THE CANADA-FRANCE-HAWAII TELESCOPE IMAGING SURVEY OF BL LACERTAE OBJECTS .2. CLUSTERING ENVIRONMENTS

The results of an extensive imaging survey of BL Lac objects conducted at the Canada-France-Hawaii 3.6 m Telescope (CFHT) are presented. Thi s paper details the results pertinent to the clustering environments o f BL Lacs; a companion paper (Wurtz, Stocke, & Yee) presented results on the host galaxies of BL Lacs obtained from the same Gunn r-band ima ges. The clustering environments of BL Lac objects in this survey are on average found to be poor clusters, comparable in galaxy density to Abell richness class <0. Using the formalism developed by Longair & Se ldner and the specific techniques employed by Yee & Green, a mean valu e for the galaxy-BL Lac two-point correlation function amplitude was f ound to be [B-gb] = 209 Mpc(1.77) +/- 386 (mean +/-1 sigma spread for H-0 = 50 km s(-1) Mpc(-1) and q(0) = 0.02). Only a few (at most six of 45) BL Lacs in this sample are found in clusters with richness class > 1, and most of these are at z > 0.4. Various subsamples of BL Lacs w ith unique qualities (e.g., presence/absence of weak emission lines, h igh/low optical core dominance or polarization, X-ray vs. radio-select ed, etc.) have statistically similar clustering properties, which furt her argues that these BL Lacs are all members of the same active galac tic nucleus (AGN) class. However, we do find that, like radio-loud qua sars, BL Lac environments are significantly richer at high redshift (f or z > 0.35, the median B-gb = 500 Mpc(1.77) compared to a median B-gb = 120 Mpc(1.77) at lower z). Correlations are also found between B-gb and host galaxy luminosity and radio core dominance. Contrary to the expectations of unification schemes for BL Lacs, the clustering enviro nments of BL Lacs, at both high and low redshift, are more similar to those of FR 2 radio galaxies and quasars than to those of FR l's. Appr oximately 20% of low-z FR 1's are in richer clusters than almost all l ow-z BL Lacs (PKS 0548 - 322 is the lone exception of a BL Lac in a ri ch cluster); similarly, 20% of FR 1's have more luminous host galaxies than any BL Lac (Paper I). This new line of evidence strongly suggest s that the unification scenario for BL Lacs with FR 1 radio galaxies r equires a critical reexamination. As a minimum the ''parent population '' of BL Lacs must be modified to exclude the brightest cluster galaxi es (BCGs) in rich clusters at low redshift. The great similarity in th e cosmic evolution of the cluster environment for radio-loud quasars, FR 2 radio galaxies, and BL Lac objects between z similar to 0.5 and 0 strongly suggests that a common physical mechanism operates to create a rapid luminosity evolution for AGNs in rich clusters. Since X-ray o bservations find rapid cosmic evolution in the intracluster medium and cluster potential well over similar timescales, the rapidly changing gas density and/or galaxy-galaxy interaction rate could be responsible for the fading of luminous AGNs in rich clusters (Stocke & Perrenod; Roos).