity of the data available hinders our interpretation, EL Lac environments are consistent with those of both F-R I and II galaxies.