Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

1. INTRODUCTION

The radio emission from radio-loud active galactic nuclei (AGN) ordinarily takes the form of collimated "jets" that connect a compact central region with kiloparsec-scale extended lobes and hot spots. For the past two decades, the structure and emission properties of these jets have been studied extensively with aperture synthesis radio imaging (see Bridle & Perley 1984, Bridle 1996). Very long baseline interferometry (VLBI) has become the primary tool to probe the most compact of these emission regions with angular resolution of 0.1 to 2 milliarcsec, corresponding to linear scales of 0.2 to 10 parsec (pc) (see Zensus & Pearson 1990, Pearson 1996).

The direct radio imaging of parsec-scale jets and the complementary study of activity in the associated AGN in all spectral regimes has broadly impacted our understanding of these objects; in particular, they have inspired and constrained the development of realistic physical models, helped to establish the relativistic jet paradigm, and influenced the thinking about general AGN unification, radio source evolution, and cosmology.

The relativistic jet model (e.g. Blandford & Königl 1979) has become the de facto generally current AGN paradigm; it postulates that AGN are fueled by a massive central black hole and accretion disk at the base of a relativistic flow in symmetric twin jets (Rees 1971, Scheuer 1974, Blandford & Rees 1974). This scenario is rooted in the ideas that the source characteristics are determined by relativistic beaming (Shklovskii 1963), by relativistic injection from galactic nuclei (Rees 1966), and by accretion of matter on a central black hole (see Begelman et al 1984); the inner jets themselves are thought to be formed magnetically from a rapidly rotating accretion disk around the black hole (Blandford & Payne 1982). The only direct evidence for the existence of bulk relativistic outflow along the radio jets comes from the detection of collimated, apparent faster-than-light ("superluminal"), outward-motion of parsec-scale jets (see Readhead 1993, Zensus & Pearson 1987). The discovery of a rapidly rotating molecular disk in NGC 4258 arguably is the most direct demonstration of the presence of a black hole in the nucleus of an active galaxy (Miyoshi et al 1995, Moran et al 1995, but see also Burbidge & Burbidge 1997). The intraday variability found in a number of compact sources hints at the presence of linear dimensions for ultra-compact jet components that are smaller than light hours (see Wagner & Witzel 1995).

The radio radiation from parsec-scale extragalactic radio sources is partially transparent synchrotron emission with characteristic spectral and polarization properties, as well as significant inverse-Compton emission (see Marscher 1990, Hughes & Miller 1991). The flat spectra of compact sources can be explained by superposition of the synchrotron spectra of the individual components that form the compact structure, the compact base of the jet, and the bright regions within the jet flow. The details of the formation of inner jets that connect the nucleus to the observed radio jet, their acceleration to near the speed of light, and the strong collimation remain poorly understood (e.g. Marscher 1995, Begelman 1995). On the other hand, considerable progress has been made in explaining the kinematic, spectral, and polarization properties of parsec-scale jets through shocks in an underlying continuous jet flow (Blandford & Königl 1979, Marscher & Gear 1985, Hughes et al 1989a). Simulations of hydro- and magneto-hydrodynamic relativistic jets have reached a quality where meaningful comparisons between numerical and observed jet properties and evolution are possible (see Wiita 1996, Norman 1996).

In the basic beaming model scenario, superluminal motion and morphologic differences in general between compact and extended radio sources are attributed to beaming and orientation effects; compact sources are thought to be basically extended lobe sources at a small inclination to the line of sight and with strongly Doppler boosted core flux density (Orr & Browne 1982). A unification of Seyfert galaxies of type 1 with type 2 is supported by optical studies of the polarization properties of broad- and narrow line regions (Antonucci 1993, Miller 1995). The original unification hypothesis for radio sources has also been extended and refined by adding opaque nuclear obscuring tori to the relativistic jets. Beamed, intermediate, and unbeamed populations of radio-moderate edge-darkened Fanaroff & Riley Class 1 galaxies and of radio-loud edge-brightened Class 2 galaxies have been identified (see Barthel 1989, 1995, Gopal-Krishna 1995, Urry & Padovani 1995). In particular, lobe-dominated, narrow-line radio galaxies such as Cygnus A are assumed to be the unbeamed counterparts to core-dominated, variable, and superluminal quasars like 3C345; the FR I radio galaxies in turn are thought to be the unbeamed pendants to BL Lac type objects.

The observational study of parsec-scale jets has seen astonishing progress since early imagery of compact radio sources at milliarcsecond resolution was achieved with very long baseline interferometry (VLBI) (see Kellermann & Pauliny-Toth 1981). Extensive VLBI surveys (see Wilkinson 1995) have made possible a morphologic classification of compact radio sources (see Pearson 1996). The systematic measurement of motion in these large samples yields apparent velocity statistics and distribution of Lorentz factors that can be compared to other indicators of relativistic motion (see Ghisellini et al 1993, Vermeulen 1995). The surveys also provide clues for unification models and for cosmology (see Vermeulen 1995, Kellermann 1993, Gurvits 1994). Realistic physical models can be tested and constrained through in-depth studies of prototypical objects, in particular when VLBI observations are combined with information from other spectral regimes.

Advances in VLBI techniques have been especially successful in the areas of detailed long-term monitoring observations, multifrequency and polarization imaging, and sensitivity-enhancing phase referencing (see the reviews in Zensus et al 1995b). High-quality VLBI observations have become routinely possible, especially with the very long baseline array (VLBA) (Napier et al 1994). This versatile and dedicated instrument offers full polarization and spectral-line imaging capability, as well as spectral coverage from 300 MHz to 45 GHz. Independent regional VLBI networks in the northern and southern hemispheres, coordinated global VLBI campaigns between the VLBA and the European VLBI Network, and the "World Array" campaigns have made possible images of outstanding fidelity (see Schilizzi 1995). At millimeter wavelengths, ad hoc arrays have been operating that result in images with submilliarcsecond resolution (Krichbaum et al 1994b, Schalinski et al 1994, Bääth 1994). The first true space VLBI mission, the Japanese VLBI Space Observatory Programme (VSOP), launched in early 1997, should provide high-fidelity images of strong sources (> 1 Jansky) at even greater resolution (Hirabayashi 1996).

This review summarizes observational properties of parsec-scale radio jets in extragalactic sources, focusing in particular on the high-luminosity core-dominated sources where the most detailed information on individual jets has been gathered. Recent accounts can also be found in several conference proceedings (Zensus & Kellermann 1994, Cohen & Kellermann 1995, Hardee et al 1996, Ekers et al 1996). Complementary perspectives are offered by Readhead (1993), Pearson (1996), Cawthorne (1991), Marscher (1995). A Hubble constant of H₀ = 100 h km s^-1 Mpc^-1, a deceleration parameter q₀ = 0.5, and a spectral index definition S are assumed throughout this paper.