VLBI hybrid maps of the compact radio structure in 3C 345 at 5.0 and 10.7 GHz show a core-jet morphology, with two components in the jet separating from the core with apparent transverse velocities ≃ 8/h c (for Ho = 100 h km/s/Mpc and qo = 0.05). These “knots” decay as they move down the jet, with lifetimes ≃ 2–3 years.

NRAO 140 is a quasar (z = 1.258) which is among only 3 or 4 such objects (and the one with the highest z) which were detected at X-ray energies prior to the operation of the Einstein Observatory (Marscher et al. 1979). We obtained contemporaneous X-ray and radio VLBI observations of the source in early 1980, to determine whether Compton scattering within the radio source is the primary X-ray emission mechanism (Marscher and Broderick 1981b). Instead, we found that the radio parameters predicted more than 103 times more X-ray flux than was observed. Since the Compton calculation is independent of distance, and since the troublesome component was partially resolved (and hence not a high-brightness-temperature emitter), we found that relativistic motion aimed nearly directly toward the observer with Lorentz factor exceeding 4, needed to be invoked in order to bring the predicted Compton flux down to the observed level (Marscher and Broderick 1981a, b). Since relativistic motion is also the preferred explanation for the apparent superluminal expansion seen in some compact radio sources (e.g., M. Cohen, this volume; Marscher and Scott 1980; Kellermann and Pauliny-Toth 1981), we predicted that the compact components in NRAO 140 should appear to separate at a speed exceeding about 4c.

The detection of apparent faster-than-light motion (v=7.6c) in the core of 3C 179 poses some problems for the simple relativistic jet explanation.

Various predictions and implications of the relativistic-jet model for compact extragalactic sources are reviewed in the light of recent radio and X-ray observations.

Unless a plasma moves at relativistic bulk speeds, the Compton radiative lifetime for relativistic electrons near a luminous object is less than the transit time from the source. Acceleration by adiabatic decompression is too slow to preserve much of the electrons' energy. However, the Compton-rocket thrust and the radiatively induced pressure gradient can accelerate a relativistic fluid to relativistic bulk speeds on a time scale comparable to that for radiative loss. Consequently, severe Compton losses are not only reduced by relativistic bulk motion, but can in fact effect such motion.

Green and Schmidt (Green 1976, and in preparation) have optically surveyed some 10000 square degrees of the northern sky to search for bright quasars. Their final sample contains about 100 quasars. The B magnitudes of the sample range from 13.1 to 16.5, with most in the range 15.0–16.2. The redshifts range from 0.03 to over 2, considerably concentrated toward smaller values (median of 0.18).

Many radio galaxies have strong emission lines in their optical spectra. The fraction with such lines is much larger than in “normal” galaxies. Radio galaxies generally also have very bright nuclei; thus those with strong emission lines are similar in both respects to Seyfert galaxies. Hence radio and Seyfert galaxies are both generally considered to be similar physical objects: active galactic nuclei. Their observational properties show they are closely related to quasars (quasi-stellar radio sources) and (radio-quiet) QSOs. A short table of the space density of these objects, culled from many sources, chiefly Schmidt (1978) and Simkim, Su and Schwarz (1980) is given below. Although all the numbers are quite uncertain, there is no doubt that the radio-loud objects are relatively rare. With less certainty, it appears that the ratio of numbers of radio galaxies to Seyfert galaxies is about the same as the ratio of numbers of quasars to QSOs.

Relations between observed optical and radio properties can, in principle, constrain the geometry and physical conditions of the broad-line regions in quasars and active nuclei. Osterbrock and colleagues (see this symposium) and J.E. Steiner (preprint) have noted differences between Hα/Hβ, [0III]/Hβ and optical Fe II emission for Seyfert 1 galaxies, broad-line radio galaxies (BLRG's) and quasars. Stockman et al. (1979) discovered a tendency for optical continuum polarization angles for quasars to be aligned with the direction of the outer radio lobes. Setti and Woltjer (1977) and Miley and Miller (1979) noted that the quasars with strongest Fe II are among the most compact radio sources (e.g. 3C 48, 0736+01, 1510–08), and Miley and Miller also note that the distribution of line widths is narrower for the more compact than for extended radio sources.

