After the initial discovery of radio-loud AGN, the advent of radio interferometry soon led to detailed images of these strong radio emitters (e.g., Bridle & Perley 1984; Bridle et al. 1994) which revealed remarkable long thin jets of plasma emanating from a central compact nucleus and feeding extended lobes, often at considerable distances from the AGN, millions of light years in the most extreme cases. The radio emission is synchrotron radiation produced by electrons spiraling around magnetic fields in the ejected plasma; figure 1 shows a radio image of the classic radio galaxy Cygnus A in which the nucleus, jets and lobes are visible. These dramatic jets and clouds of radio-emitting plasma were interpreted as exhaust material from the powerful central engine (Scheuer 1974; Blandford & Rees 1974).
Figure 1. 6-cm radio image of the classic radio galaxy Cygnus A (courtesy Chris Carilli)
3.1. Too fast to believe - the remarkable jets in radio-loud AGN
The sharpest radio images, made repeatedly over many years using networks of radio telescopes spanning the globe, resulted in `movies' of the motion of material in the jets. The blobs of plasma in these jets were apparently being ejected at many times the speed of light, c, appearing to violate fundamental laws of physics. It was quickly realised that such superluminal motion, was an optical illusion caused by the plasma moving at relativistic speeds, i.e. 0.7 c, and being ejected towards us at an angle close to our line of sight (e.g., Blandford & Rees 1978). Relativistic motion appears to be present for jet matter over hundreds of thousands of light years and the detailed physical driving mechanisms remain an area of active study. The relativistic motion of jet matter has an enormous impact on the appearance of these objects and is possibly the single-most important contributor to the variety of observed morphological types.
The fast motion of jet material also causes extreme apparent brightening, or Doppler boosting, of the radiation and greatly amplifies any flickering, or variability, in the light levels. Today, the wide range of observed radio structures, brightnesses and levels of variability can be understood in terms of the angle at which we view the high-speed plasma jet. Radio galaxies like Cygnus A are orientated perpendicular to our line of sight, lying in the plane of the sky, appear rather symmetrical, and as expected, show no variability or superluminal motion. At the other extreme are bright, compact and highly variable BL Lac objects, which are being observed head-on. Figure 2 shows a sketch of this model in which a jet of plasma is ejected from either side of the central engine at relativistic speeds; object classification depends on the angle of the jet to our line of sight. Objects viewed at intermediate angles are seen as either extended, `lobe-dominated' quasars or relatively compact, `core-dominated' quasars (see also Urry & Padovani 1995).
Figure 2. Top: Radio-loud unification scheme, in which the observed AGN type depends on the observer's viewing angle to the ejection axis of the radio jet. Bottom: Sketch of an AGN central engine with a central black hole surrounded by (a) an accretion disc that emits cones of ultraviolet ionising radiation and defines the radio jet launch direction, (b) a torus of dust a gas that accounts for the different observed kinds of radio-quiet AGN by blocking our view of the accretion disc and dense, rapidly-moving ionised gas clouds in the broad-line region (BLR) when viewed edge-on (type 2 objects). Less-dense ionised clouds in the narrow-line region (NLR) lie above the plane of the torus and are visible from all angles.
3.2. Obscuring-Doughnuts in Radio-Quiet AGN
Radio-quiet quasars and Seyferts are known to be ~ 10 times more common, but 100 to 1000 times weaker at radio wavelengths and significantly less extended than their radio-loud cousins (Goldschmidt et al. 1999), but orientation still has important effects, this time on the optical properties. Optical spectroscopy provides a powerful diagnostic tool for the physical conditions in astronomical objects; as described earlier chemical elements have a characteristic spectral signature and physical conditions within a gas can be inferred from distortions of this chemical bar-code. In particular, broadening of the spectral lines indicates a spread in gas-cloud velocities, whilst the relative brightnesses of spectral lines indicate the intensity of ultraviolet radiation incident upon the gas.
Measurements of the optical spectra of Seyfert nuclei show spectral lines from gas ionised (i.e. gas in which atoms have been stripped of one or more electrons) by strong ultraviolet radiation that is too intense to be produced by a collection of stars and is instead thought to originate from the accretion disk. All Seyfert nuclei contain a region of ionised gas, the Narrow Line Region (NLR), extending over several hundred light years where the spectral line-widths correspond to gas velocties of a few hundred km s-1 and densities are moderate (electrons per unit volume ne ~ 103 - 106 cm-3). Closer in, within ~ 0.1 light year of the black hole, is the Broad-Line Region (BLR), a much denser region of gas (ne ~ 109 cm-3) that shows gas velocities up to 10,000 km s-1. Seyferts were originally classified into two types; type-1 Seyferts that show evidence for both a BLR and an NLR, and type-2 Seyferts that show only an NLR (Khachikian & Weedman 1971, 1974).
The mystery of the missing BLRs in type-2 Seyferts was solved elegantly in 1985 when Antonucci & Miller discovered a hidden BLR in the scattered light spectrum of the archetypal Seyfert 2 galaxy NGC 1068, which closely resembled that of a Seyfert type 1. This discovery led to the idea that the BLR exists in all Seyferts and is located inside a doughnut, or torus, of molecular gas and dust; our viewing angle with respect to the torus then explains the observed differences between the unobscured, broad-line Seyfert 1s, viewed pole-on, and the obscured, narrow-line Seyfert 2s, viewed edge-on. Hidden Seyfert 1 nuclei can then be seen in reflected light as light photons are scattered into the line of sight by particles above and below the torus acting like a "dentist's mirror" (Antonucci & Miller, 1985; Tran 1995; Antonucci 1993; Wills 1999). The lower panel of Figure 2 shows a sketch of a Seyfert nucleus with the different types of AGN observed as angle between line of sight and torus axis increases. Figure 3 shows an image of the molecular torus in NGC 4151, surrounding the mini, quasar-like radio jet emanating from the centre of the galaxy, as predicted by the unification scheme.
Figure 3. Left: Radio image of neutral hydrogen gas in the spiral Seyfert host galaxy NGC 4151 (Mundell et al. 1999); Right: composite image of the central regions of NGC 4151 showing a 1.4-GHz radio image of the well-collimated plasma jet surrounded by an obscuring torus of molecular hydrogen imaged at 2.2 µm (Fernandez et al. 1999) and an inferred inner ring of neutral hydrogen from absorption measurements (Mundell et al. 2002).
Radio quiet quasars also have broad and narrow lines and are considered to be the high luminosity equivalents of Seyfert type 1 galaxies. A population of narrow-line quasars, high luminosity equivalents to obscured Seyfert 2s, are predicted by the unification scheme but, until now, have remained elusive. New optical and infrared sky surveys are beginning to reveal a previously undetected population of red AGN (Cutri et al. 2001) with quasar type 2 spectra (Djorgovski et al. 1999) and weak radio emission (Ulvestad et al. 2000). A significant population of highly obscured but intrinsically luminous AGN would alter measures of AGN evolution, the ionisation state of the Universe and might contribute substantially to the diffuse infrared and X-ray backgrounds.
3.3. Further unification?
The presence of gas emitting broad and narrow optical lines in radio-loud AGN and the discorvery of mini radio jets in Seyferts (e.g. Wilson & Ulvestad 1982) led to further consistency between the two unification schemes. Nevertheless, the complete unification of radio-loud and radio-quiet objects remains problematic, particularly in explaining the vast range in radio power and jet extents, and might ultimately involve the combination of black hole properties, such as accretion rate, black hole mass and spin, and orientation (Wilson & Colbert, 1995; Boroson 2002).