BL Lac Host Galaxies Research Articles

The redshift and the host galaxy of the neutrino candidate 4FGL J0955.1+3551 (3HSP J095507.9+355101)

ABSTRACT The BL Lac object 4FGL J0955.1+3551 has been suggested as a possible source of ultra-energetic neutrinos detected by the IceCube observatory. The target was observed in 2020 January at the Large Binocular Telescope. Our spectroscopy (4100–8500 Å) yields a firm redshift z = 0.557 as deduced by the absorption lines of the host galaxy. The upper limit of the minimum equivalent width on emission lines is ∼0.3 Å. From the source image, we are able to resolve the host galaxy for which we measure an absolute magnitude M(R) = −22.9 and Re = 8 kpc, which are values which are typical of the host galaxies of BL Lacs.

Resolving the host galaxy of a distant blazar with LBT/LUCI 1 + ARGOS

BL Lac objects emitting in the Very High Energy (VHE) regime are unique tools to peer into the properties of the Extragalactic Background Light (EBL). However, due to the typical absence of features in their spectra, the determination of their redshifts has proven challenging. In this work we exploit the superb spatial resolution delivered by the new Advanced Rayleigh guided Ground layer adaptive Optics System (ARGOS) at the Large Binocular Telescope to detect the host galaxy of HESS J1943+213, a VHE emitting BL Lac shining through the Galaxy. Deep H-band imaging collected during the ARGOS commissioning allowed us to separate the contribution of the nuclear emission and to unveil the properties of the host galaxy with unprecedented detail. The host galaxy is well fitted by a S\'ersic profile with index of n~2 and total magnitude of H~16.15 mag. Under the assumption that BL Lac host galaxies are standard candles, we infer a redshift of z~0.21. In the framework of the current model for the EBL, this value is in agreement with the observed dimming of the VHE spectrum due to the scatter of energetic photons on the EBL.

The Blazar Sequence 2.0

This paper discusses the spectral energy distribution (SED) of all blazars with redshift detected by the Fermi satellite and listed in the 3LAC catalog. The so called “blazar sequence” from the phenomenological point of view will be updated, with no theory or modelling. It will be shown that: (i) pure data show that jet and accretion power are related; (ii) the updated blazar sequence maintains the properties of the old version, albeit with a less pronounced dominance of the γ-ray emission; (iii) at low bolometric luminosities, two different types of objects have the same high energy power: low black hole mass flat spectrum radio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a very large dispersion of SED shapes; (iv) in low power BL Lacs, the contribution of the host galaxy is important. Remarkably, the luminosity distribution of the host galaxies of BL Lacs are spread in a very narrow range; (v) a simple sum of two smoothly joining power laws can describe the blazar SEDs very well.

First broadband characterization and redshift determination of the VHE blazar MAGIC J2001+439

We aim to characterize the broadband emission from 2FGL J2001.1+4352, which has been associated with the unknown-redshift blazar MG4 J200112+4352. Based on its gamma-ray spectral properties, it was identified as a potential very high energy (VHE; E > 100 GeV) gamma-ray emitter. The source was observed with MAGIC first in 2009 and later in 2010 within a multi-instrument observation campaign. The MAGIC observations yielded 14.8 hours of good quality stereoscopic data. The object was monitored at radio, optical and gamma-ray energies during the years 2010 and 2011. The source, named MAGIC J2001+439, is detected for the first time at VHE with MAGIC at a statistical significance of 6.3 {\sigma} (E > 70 GeV) during a 1.3-hour long observation on 2010 July 16. The multi-instrument observations show variability in all energy bands with the highest amplitude of variability in the X-ray and VHE bands. We also organized deep imaging optical observations with the Nordic Optical Telescope in 2013 to determine the source redshift. We determine for the first time the redshift of this BL Lac object through the measurement of its host galaxy during low blazar activity. Using the observational evidence that the luminosities of BL Lac host galaxies are confined to a relatively narrow range, we obtain z = 0.18 +/- 0.04. Additionally, we use the Fermi-LAT and MAGIC gamma-ray spectra to provide an independent redshift estimation, z = 0.17 +/- 0.10. Using the former (more accurate) redshift value, we adequately describe the broadband emission with a one-zone SSC model for different activity states and interpret the few-day timescale variability as produced by changes in the high-energy component of the electron energy distribution.