A sample of 13 out of 15 low redshift (0.1 ≤ z ≤ 0.6) radio-loud quasars have been resolved on large-scale (19 arcsec mm−1), sky-limited (µR ∼ 26.5 mag sec−2) Kodak IIIa-F photographs obtained with the ESO 3.6-m telescope. The QSO images were analyzed by digitally subtracting the plate background and the point-spread function defined by images of nearby (≤ 1 arcmin) field stars having magnitudes comparable to the quasars (Δm ≲ 0.3 mag). The resolved nebulosities underlying the QSO images have isophotal diameters in the range θ ∼ 7 to 40 arcsec, with surface brightnesses µR ∼ 22–26 mag sec−2, and integrated apparent red magnitudes ∼ 16–21.

CCD photometry of five BL Lac objects indicates that at least three, and possibly four, are located at the centers of giant elliptical galaxies. The redshift for one of these objects, 1218+304, is estimated. A lower limit is placed on the redshift of 1219+28, for which no associated galaxy has been detected. Separation of the galaxy emission from the total observed flux makes possible comparison of the optical — far red flux from the point source alone with radio and X-ray data. This comparison suggests the emission from 1727+50 and 1218+304 can be interpreted as due to direct synchrotron emission. Observations of a small group of galaxies associated with the BL Lac object 1400+162 are also discussed.

As one part of our joint study of the X-ray properties of BL Lac objects, we compare the measured X-ray flux densities with those predicted using the synchrotron self-Compton (SSC) formalism (Jones ET AL. 1974). Naive application of the formalism predicts X-ray fluxes from 10−3 to 105 those observed. We therefore ask what we can learn by simply assuming the SSC mechanism, and looking for ways to reconcile the observed and measured X-ray fluxes. This paper reports our investigation of beaming factors due to relativistic ejection of a radiation source which is isotropic in its own rest frame. We conclude that large Lorentz factors, Γ ≳ 10, do not apply to BL Lac objects as a class.

After several years of relative quiescence, the flux of BL Lac has increased dramatically at centimeter wavelength, starting about epoch 1979.9 (Fig. 1). We have begun a series of VLBI observations to monitor the milliarcsecond structure at λ6 and λ2.8 cm wavelengths, using a five element VLBI array consisting of telescopes at Bonn, West Germany; Westford, MA; Green Bank, WV; Ft. Davis, TX; and Owens Valley, CA. The first two observations, in 1980 May and September, were at 5 GHz and were not of sufficient resolution to distinguish individual components in the source (Mutel, Phillips and Aller 1981). They did show, however, that the source was highly elongated along position angle ∼ 10° and was expanding that axis with a velocity of ∼4 c. The position angle is the same as several previous VLBI observations of this source, both during quiet periods and during previous flux outbursts (Pearson and Readhead 1981; Shaffer 1978 and references therein).

The normal spiral galaxy M81, which has some characteristics of a Seyfert (Peimbert, Torres-Peimbert, 1981), has a flat spectrum in the radio range (de Bruyn et al., 1976), variable on the time scale of days (Crane et al., 1976), and detectable radiation at infrared (Rieke, Lebofsky, 1978) and X-ray wavelengths (Elvis, van Speybroeck, 1981). At a distance of ∼3.3 Mpc, M81 is the nearest extragalactic object with a nucleus detectable with VLBI (Kellermann et al., 1976). We report here on simultaneous VLBI observations made with the Mark III system at 2.3 and 8.3 GHz. Observations on 14 and 16 March 1981 utilized the 100 m diameter telescope in Effelsberg, W. Germany (MPIR); the 43 m telescope at Green Bank, WV (NRAO); and the 40 m telescope near Big Pine, CA (OVRO).

