The CFHT Imaging Survey of BL Lacertae Objects.

John T. Stocke, University of Colorado.

Several years ago (so long ago that our instrument of choice was FOCAM), myself, Ron Wurtz (a PhD student at the University of Colorado), Howard Yee (Toronto) and Erica Ellingson (Colorado) embarked on a major imaging survey of BL Lacertae Objects. Interest in this very rare class of AGN has been heightened recently by their prominence among Gamma-ray emitters detected by Compton GRO and with ground-based Cerenkov light/air shower detectors on Mt. Hopkins in Arizona (e.g., the only two extragalactic detections at 1023 eV are the nearby BL Lac Objects Markarian 421 & 501). We conducted this survey for two reasons: (1) BL Lacs had not been studied extensively at optical wavelengths as had their more numerous "relatives" the quasars (e.g., extensive work by John Hutchings and others) and (2) any spatially extended emission seen for BL Lacs is almost certainly not affected by relativistic beaming, which is thought to play a major role in the observable properties of these objects. The "unbeamed" properties of these rare objects would then allow a direct comparison with other classes of AGN. Although some of what we found was expected based upon previous work (e.g., we found no differences between radio and X-ray loud BL Lacs), what also emerged was that the "standard view" of these objects as low-power radio galaxies (Fanaroff-Riley class 1 = FR 1s) viewed directly down the axis of a relativistic jet is over-simplified at best or at worst, just plain wrong (see Urry & Padovani 1995 PASP 107, 803).

Deep wide-field FOCAM images of 50 BL Lac Objects were obtained in Jan and Aug of 1991 primarily using a Gunn r-band filter. Examples of the data quality are shown in Figures 6 & 7, for two high-z BL Lacs (1207+39 at z=0.615 and 1407+59 at z=0.495), which show the point source surrounded by a luminous elliptical host galaxy (M(r)=-23.9 in both cases) and by a rich cluster of galaxies (Abell classes 2 & 1 respectively). However, these objects are somewhat extreme in that most BL Lacs surveyed were found in somewhat less luminous elliptical hosts (<M(r)=-23.3) and in significantly poorer clusters (Abell class < 0). Our survey also found a significant evolution in cluster richness surrounding BL Lac Objects that is identical in amount and redshift range (z=0 to 1/2) to that found for quasars by Ellingson, Yee & Green (1991 ApJ 371, 49).

These statistics are difficult to reconcile with the beamed FR 1 picture espoused by Urry, Padovani & others because many FR 1s are found in brightest cluster galaxies (BCGs) in rich clusters of galaxies at the present epoch. Typical current-epoch BCGs are at least 1/2 mag brighter than the low-z BL Lacs in the CFHT survey sample. Further, with a sample size of 50 objects we could now perform a detailed statistical comparison between BL Lac host galaxies and environs and that of other AGN classes. By far the best matches came not with the FR 1s but with the FR 2s, the higher-power "classical double" radio sources typical among quasars. But the extended radio power levels of most BL Lacs are fully consistent with FR 1 power levels (see e.g., Perlman & Stocke 1993 ApJ 406, 430), leaving us with a quandry: Is the beamed FR 1 model for BL Lacs fatally flawed in some way we don't clearly understand or can it remain consistent with the new and old data through a minimal modification of the "parent population" of BL Lacs which specifically excludes BCGs in rich clusters ? While the latter suggestion seems explicitly "ad hoc" there is some theoretical support for this idea from recent work by Geoff Bicknell (1994 ApJ 422,542; 1995 ApJS 101, 29). Whatever the resolution to the current quandry involving the enigmatic BL Lac Objects, we are faced with a result in which the environs of all radio-loud AGN (BL Lacs, quasars & luminous radio galaxies) change in the same way over the same timespan. This strongly suggests a common physical mechanism for this evolution, which could well be the evolution of the intracluster gas medium in rich clusters, which also changes significantly over the same timeframe (z=0 to 1/2; Gioia et al. 1990 ApJ 356, 35).

A detailed discussion of these results may be found in Wurtz, Stocke & Yee (1996 ApJS 103, 109) for host galaxies and Wurtz, Stocke, Ellingson & Yee (1997 May ApJ, in press) for cluster environments. As always, more data are needed and now are very possible with higher resolution imaging at CFHT than what we used several years ago.
Copyright © 1997, Canada-France-Hawaii Telescope Corp. All rights reserved.