Properties of optically selected BL Lacertae candidates from the SDSS

\textbf{Context.} Deep optical surveys open the avenue for find large numbers of BL Lac objects that are hard to identify because they lack the unique properties classifying them as such. While radio or X-ray surveys typically reveal dozens of sources, recent compilations based on optical criteria alone have increased the number of BL Lac candidates considerably. However, these compilations are subject to biases and may contain a substantial number of contaminating sources. \textbf{Aims.} In this paper we extend our analysis of 182 optically selected BL Lac object candidates from the SDSS with respect to an earlier study. The main goal is to determine the number of bona fide BL Lac objects in this sample. \textbf{Methods.} We examine their variability characteristics, determine their broad-band radio-UV SEDs, and search for the presence of a host galaxy. In addition we present new optical spectra for 27 targets with improved S/N with respect to the SDSS spectra. \textbf{Results.} At least 59% of our targets have shown variability between SDSS DR2 and our observations by more than 0.1-0.27 mag de- pending on the telescope used. A host galaxy was detected in 36% of our targets. The host galaxy type and luminosities are consistent with earlier studies of BL Lac host galaxies. Simple fits to broad-band SEDS for 104 targets of our sample derived synchrotron peak frequencies between $13.5 \leq \mathrm{log}_{10}(\nu_{\mathrm{peak}}) \leq 16$ with a peak at $\mathrm{log}_{10} \sim 14.5$. Our new optical spectra do not reveal any new redshift for any of our objects. Thus the sample contains a large number of bona fide BL Lac objects and seems to contain a substantial fraction of intermediate-frequency peaked BL Lacs.

Dynamical black hole masses of BL Lac objects from the Sloan Digital Sky Survey

We measure black hole masses for 71 BL Lac objects from the Sloan Digital Sky Survey with redshifts out to z~0.4. We perform spectral decompositions of their nuclei from their host galaxies and measure their stellar velocity dispersions. Black hole masses are then derived from the black hole mass - stellar velocity dispersion relation. We find BL Lac objects host black holes of similar masses, ~10^{8.5} M_sun, with a dispersion of 0.4 dex, similar to the uncertainties on each black hole measurement. Therefore, all BL Lac objects in our sample have the same indistinguishable black hole mass. These 71 BL Lac objects follow the black hole mass - bulge luminosity relation, and their narrow range of host galaxy luminosities confirm previous claims that BL Lac host galaxies can be treated as standard candles. We conclude that the observed diversity in the shapes of BL Lac object spectral energy distributions is not strongly driven by black hole mass or host galaxy properties.

Detection of the host galaxy of S5 0716+714

We have acquired a deep i-band image of the BL Lacertae object S5 0716+714 while the target was in an low optical state. Due to the faintness of the nucleus, we were able to detect the underlying host galaxy. The host galaxy is measured to have an I-band magnitude of 17.5 +- 0.5 and an effective radius of (2.7 +- 0.8) arcsec. Using the host galaxy as a ``standard candle'', we derive z = 0.31 +- 0.08 (1 sigma error) for the host galaxy of S5 0716+714. This redshift is consistent with the redshift z = 0.26 determined by spectroscopy for 3 galaxies close to S5 0716+714. The effective radius at z = 0.31 would be 12 +- 4 kpc, which is consistent with values obtained for BL Lac host galaxies. An optical spectrum acquired during the same epoch shows no identifiable spectral lines.

