A widely accepted model for BL Lac objects is that they are radio galaxies with a relativistic jet pointing almost directly towards us. But we need a clear trigger mechanism for these jets. One possibility is the close interaction between the BL Lac host and the closeby galaxies (e.g. Heckman et al. 1986). This interaction has been seen many times in the case of quasars (Hutchings et al. 1989) but not so much is known about the close surroundings of the BL Lac objects although there has been some pioneer work like Stickel et al. (1993). The problem has usually been that the images are not deep enough and that the seeing has not been so good. To clarify the situation we have started an observing program to get very deep images in the sub-arcsecond seeing conditions from the whole 1 Jy sample (Stickel et al. 1991) of BL Lac objects. The aims of this study are: 1. to search for very close companions to the BL Lacs, 2. to study the large scale galaxy clustering around the BL Lacs and 3. to study the BL Lac hosts themselves.

The major distinguishing features of BL Lacertae Objects are weak or absent line emission and strong and variable optical, infrared, and radio polarization (Angel and Stockman 1980; Kollgaard 1994). The radio emission and much of the optical emission is believed to be synchrotron radiation. In nearly every BL Lacertae object in which polarization structure has been detected, the polarization position angles χ in knots in the jets are nearly parallel to the VLBI structural axis. Assuming the jet components to be optically thin, the inferred magnetic fields B are nearly perpendicular to the jet direction θ; perhaps the most natural interpretation of this is that the knots are associated with shocks that compress an initially tangled B field as they propagate down the VLBI jet, enhancing B transverse to the compression (Laing 1980; Hughes, Aller, & Aller 1989). The superluminal speeds observed in the jets of BL Lacertae objects are on average lower than those observed in a comparably core-dominated population of quasars (Gabuzda et al. 1994). The distribution of core polarization position angles χcore at λ = 6 cm is bi-modal, suggesting that χcore is roughly perpendicular to θ when the cores are quiescent, and aligns with θ at epochs when polarization from newly emerging shock components is blended with the core polarization (Gabuzda et al. 1994).

The major distinguishing features of BL Lacertae Objects are weak or absent line emission and strong and variable optical, infrared, and radio polarization (Angel and Stockman 1980; Kollgaard 1994). The radio emission and much of the optical emission is believed to be synchrotron radiation. There are now some 20 BL Lacertae objects for which VLBI polarization (VLBP) images have been made at λ = 6 cm (Gabuzda et al. 1994 and references therein). In nearly every BL Lacertae object in which polarization structure has been detected, the polarization position angles in knots in the jets are nearly parallel to the VLBI structural axis. Assuming the jet components to be optically thin, the magnetic fields inferred by this orientation are nearly perpendicular to the direction of the jet; perhaps the most natural interpretation of this is that the knots are associated with shocks that compress an initially tangled magnetic field as they propagate down the VLBI jet, enhancing the magnetic field transverse to the compression (Laing 1980; Hughes, Aller, & Aller 1989).

The study of the extended emission and polarization properties of BL Lacs is an important step for the identification of their parent population. FRI radio sources, the supposed parent population of BL Lacs, have weaker extended radio luminosity and a dominant inferred magnetic field perpendicular to the jet, while FRII radio sources, the supposed parent population of quasars, have stronger extended radio power and an inferred magnetic field parallel to the jet. The only complete sample of radio selected BL Lacs (1 Jy sample, Stickel et al. 1991, ApJ, 374, 431) contains 34 objects. Unfortunately, about half of 1 Jy BL Lacs do not have very high dynamic range images, necessary to detect the low emissivity radio emission surrounding the bright compact source, either because the object was never observed, or because the observation was carried out at the beginning of 1980s with low sensitivity. In 1994 we started a programme using the VLA (A, B, and D configuration, see Table 1) and the WSRT (W in Table 1) to complete the high sensitivity radio imaging of the 1 Jy sample. We aim to investigate morphology and polarization properties, as well as the luminosity of the extended emission. This contribution presents the L band observations. The results are very preliminary, some of the data reduction is still in progress as well as the statistical analysis. The sources in Table 1 have been roughly classified as extended (E), or point-like (P) if no extended feature was detected. Among the 15 sources observed at the highest resolution 13 were classified as extended. In many sources we detect significantly much more extended flux than previously reported from earlier observations. Almost all the BL Lac objects we observed at the highest resolution show some extended features; furthermore, in a few cases, we detected emission on the arcminute scale. The power of the extended luminosity covers 3 orders of magnitude, 3 objects (0537–441, 0820+225, and 2240–260) have values typical of a FRII radio source. These new data will be used for an updated statistical analysis of the properties of the extended emission in the 1 Jy sample of BL Lac objects.

