1. INTRODUCTION

The mystery of active galactic nuclei (AGN) is that they produce very high luminosities in a very concentrated volume, probably through physical processes other than the nuclear fusion that powers stars. AGN are thus special laboratories for extreme physics which we would like to understand. They are also our principal probes of the Universe on large scales, so understanding them is essential to studying the formation and evolution of the Universe.

At present the approximate structure of AGN is known but much of the detailed physics is literally hidden from view because of their strongly anisotropic radiation patterns. The prevailing (but not necessarily correct) picture of the physical structure of AGN is illustrated in Fig. 1 (Holt et al. 1992). At the center is a supermassive black hole whose gravitational potential energy is the ultimate source of the AGN luminosity. Matter pulled toward the black hole loses angular momentum through viscous or turbulent processes in an accretion disk, which glows brightly at ultraviolet and perhaps soft X-ray wavelengths. Hard X-ray emission is also produced very near the black hole, perhaps in connection with a pervasive sea of hot electrons above the disk. If the black hole is spinning, energy may be extracted electromagnetically from the black hole itself.

Figure 1. A schematic diagram of the current paradigm for radio-loud AGN (not to scale). Surrounding the central black hole is a luminous accretion disk. Broad emission lines are produced in clouds orbiting above the disk and perhaps by the disk itself. A thick dusty torus (or warped disk) obscures the broad-line region from transverse lines of sight; some continuum and broad-line emission can be scattered into those lines of sight by hot electrons that pervade the region. A hot corona above the accretion disk may also play a role in producing the hard X-ray continuum. Narrow lines are produced in clouds much farther from the central source. Radio jets, shown here as the diffuse jets characteristic of low-luminosity, or FR I-type, radio sources, emanate from the region near the black hole, initially at relativistic speeds. For a 108 M black hole, the black hole radius is ~ 3 x 1013 cm, the accretion disk emits mostly from ~ 1-30 x 1014 cm, the broad-line clouds are located within ~ 2-20 x 1016 cm of the black hole, and the inner radius of the dusty torus is perhaps ~ 1017 cm. The narrow-line region extends approximately from 1018-1020 cm, and radio jets have been detected on scales from 1017 to several times 1024 cm, a factor of ten larger than the largest galaxies.

Strong optical and ultraviolet emission lines are produced in clouds of gas moving rapidly in the potential of the black hole, the so-called ``broad-line clouds'' (dark blobs in Fig. 1). The optical and ultraviolet radiation is obscured along some lines of sight by a torus (as shown in Fig. 1) or warped disk of gas and dust well outside the accretion disk and broad-line region. Beyond the torus, ⁽¹⁾ slower moving clouds of gas produce emission lines with narrower widths (grey blobs in Fig. 1). Outflows of energetic particles occur along the poles of the disk or torus, escaping and forming collimated radio-emitting jets and sometimes giant radio sources when the host galaxy is an elliptical, but forming only very weak radio sources when the host is a gas-rich spiral. The plasma in the jets, at least on the smallest scales, streams outward at very high velocities, beaming radiation relativistically in the forward direction.

This inherently axisymmetric model of AGN implies a radically different AGN appearance at different aspect angles. In practice, AGN of different orientations will therefore be assigned to different classes. Unification of these fundamentally identical but apparently disparate classes is an essential precursor to studying the underlying physical properties of AGN. The ultimate goal is to discover which are the fundamentally important characteristics of AGN - e.g., black hole mass, black hole spin, accretion rate, host galaxy type, interaction with neighboring galaxies - and how they govern the accretion of matter, the formation of jets, and the production of radiation in these bizarre objects.

This review covers the unification of radio-loud AGN, i.e., those with prominent radio jet and/or lobe emission. Comparable unification schemes for radio-quiet objects (Rowan-Robinson 1977; Lawrence and Elvis 1982; Antonucci and Miller 1985), which have not been explored using the same statistical techniques, have recently been reviewed by Antonucci (1993; his review includes radio-loud AGN as well) and are not discussed here. In the following sections, we describe current AGN classification schemes (Sec. 2) and the two principal causes of anisotropic radiation, obscuration (Sec. 3) and relativistic beaming (Sec. 4). We establish the motivation for current unification schemes for high- and low-luminosity ⁽²⁾ radio-loud AGN (Sec. 5) and then discuss them quantitatively (Sec. 6). We discuss the possible connections among high- and low-luminosity AGN and other aspects of the unification paradigm (Sec. 7), including potential problems, complications and future tests (Sec. 8). In the final section (Sec. 9), we briefly summarize the status of unification and pose what we believe are the ten most important questions at the current time. In the Appendices, we present equations governing the various beaming parameters (A), the Doppler enhancement (B), and the ratio of core to extended flux (C), and a glossary of acronyms used in the paper (D). Throughout this review the values H₀ = 50 km s^-1 Mpc^-1 and q₀ = 0 have been adopted (unless otherwise stated) and the spectral index is defined such that F ^-.

² In all that follows, we compute observed luminosities assuming spherical symmetry, i.e., we assume uniform emission into 4 steradians. If an AGN radiates anisotropically, it may be called a ``high luminosity'' source even though its intrinsic luminosity is low. Back.