The stellar content of low redshift BL Lacertae host galaxies from multicolour imaging

We present B-band imaging of 18 low redshift (z<0.3) BL Lac objects for which their host galaxies were previously resolved in the R-band and the near-infrared H-band. For a subset of the objects, also U- and V-band imaging is presented. These data are used to investigate the blue-red-near-infrared colours and the colour gradients of the host galaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity. In all cases galaxies are well represented by an elliptical model, with average absolute magnitude M_B=-21.6+-0.7 and average scale length R_e=7.6+-3.2 kpc. The best-fit B-band Kormendy relation is in reasonable agreement with that obtained for normal ellipticals and radio galaxies. This structural and dynamical similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The distributions of the integrated blue/near-infrared colour (with average B-H=3.5+-0.5) and colour gradient (with average Delta(B-R)/Delta(log r)=-0.14+-0.75) of the BL Lac hosts are much wider than those for normal ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are likely caused by a young stellar population component, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. This result is corroborated by stellar population modelling, indicating a presence of young/intermediate age populations in the majority of the sample, in agreement with low redshift quasar hosts. The lack of strong signs of interaction may require a significant time delay between the event with associated star formation episodes and the start of the nuclear activity.

The luminous host galaxies of high redshift BL Lac objects

We present the first near-infrared Ks-band (2.1 µm) imaging study of a sizeable sample of 13 high redshift (0.6 0.5 BL Lacs substantially increase the number of detected hosts (from ∼20 to ∼30). This dataset allows us to explore the evolution of BL Lac hosts from z ∼ 1 to the present epoch. We find that the host galaxies of high redshift BL Lacs are large (average bulge scale lengthR(e) �∼ 7 kpc) and similar to those hosting low redshift BL Lacs, indicating that there is no evolution in the host galaxy size. On the other hand, these host galaxies are very luminous (averageM(K)� = −27.9 ± 0.7). They are ∼3 mag brighter than the typical galaxy luminosity L*, and ∼1-1.5 mag more luminous than brightest cluster galaxies at low redshift. They are also ∼1 mag brighter than radio galaxies at low redshift and they appear to deviate from the K − z relationship of radio galaxies. On the other hand, these high luminosities agree with the few optical studies of high redshift BL Lacs and are similar to those of flat spectrum radio quasars studied by us in the near-infrared. The nuclear luminosity and the nucleus-galaxy luminosity ratio of the high redshift BL Lacs are much larger than those found for low redshift BL Lacs and similar to those observed in flat spectrum radio quasars at similar redshift. This mainly reflects the selection effects in the surveys and may be due to either an higher intrinsic nuclear luminosity, or due to enanched luminosity because of strong beaming. Contrary to what is observed in low redshift BL Lacs, the luminosities of the host galaxy and of the nucleus appear fairly well correlated, as expected from the black hole mass - bulge luminosity relationship found in nearby spheroids, if the nuclear emission works at the same regime. Our observations indicate that high redshift BL Lacs radiate with a wide range of power with respect to their Eddington luminosity, and this power is intermediate between the low level observed in nearby BL Lacs and the higher level occurring in luminous radio-loud quasars. The comparison with BL Lac host galaxies at lower redshift suggests that there is a ∼2 mag brightening of the hosts. We argue that the large luminosity of the hosts is due to a strong selection effect in the surveys of BL Lacs that makes observable only the most luminous sources at z > 0.5 and produces a correlation between the nuclear and the host luminosity that emerges at high redshift. However, this may also suggest a strong luminosity evolution which is inconsistent with a simple passive evolution of the stars in the host galaxies, and requires a contribution from recent star formation episodes that takes place at z > 0.5.