BL Lac objects are an enigmatic class of active galactic nuclei. They are characterized by high luminosity, a flat radio spectrum that steepens at higher energies, relatively high optical and radio polarization, rapid variability and an optical continuum with weak or absent emission lines (see Urry and Padovani, 1995 for a recent review).

These properties have been interpreted in terms of a relativistic jet closely aligned to the line of sight (Blandford and Rees, 1978, Ghisellini et al., 1993). This model, known as the beaming model, implies that there must be a so called “parent population” of radio sources intrinsically identical to BL Lac objects, but with the jets oriented at large angles to the line of sight. Browne (1983) was the first to propose the low luminosity FR I radio galaxies as the most likely candidates for the “parent population” of the core dominated BL Lac objects. An outcome of the beaming model is that all the properties not depending on orientation should be shared by the BL Lac objects and the FR I radio galaxies.

First results from a large homogeneous superluminal motion survey are presented. The data do not show compelling evidence for the existence of intrinsically different populations of galaxies, BL Lac objects, or quasars. βapp in the range 1–5 h –1 occur with roughly equal frequency; higher values, up to βapp = 10 h –1, are rather more scarce than appeared to be the case from earlier work, which evidently concentrated on sources which are not representative of the general population. The βapp distribution suggests that there might be a skewed distribution of Lorentz factors over the sample, with a peak at γb ≈ 2 h –1 and a tail up to at least γb ≈ 10 h –1. There appears to be a clearly rising upper envelope to the βapp distribution when plotted as a function of observed 5 GHz luminosity; a combination of source counts and the apparent velocity statistics in a larger sample could provide much insight into the properties of radio jet sources.

A large fraction of the sources in flux density limited radio samples have angular sizes < 2 arcsec (and hence projected linear sizes ≤ 10–15 kpc for H 0 = 100 Km/(sec Mpc), and steep (α > 0.5, S∝ v –α) high frequency spectra (Kapahi, 1981; Peacock and Wall 1982). The proportion of these Compact Steep–spectrum Sources (CSSs) is high (15–30% depending on the selection frequency) amongst distant (z > 0.2) radio sources of high power, both galaxies and quasars. We include in this class the GHz Peaked Spectrum Radio Sources (GPS), sub–kpc objects whose radio spectra are peaked at GHz frequencies (see, e.g., O'Dea et al, 1991).

The GHz-Peaked-Spectrum (GPS) radio sources are powerful (Lradio ≈ 1045 erg sec–1) and compact (10–100 mas, 10–1000 parsecs) sources characterized by a simple convex spectrum which peaks near 1 GHz (O'Dea et al. 1991 and references therein).

Optical and radio observations lead to the conclusion that GPS sources are formed when the radio source is confined to the narrow line region (or an even smaller scale) by a dense and clumpy medium. This could lead to 2 different evolutionary scenarios: as first suggested by Phillips and Mutel (1982) GPS radio sources could be classical double radio sources at the very first stage of their life, or alternatively they will never become as large as the classical doubles since the dense and turbulent environment is able to confine and trap the radio emitting region on the scale of the NLR (Baum et al. 1990).