Low-resolution, radio-intensity maps of the Galactic Center are dominated by two structures: the extended, nonthermal region between ℓ = 0°.0 and 0°.3 called Sagittarius A(East); and the bright thermal source at ℓ = −0°.05 and b = −0°.05 called Sagittarius A(West). SgrA(East) is probably a supernova remnant. SgrA(West) is a dense, gaseous region which is the very center of our Galaxy. Within SgrA(West) is a pc-sized cluster of infrared sources — late-type stars and gas condensations — swirling about with a dispersion of 250 km s−1. The cluster contains a unique radio source with the following properties: a size not exceeding 1015 cm, a radio luminosity exceeding 1034 erg s−1, and rather stable properties on a time scale of 108 s. This compact, nonthermal object (SgrAcn below) may be surrounded by a 0.1pc infrared source. Models for SgrAcn include both 106 M⊙ black holes related to the central objects in radio galaxies and quasars (Oort 1977, van Buren 1978) and 1 M⊙ degenerate stars related to pulsars and X-ray binaries (Reynolds and McKee 1980). We present below the radio intensity spectrum as a basic datum for further development of either class of models.

The supernovae SN1979c in NGC 4321 and SN1980k in NGC 6946 have both been detected at centimeter wavelengths at the VLA. The radio emission turns on very rapidly, but may be delayed by as much as a year with respect to the optical outburst. In both supernovae, the 20 cm radiation peaks after the 6 cm, and the radio emission has a very slow post-maximum decay.

Observations of complete samples of extragalactic radio sources at low and intermediate flux densities are described. Many types of source are found. The angular sizes form a smooth extrapolation from higher flux densities, and can be predicted from the known properties of samples at high flux density either with linear size evolution (for Ω = 1 or Ω = 0 Universes) or without linear size evolution (for Ω = 0). The question of whether such evolution is required therefore remains open.

Possible evidence that the linear sizes of extragalactic radio sources were smaller at earlier epochs was first provided by the angular size redshift (θ-z) relation for double radio quasars (Legg 1970, Miley 1971, Wardle and Miley 1974). But because of the strong correlation between redshift (z) and radio luminosity (P) in flux limited radio samples, it is hard to decide if the observed decrease in sizes with z is caused by an epoch dependence of linear sizes (ℓ) or by an inverse correlation between P and ℓ. Several authors (eg. Stannard and Neal 1977, Wardle and Potash 1977, Hooley ET AL. 1978, Wills 1979, Masson 1980) have attempted to separate the two effects by comparing the properties of quasars from the 3CR survey with those from the 4C and Parkes samples. Although most of these studies appear to marginally favour a P-ℓ correlation, none of them can rule out even a fairly strong evolution in ℓ with z. Apart from the small numbers involved, the difficulty is that 3C and 4C quasars do not differ a great deal in their redshifts or luminosities. A complete sample of quasars at much weaker flux levels would be quite valuable in this regard.

A number of investigations have been made of the dependence of largest angular size (LAS) of quasars on their redshifts Z (Miley 1971; Wardle and Miley 1974; Hooley ET AL. 1978; Wills 1979; Masson 1980). The earlier workers found a relation LAS ∝ Z−1 indicating a decrease in linear size of quasars as (1+Z)−n, where 1 < n < 3. The latter workers have argued that the apparent decrease is perhaps caused by an inverse correlation between LAS and radio luminosity. It is also interesting to investigate the dependence of size of hot-spots on luminosity and Z.

For the last 20 years, a huge observational effort has been devoted to the identification of all 3CR radio sources lying in directions away from the Galactic plane. The project is now more-or-less complete thanks to high-resolution radio observations with aperture synthesis radio telescopes, in particular the Cambridge 5-km telescope, and the use of a CCD camera on the Palomar 5-metre telescope (see e.g. Gunn et al 1980).

Calculations modeling the effects of stellar evolution on elliptical galaxies (e.g., Bruzual and Kron 1980, Bruzual 1981) suggest that the shape of the spectral flux distribution should remain almost constant in the red and near-infrared out to at least z ∼ 2. Thus, it should be possible to derive the redshift of a distant elliptical galaxy by fitting a model galaxy spectrum to broadband near-infrared (RIJHK) photometry.