The optical-near-infrared colour of the host galaxies of BL Lacertae objects

We present H-band imaging of 23 low redshift (z < 0.3) BL Lac objects that were previously investigated by us in the R-band. For all the observed objects, the host galaxy is well represented by an elliptical model, with average scale length R(e) = 7.2 +- 3.6 kpc and average absolute magnitude M(H) = -25.9 +- 0.6. BL Lac hosts are therefore luminous elliptical galaxies indistinguishable from those of low redshift radio galaxies and inactive ellipticals. The best-fit H-band Kormendy relation of mu(e) = 3.7 log r(e) (kpc) + 14.8 mag arcsec^-2 agrees with that obtained for normal ellipticals and radio galaxies. This similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The new observations are combined with literature data of 18 objects, and this extended sample is used to investigate the optical-NIR colour and colour gradient properties of the host galaxies of BL Lacs and to perform a direct comparison with other elliptical galaxies with and without nuclear activity. The optical/near-infrared colour (R-H = 2.2 +- 0.4) and colour gradient (Delta(R-H)/Delta(log r) = -0.38 +- 0.60) distributions of the BL Lac hosts are much wider than those for normal ellipticals with old stellar populations, and many BL Lacs have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are most likely caused by a young stellar population, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. However, the lack of obvious signs of interaction may require a significant time delay between the event with associated SF episodes and the onset of nuclear activity.

Evolution of BL Lacertae host galaxies

We present and discuss deep, high-resolution I-band images of 24 BL Lac objects between and 1.3 taken with the Nordic Optical Telescope (NOT) and the ESO-NTT and VLT telescopes. In addition, new redshifts for the BL Lac objects PKS 0406+121, PKS 0426–380 and PKS 1519–273 are reported. In 17/24 (71%) of the BL Lac objects, we detected an underlying nebulosity, in 11/17 for the first time. We assigned the underlying nebulosity to the BL Lac host galaxy in 11 cases spanning the redshift range . The remaining 6 BL Lac objects have either intervening galaxies (S4 0218+35, PKS 0426–380), no redshift (MH 2133–449) or are probably misidentified (Q 0230+3429, B2 0937+26, MS 2347.4+1924). Restricting ourselves to the 11 BL Lac objects ( ), where a core and host galaxy was detected, we find that their host galaxies are luminous () and large ( kpc). They are on average about 0.6 mag brighter than BL Lac host galaxies at indicative of evolution, whereas their half-light radii are similar. By combining our data with literature data at low-redshift and applying evolutionary models to them, we show that the properties of the host galaxies of BL Lac objects up to are compatible with passively evolving elliptical galaxies formed at a redshift of (13 Gyrs ago in our adopted cosmology). Our results, however, are affected by an unavoidable luminosity bias and need to be confirmed. Future prospects are described. If they could be confirmed, host galaxies of low-luminosity radio-loud AGN (BL Lac/FR I) have very similar properties to the hosts of radio-quiet QSOs and high-luminosity radio-loud AGN (radio-loud QSO/FR II) over a wide redshift range. This supports the picture of the “Grand Unification” in which AGN activity is a transient phenomenon in galaxy evolution.

Evolution of BL Lac host galaxies

Deep, high-resolution I-band images of 24 BL Lac objects between z = 0.3 and 1.3 taken with the NOT and the ESO-NTT and VLT telescopes are discussed. In 17/24 (71%) of the BL Lac objects, we detected an underlying nebulosity, in 11/17 the host of the BL Lac in the redshift range z = 0.3-1 (&lt; z &gt; = 0.6). The host galaxies are luminous (MI = −25.2±0.8) and large (re = 10.5±7 kpc). They are on average about 0.6 mag brighter than BL Lac host galaxies at z ∼ 0.3 indicative of evolution, whereas their half-light radii are similar. By combining our data with literature data at low-redshift and applying evolutionary models to them, we find that the properties of the host galaxies of BL Lac objects up to z ∼ 1 are compatible with passively evolving elliptical galaxies formed at a redshift of z ≥ 2. Host galaxies of low-luminosity radio-loud AGN (BL Lac/FR I) have very similar properties to the hosts of radio-quiet QSOs and high-luminosity radio-loud AGN (radio-loud QSO/FR II) over a wide redshift range. This supports the picture of the “Grand Unification”, in which AGN activity is a transient phenomenon in galaxy evolution.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Blue colours of BL Lac host galaxies