Compact Steep-Spectrum (CSS) and GHz-Peaked Spectrum (GPS) radio sources are intrinsically small objects (Fanti et al. 1990) with high frequency steep spectra (α > 0.5 with S∝ v –α), found at moderate or high redshifts (z>0.2 but many objects have z>1.5). Their characteristics have been interpreted in terms of youth (e.g. Fanti et al. 1995) or “frustration” (e.g. van Breugel et al. 1984). Their radio spectra turn over at tens or a few hundreds of MHz (CSS) or at higher frequencies (GPS), interpreted as due to synchrotron self-absorption. They represent a significant fraction of flux limited catalogues (15–30 %, depending on the frequency).

Recently we focussed our attention on a sample of Compact Steep-spectrum Sources (CSSs) selected because of the large bent radio jets seen in the inner region of emission. The largest distortions are often seen in sources dominated by jets, and there are suggestions that this might to some extent be due to projection effects. However, superluminal motion is rare in CSSs. The only case we know of so far is 3C147 (Alef at al. 1990) with a mildly superluminal speed of ≃ 1.3v/c. Moreover, the core fractional luminosity in CSSs is ≃ 3% and ≤ 0.4% for quasars and radio galaxies respectively. Similar values are found for large size radio sources i.e. both boosting and orientations in the sky are similar for the two classes of objects. An alternative possibility is that these bent-jet sources might also be brightened by interactions with the ambient media. There are clear indications that intrinsic distortions due to interactions with a dense inhomogeneous gaseous environment play an important role. Observational support comes from the large RMs found in CSSs (Taylor et al. 1992; Mantovani et al. 1994; Junor et al. these proc.) and often associated with strong depolarization (Garrington & Akujor, t.p.). The CSSs also have very luminous Narrow Line Regions emission, with exceptional velocity structure (Gelderman, t.p.).

Two important factors for understanding the physical nature of compact steep spectrum (CSS) radio sources are determining the correct radio morphological classification of these objects together with their characteristics in wavebands different from the radio (Fanti et al. 1995, A&A, 302, 317). Seven CSS sources (linear dimensions < 30kpc for Ho = 50 kms–1Mpc–1 and α > 0.5, S ≃ v –α) have been found in a complete sample of strong southern radio sources. This group of CSS sources is particularly interesting because some optical and X-ray information is already available as part of a more general study of southern radio sources (Morganti et al. & Siebert et al. these Proceedings). The spectra of all the sources were presented in Tadhunter et al. (1993, MNRAS, 263, 999.) Here we present VLBI observations for three of these sources (0252-71, 1306-09 and 1814-63). The remaining four have already been imaged with VLBI (King et al. these Proceedings).

A sample of 38 southern peaked-spectrum radio sources from the Parkes Catalogue have been observed using single-baseline VLBI. Thirty three objects were successfully detected on baselines of > 30Mλ at 2.3 GHz. For 21 of these sources, the flux density in the compact components contributes more than half the total flux density of the radio emission. Twenty sources showed structure more complex than a point-source.

Compact steep spectrum (CSS) sources show all the features of the powerful extended radio galaxies and quasars but on a scale at least ten times smaller. The debate continues over whether this is primarily due to a difference in age or the result of denser gas surrounding CSS sources (Fanti et al. 1990); although two of the most recent investigations favour the view that CSS sources are young (Readhead et al. and Fanti et al., these proceedings)

A majority of compact steep-spectrum (CSS) and GHz-peaked spectrum (GPS) sources show low integrated polarizations (≲ 1%) at or below 5 GHz. To investigate whether this is mostly due to large rotation measures (RMs) arising from their being cocooned in dense gaseous envelopes we have observed 26 CSSs and GPSs at 8 and 15 GHz with the VLA A-array. The sample includes equal numbers whose hosts are galaxies and quasars.

Compact Steep-Spectrum (CSS) radio sources are defined by their powerful yet subgalactic radio structure. High resolution radio interferometers reveal subarcsecond double, triple, and core-jet radio morphologies resembling miniature FR II sources. There are three promising scenarios for how such powerful radio sources could be so intrinsically small: 1) they are young sources which have not had time to grow large, 2) they are confined sources with exceptionally dense gas in the host which either smothers an existing source or frustrates its initial growth, or 3) they are enhanced sources with an intrinsic power much lower than one might expect based on their radio luminosity. To investigate these scenarios, we are continuing a comprehensive optical study of a sample of Compact Steep Spectrum objects.

