The Narrow-line Region

Jet-Excited Regions

The various classes of bright and compact extragalactic radio sources form an fascinating subset of all radio sources which holds the key to our understanding to the evolution of radio jets in general. These include the steep-spectrum radio sources (CSS) (Fanti et al. 1990), the Gigahertz-peaked sources (GPS) (O'Dea et al. 1990, and references therein), the compact symmetric objects (CSO) (Wilkinson et al. 1994, and references therein) and the compact double sources (CD) (Phillips and Mutel, 1982). Following the Caltech-Jodrell Bank and Bologna-Jodrell-Dwingeloo surveys (Wilkinson et al. 1994; Fanti et al. 1995), these classes of radio sources are now understood to represent an appreciable fraction (10-30%) of the luminous radio sources. Not only are these sources very luminous at radio frequencies, but they are also very luminous in the optical. The spectra of Gelderman and Whittle (1994) and Gelderman (1995) reveal the broad emission lines of the AGN itself as well as intense ``narrow line'' emission reminiscent of Seyfert II galaxies. The similarity of properties of the different classes of compact sources argues strongly that the same physical processes are at work in all of them.

Following Begelman (1996), Bicknell, Dopita and O'Dea (1997) assumed that all classes of compact, young radio sources are manifestations of a jet-driven outflow in a galaxy which is also undergoing an accretion or merging event. As a result of the accretion, the ambient density of the galactic medium will be strongly peaked toward the centre. A reasonably physical model density distribution could that produced by a steady inflow, , or, at later phases, that produced by free fall towards a massive nuclear core; . Bicknell, Dopita and O'Dea (1997) computed the evolution of jets with a generalised form of the density distribution; .

As the jet propagates from the AGN, it is either disrupted by Kelvin-Helmholz instabilities within it or by the termination shock, and back-fills the cavity it has produced, giving rise to a symmetric pair of relativistic plasma bubbles on either side of the nucleus. As these bubbles expand, wall shocks are pushed into the galactic medium by the cocoon overpressure. Provided that these shocks can become radiative within a dynamical expansion timescale, then the optical emission of these shocks and the general outflow associated with them will become visible as a shock-excited narrow-line region (NLR). In this model, the fluxes of the optical emission lines such as [O III] or the H + [N II] complex, is almost proportional to the mechanical energy flux through the shocked region, i.e. to the total luminosity of the shocks,

 
However, the shock luminosity is equal to the rate of work done by the expansion of the radio lobe, which is given by:

 
where is the jet energy flux. Thus, with = 2, the total luminosity in the NLR is 1/2 of the jet energy flux.

Note that equation (12) allows a maximum jet energy flux of about 30% of the bolometric luminosity of the BH. Thus the NLR will have a maximum luminosity of 15% of the bolometric luminosity of the BH. On the other hand, equation (11) would imply that the radiative wind carries about 5-10% of the bolometric luminosity, which would be likewise capable of shock exciting an NLR . In this case, the NLR accounts for about 2-5% of the bolometric luminosity of the BH. This limit is appropriate for the mass-entrained jets of radio-quiet objects, and in general, we might expect the luminosity of the NLR to lie between these two limits.

In the case of objects with ``buried'' or dust enshrouded narrow-line regions, the extended far-infrared luminosity may possibly be used as a tracer of mechanical energy input. A shock with a velocity of 500 km.s puts 13% of its total luminosity in the Ly line, and its precursor adds another 9% for a grand total of 22%. However, faster shocks emit a greater fraction of their luminosity in this line, thanks mostly to increasing amounts of collisional excitation in the partially ionised recombination zone of the shock. For a 1000 km.s shock, the percentage of total luminosity in Ly rises to 24% and in the precursor to 9%. Thus, a total of 34% of the mechanical energy can be converted into Ly, which is then largely trapped in the surrounding medium, absorbed by dust grains, and re-radiated as far-IR emission. If we also take into account the competition of the dust grains in directly absorbing hard UV and soft X-rays in the precursor, the percentage of the mechanical energy flux through the shock which is converted to far IR dust emission may rise to as much as 50%. Nontheless, these figures mean that only 2-7% of the bolometric luminosity of the BH is ultimately converted to far-infrared photons. Direct absorption of the radiative energy of the BH will remain the dominant source of far-infrared photons.

In Figure 1, we plot the [O III] line flux in the NLR against the radio flux associated with the AGN for a wide range of AGN subtypes. Note the clear bifurcation between the radio-loud and the radio-quiet classes, with the exception of the radio-intermediate IRAS galaxies taken from McGregor, van Breugel and Kewley (1997). This rare class of objects displays post-starburst continua (a spectrum dominated by A-type stars, with prominent Balmer absorption), along with very luminous NLR emission presumably powered by lobes with a large admixture of thermal gas, giving them properties between the radio-quiet and the radio-loud branches. In the context of the model presented here, Figure 1 could be considered as a ``Hertzprung-Russell'' diagram for AGN. In gas-rich post-merger systems as the BH grows, the AGN moves up the radio-quiet sequence. At the same time, some of the gas is converted to stars which eventually form the nuclear bulge of the galaxy. When the accretion rate into the nuclear region drops below the Eddington-limited value, the relativistic jet escapes, and the AGN moves across though the radio-intermediate IRAS stage to the radio loud QSO region. At this point, the merged galaxy would be classified as an elliptical. Because this galaxy now has a massive BH at its centre, subsequent mergers of low-mass gas-rich systems such as Ir galaxies cannot as easily produce super-Eddington inflow, but may produce inflow which approaches the Eddington rate. These systems will generate radio-loud QSOs, GPS or CSS radio sources, and subsequently FR II radio sources. Accretion due to cooling flows can only remain sub-Eddington. In these sources the NLR is LINER like, and, as I show in the next section, this sub-Eddington accretion phase is likely to be related to BL Lac or low-power FR 1 activity.