Near-infrared and optical imaging of BL Lac host galaxies is used to investigate their colour properties. We find that the R–H colour and colour gradient distributions of the BL Lac hosts are much wider than those for normal ellipticals, and many objects have very blue hosts and/or steep colour gradients. The blue colours are most likely caused by recent star formation. The lack of obvious signs of interaction may, however, require a significant time delay between the interaction event with associated star formation episodes and the onset of the nuclear activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Evolution of Quasars

F01 Unification and Evolution of 3CR Radio Galaxies and Quasars as Seen by ISO F02 BL Lac Host Galaxies at z > 0.5 F03 Spectral Energy Distribution of Blazars F04 Power Unification of Black Holes F05 Evolution of the Dust Emission of Palomar-Green Quasars F06 Mid-Infrared Selection of Quasars F07 VLT-Spectropolarimetry of BAL QSOs F08 Evolution of Luminous Quasars from the Hamburg/ESO Survey F09 Quasars from the Variability and Proper Motion Survey F10 Formation and Acceleration of Jets in High Red-shifted Quasars F11 Evolution of Black Holes F12 Dust and Molecular Emission from High-redshift Quasars F13 Iron Emission in z ∼ 6 Quasars

TheHubble Space TelescopeSurvey of BL Lacertae Objects. IV. Infrared Imaging of Host Galaxies

The Hubble Space Telescope NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band by Urry and coworkers and Scarpa and coworkers. Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average absolute magnitude MH = -26.2 ± 0.45 mag and effective radius re = 10 ± 5 kpc. The rest-frame integrated color of the host galaxies is on average R-H = 2.3 ± 0.3, consistent with the value for both radio galaxies and normal, nonactive elliptical galaxies and indicating that the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient Δ(R-H)/Δ log r = -0.2 mag, again consistent with results for normal nonactive elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is μe = 3.8 log re + 14.8, fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests that the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes that can be actively accreting for some fraction of their lifetime.

TheHubble Space TelescopeSurvey of BL Lacertae Objects. III. Morphological Properties of Low‐Redshift Host Galaxies

We report on the optical properties of a sample of 30 BL Lac host galaxies in the redshift range 0.03 < z < 0.2, as derived from Hubble Space Telescope observations. All galaxies are fully resolved in the WFPC2 (F702W filter) images, allowing a quantitative analysis in two dimensions. Most and possibly all of these galaxies have characteristics very similar to those of normal giant ellipticals. The luminosity, ellipticity, isophote twisting, and amount of disky or boxy isophotes are consistent with those found in nonactive ellipticals and in radio galaxies. In all cases the BL Lac nucleus is well centered in the main body of its host galaxy, a result that argues strongly against the microlensing hypothesis for any significant fraction of the population. A search for faint substructures in the host galaxies has not revealed notable signatures of tidal distortions or subcomponents (faint disks, bars, X features, etc.), and with only one exception, there are no prominent dusty features in the central regions. Instead, the BL Lac host galaxies are smooth and unperturbed, suggesting that strong external gravitational interactions are not important to ongoing activity. A careful examination of the environment around the nucleus, however, shows a high incidence of close companion objects, whose nature remains unclear pending spectroscopic observations.