The quasar 3C216 has the distinction of being classified as a blazar as well as a compact steep-spectrum radio source (Akujor et al, 1993). It has a grossly mis-aligned large-scale radio structure. Previous VLBI images include those of the core region (Barthel et al 1988; Venturi et al 1993) with 1 mas resolution, and the sub-kpc scale core-jet with resolution ca. 25 mas (Fejes et al, 1992; Akujor et al, 1993). In order to investigate the connection between these structures seen on different scales, we have made a 1.6 GHz observation of the core-jet region with a “World Array” VLBI network. This consisted of 20 antennas, at Effelsberg (Germany), Jodrell “Lovell” (UK), Westerbork (Netherlands), Medicina and Noto (Italy), Onsala-26m (Sweden), Torun-15m (Poland), Simeiz (Ukraine), Bear Lakes (Russia), Seshan (China), Hartebeesthoek (S. Africa), Green Bank-43m, VLA and the VLBA antennas at HN, NL, PT, KP, LA, BR, OV (USA). The observations were made on 18 November, 1992, using the Mk2 recording system, and these were amongst the last to be correlated using the JPL-Caltech Block2 processor. The NRAO AIPS package was used for the data analysis.

3C286 (1328+307) is a powerful radio source identified with a quasar at z=0.849. There is a foreground galaxy responsible for an H I absorption line system at z=0.6922 (Brown & Roberts 1973), centered approximately 2.″5 to the southeast of 3C286. The radio source has a steep spectrum (α = −0.61, Sv ∝ v α between 1.4 and 15 GHz) which turns over at about 100 MHz. Subarcsecond resolution radio images show a misaligned triple structure, dominated by the central component (Spencer et al. 1989) which accounts for at least 95% of the total flux density at all frequencies. 3C286 is one of the strongest extragalactic sources in polarized emission (0.84 Jy at 5 GHz and 1.41 Jy at 1.4 GHz) and with a rotation measure close to 0 rad m–2 (Rudnick and Jones 1983). Hence the observed orientation of the electric field vector is essentially independent of frequency.

A series of VLBI surveys of complete samples of radio sources selected at 5 GHz (Pearson & Readhead 1988, hereafter PR; Xu et al. 1995 and references therein) has revealed that ≃ 10% of the objects are “Compact Symmetric Objects” (CSOs), in which high-luminosity radio emission regions are seen on both sides of the center of activity on scales less than one kiloparsec (Phillips & Mutel 1982; Readhead et al. 1984; Conway et al. 1992; Wilkinson et al. 1994). In order to be sure that an object is a CSO, either the center of activity must be pinpointed (Taylor et al. 1996) or compelling morphological evidence of symmetric structure must be found.

PKS 1413+135 is an enigma: while classified as a BL Lac due to its polarized near-IR continuum (Stocke et al. 1992) and optical spectrum (Beichmann et al. 1981), it appears to lie within a spiral host (McHardy et al. 1991, Stocke et al. 1992). In addition, the AGN is highly obscured (Beichmann et al. 1981, Carilli et al. 1992, Stocke et al. 1992, Wiklind & Combes 1994, 1995). Yet there is no evidence that the absorbing gas is being heated and re-emitting the AGN radiation in the form of thermal IR or emission lines (as in, e.g., Sey 2s). This led Stocke et al. (1992) to suggest that the AGN might be background to the optical galaxy.

Compact Symmetric Objects (CSO's) are strong, compact (< lkpc) objects with radio structure and luminosity similar to classical double radio sources (i.e. two lobes and a weak core), but thousands of times smaller (Wilkinson et al 1994). It has been proposed that CSO's are either young radio sources which will later evolve into classical sources, or sources whose growth is ‘frustrated’ by dense surrounding gas. In a recent survey of z < 0.1 CSO's searching for HI absorption, approximately 50% were detected (see Conway et al 1996), a much higher percentage than found in surveys of general radio loud ellipticals with similar sensitivity (see van Gorkom et al 1989).