LINERs - The Signature of Accretion Disks

The low-ionization nuclear emission-line regions, LINERs, were first recognised as a distinct class of AGN by Heckman (1980). Where these emission line regions can be resolved, they appear as an extended region strongly peaked toward the nucleus. The velocity broadening of these regions is generally a few hundred km.s, with evidence for a considerable degree of rotational support in many objects. The emission line ratio criteria given by Heckman define a distinct (if somewhat arbitrary) region of excitation space for these objects. According to his definition the [O II] lines are stronger than [O III] ; the [O I] /[O III] ratio is less than about 0.33; and the [N II] /H ratio is larger than about 0.6. A survey of ``normal'' elliptical galaxies by Phillips et al. (1986) showed that some low-level LINER activity is found in an appreciable fraction of these. The recent work of Ho, Filippenko, & Sargent (1995a) has shown that low level LINER activity is even more ubiquitous than had been suspected in both elliptical and spiral galactic nuclei. Elliptical LINER galaxies cover a range of radio properties, including radio quiet objects, weak nuclear sources, and Fanaroff-Riley (FR) Class I sources. However, no example of a LINER is yet known to be associated the high-power FR II radio sources. In terms of the correlation between the optical and the radio luminosities, a similar trend emerges (Baum, Zirbel, & O'Dea 1995). There is a good correlation for the FR II class, but this disappears smoothly at the FR I-II transition. These correlations suggest that the optical emission makes a transition from predominantly jet-excited to some other excitation mechanism at the FR I-II transition. An appreciable fraction of late type galaxies with LINER nuclei was found by Ho, Filippenko, & Sargent (1995) to have an underlying broad component of H, emphasizing the connection between LINERs and other types of AGN. Finally, LINERs, or at least objects having LINER-like spectra, are also found in the cooling flows associated with first-ranked elliptical galaxies in clusters (Heckman 1981; Cowie et al. 1983; Johnstone, Fabian, & Nelsen 1987; Heckman et al. 1989).

In the light of the model presented in this paper, this correlation with radio properties and host galaxies strongly suggests that LINERs are found in the objects with sub-Eddington (and possibly, advective; Reynolds et al. 1996) accretion rates. It is in these cases that the details of the extended accretion disk around the BH can be observed. A fine nearby example of this is presented by M87. From optical and UV spectroscopy with the HST, Dopita et al. (1997) have been able to demonstrate that the LINER emission arises by shock dissipation in a turbulent accretion flow derived from the M87 cooling flow as it settles into an organised flat accretion disk within a few parsecs of the nucleus. We know that this accretion continues down to the central massive object, since we see the optical synchrotron jets emerging orthogonally to the disk plane. The mechanical energy flux of the jets of the FR I source has been calculated by Bicknell & Begelman (1996) to be ergss, which implies an accretion rate onto the BH of about 2.10 yr. Since Ford et al. (1994) estimate the mass of the black hole to be , the rate of nuclear mass accretion in this object is only about of the Eddington rate. Clearly, M87 is an excellent example of a sub-Eddington accretion flow. In addition, Tsvetanov et al. (1997a, b; in preperation) have demonstrated that the nuclear spectrum of M87 is a power-law continuum, and is strongly variable on a timescale of weeks or months. These properties emphasise the link between FR I sources, LINER nuclei, and BL Lac objects.

Further (unpublished) HST spectrophotometric data on NGC1052, and M81; prototypes of LINERs in an elliptical and a spiral, respectively, show the strong UV lines expected in the case of shock excitation. In the case of NGC1052, it has long been asserted that it is a shock excited disk (Koski & Osterbrock, 1976; Fosbury et al. 1978). In addition, M81 shows some evidence for a broad-line nucleus and a dense inner accretion disk.

This strongly suggests that all LINERs are excited in the same way as the nuclear regions of M87, and that LINER activity is therefore a measure of the shock dissipation occuring in the disorganised accretion flows around BHs accreting at sub-Eddington rates. In the case of elliptical host galaxies, the central BHs may be quite massive, with the sub-Eddington inflow derived from a large-scale galactic or cluster cooling flow. In the case of spiral host galaxies, it may be that the accretion flow is simply derived from the ISM in the galaxy, but that the absence of a large-scale disturbance (such a a bar instability) ensures that this accretion flow is very weak.

© Copyright Astronomical Society of Australia 1997