TheHubble Space TelescopeSurvey of BL Lacertae Objects. II. Host Galaxies

We have used the Hubble Space Telescope (HST ) WFPC2 camera to survey 132 BL Lac objects com- prising seven complete radio-, X-ray¨, and optically selected samples. We obtained useful images for 110 targets spanning the redshift range These represent an unbiased subsample of the original 0 ( z ( 1.3. 132 since they were snapshots selected to —ll random holes in the HST schedule. The exposure times ranged from a few hundred to D1000 s, increasing with redshift. Most images were taken in the F702W —lter; those already observed in F814W during Cycle 5 were reobserved in F606W to give broader wave- length coverage. The data were analyzed uniformly, and both statistical and systematic errors were esti- mated (the latter dominate). In of the BL Lac images, host galaxies are detected, including nearly all 2 for z 0.5 (58 of 63). In contrast, only one-quarter of the BL Lac objects with z ( 0.5 (six of 22) were resolved because of the relatively short exposure times, and these tend to be very luminous host galaxies. The highest redshift host galaxy detected is in a BL Lac object at z 0.664. HST data add critical mor- phological information in the range a few tenths to a few arcseconds. In 58 of the 72 resolved host galaxies, a de Vaucouleurs pro—le is signi—cantly preferred, at con—dence, over a pure exponential Z99% disk; the two —ts are comparable in the remaining 14 cases because of their generally lower signal-to- noise ratios. These results limit the number of disk systems to at most 8% of BL Lac objects (at 99% con—dence) and are consistent with all BL Lac host galaxies being ellipticals. The detected host galaxies are luminous ellipticals with a median absolute K-corrected magnitude of mag (rms M R D (23.7 ^ 0.6 dispersion), at least 1 mag brighter than M* and comparable to brightest cluster galaxies. The galaxy morphologies are generally smooth and undisturbed, with small or negligible ellipticities

TheHubble Space TelescopeSurvey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies

We report on a large Hubble Space Telescope imaging survey of BL Lac objects, at spatial resolution 10 times better than previous ground-based surveys. We focus on data reduction and analysis, describing the procedures used to model the host galaxy surface brightness radial profiles. A total of 69 host galaxies were resolved out of 110 objects observed, including almost all sources at z 0.5. We classify them morphologically by fitting with either an exponential disk or a de Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69 cases, it is invariably the elliptical morphology. This is a very strong result given the large number of BL Lac objects, the unprecedented spatial resolution, and the homogeneity of the data set. With the present reclassification of the host galaxy of 1418+546 as an elliptical, there remain no undisputed examples of a disk galaxy hosting a BL Lac nucleus. This implies that, at 99% confidence, fewer than 7% of BL Lac objects can be in disk galaxies. The apparent magnitude of the host galaxies varies with distance as expected if the absolute magnitudes are approximately the same, with a spread of ±1 mag, out to redshift z ~ 0.5. At larger redshifts, only six of 23 BL Lac objects are resolved so the present data do not constrain possible luminosity evolution of the host galaxies. The collective Hubble diagram for BL Lac host galaxies and radio galaxies strongly supports their unification.

Near-infrared imaging of the host galaxies of BL Lacertae objects

Near-infrared images have been obtained of nine BL Lacertae objects in order to investigate their host galaxy properties. From numerical modelling of the data we find that five of the nine BL Lacs have contributions from extended galaxy emission in their profiles. Tentative morphologies were derived for two of the BL Lacs, namely B2 0752+258, for which a disc morphology is slightly preferred, and S4 0954+65, for which an elliptical morphology is preferred. From our modelling we derive host galaxy absolute magnitudes of MH=−25.6±0.5 for B2 0752+258, and MH=−26.3±0.8 for S4 0954+65. We also find an average K-band absolute K-corrected host galaxy magnitude, for the BL Lacs, of 〈MK〉=−26.3±0.6 asssuming an elliptical galaxy model, and 〈MK〉=−26.1±0.9 assuming a disc galaxy model. The derived absolute magnitudes are similar to those found for the putative parent population of FRI radio galaxies, predicted by unified schemes. For those BL Lacs in which host galaxies have been previously detected at optical wavelengths we derive rest frame colours which are generally consistent with those of ‘normal’ galaxies. However, for TEX 0836+182 we find a rather blue colour which, together with the elongated morphology, may be indicative of interaction or spiral structure. Our results suggest that the host galaxies of BL Lacs are indeed similar to those of FRI radio galaxies, adding further weight to the unification scenario. In our present infrared frames of relatively modest resolution, we do not find that infrared observations are any more or less effective than optical observations at revealing the underlying host galaxy.

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