THE CFHT IMAGING SURVEY OF BL LACERTAE OBJECTS II:

The results of an extensive imaging survey of BL Lac Objects conducted at the Canada­France­Hawaii 3.6m Telescope (CFHT) are presented. This paper details the results pertinent to the clustering environments of BL Lacs; a companion paper (Wurtz, Stocke & Yee 1996; Paper 1) presented results on the host galaxies of BL Lacs obtained from the same Gunn r-band images.

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 & Seldner (1979) and the specific techniques employed by Yee & Green (1987), a mean value for the galaxy­BL Lac two­point correlation function amplitude was found to be: <B_gb> = 209 Mpc^1.77 + 386 (mean + 1 sigma spread for H₀ = 50 km s^-l Mpc^­l and q₀ = 0.02). Only a few (at most six of 45) BL Lacs in this sample are found in richness class > 1 clusters and most of these are at z > 0.4.

Various subsamples of BL Lacs with unique qualities (e.g., presence/absence of weak emission lines, high/low optical core dominance or polarization, X­ray vs. radio selected, etc.) have statistically similar clustering properties further arguing that these BL Lacs are all members of the same AGN class. However, we do find that, like

radio­loud quasars, BL Lac environments are significantly richer at high redshift (for z > 0.35 the median B_gb = 500 Mpc^1.77 compared to a median B_gb = 120 Mpc^l.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 environments 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 1's. Approximately 20% of low­z FR 1's are in richer clusters than almost all low­z BL Lacs (PKS 0548­322 is the lone exception of a BL Lac in a rich cluster); similarly 20% of FR 1's have more luminous host galaxies than any BL Lac (Paper I). This new line of evidence strongly suggests that the unification scenario for BL Lacs with FR 1 radio galaxies requires a critical reexamination. As a minimum the "parent population" of BL Lacs must be modified to exclude the brightest cluster galaxies (BCGs) in rich clusters at low redshift.

The great similarity in the cosmic evolution of the cluster environment for radio-loud quasars, FR 2 radio galaxies, and BL Lac objects between z ~ 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 observations find rapid cosmic evolution in the intracluster medium and cluster postential well over similar timescales, the rapidly changing gas density and/or galaxy-galaxy interaction rate could be responsible for the rading of luminous AGNs in rich clusters (Stocke & Perrenod; Roos).

Colorado Astrophysics Preprint #303
to appear in Astrophysical Journal