The Canada‐France‐Hawaii Telescope Imaging Survey of BL Lacertae Objects. II. Clustering Environments

Abstract

The results of an extensive imaging survey of BL Lac objects conducted at the Canada-France-Hawaii 3.6 m Telescope (CFHT) are presented. This paper details the results pertinent to the clustering environments of BL Lacs; a companion paper (Wurtz, Stocke, & Yee) 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 1, 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, X-ray vs. radio-selected, etc.) have statistically similar clustering properties, which further argues that these BL Lacs are all members of the same active galactic nucleus (AGN) class. However, we do find that, like radio-loud quasars, BL Lac environments are significantly richer at high redshift (for z > 0.35, the median Bgb = 500 Mpc1.77 compared to a median Bgb = 120 Mpc1.77 at lower z). Correlations are also found between Bgb 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 low-z FR 1's are in richer clusters than almost all low-z BL Lacs (PKS 0548-322 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 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).