At this stage, a number of properties of AGN were recognized. Most astronomers accepted the cosmological redshift of QSOs, and the parallel between Seyfert galaxies and QSOs suggested a common physical phenomenon. Questions included the nature of the energy source, the nature of the continuum source and emission-line regions, and the factors that produce an AGN in some galaxies and not others.
4.1. Emission Lines
The basic parameters of the region of gas emitting the narrow emission lines were fairly quickly established. In one of the first physical analyses of "emission nuclei" in galaxies, Woltjer (1959) derived a density Ne 104 cm-3 and temperature T 20,000 K from the [S II] and [O III] line ratios of Seyfert galaxies. The region emitting the narrow lines was just resolved for the nearest Seyfert galaxies, giving a diameter of order 100 pc (e.g., Walker 1968; Oke and Sargent 1968). Oke and Sargent derived a mass of ~ 105 M and a small volume filling factor for the narrow line gas in NGC 4151. Burbidge, Burbidge, and Prendergast (1958) found that the nuclear emission lines of NGC 1068 were much broader than could be accounted for by the rotation curve of the galaxy, and concluded that the material was in a state of expansion.
A key question was why, in objects showing broad wings, these were seen on the permitted lines but not the forbidden lines. (Seyfert galaxies with broad wings came to be called "Seyfert 1" or "Sy 1" and those without them "Sy 2" [Khachikian and Weedman 1974].) Were these wings emitted by the same gas that emits the narrow lines? Woltjer (1959) postulated a separate region of fast moving, possibly gravitationally bound gas to produce the broad Balmer line wings of Seyfert galaxies. Souffrin (1969a) adopted such a model in her analysis of NGC 3516 and NGC 4151. Alternatively, broad Balmer line wings might be produced by electron scattering (Burbidge et al. 1966). Oke and Sargent (1968) supported this possibility for NGC 4151. Their analysis of the emission-line region gave an electron scattering optical depth e ~ 0.1. Multiple scattering of Balmer line photons by the line opacity might increase the effective electron scattering probability, explaining the presence of wings only on the permitted lines. However, analysis of electron scattering profiles by other authors (e.g., Weymann 1970) indicated the need for a dense region only a tiny fraction of a light year across. Favoring mass motions were the irregular broad line profiles in some objects (Anderson 1971), which demonstrated the presence of bulk velocities of the needed magnitude. In addition, Shklovskii (1964) had argued for an electron scattering optical depth es < 1 in 3C 273 to avoid excessive smoothing of the continuum light variations. The picture of broad lines from a small region of dense, fast moving clouds ("Broad Line Region" or BLR) and narrow lines from a larger region of slower moving, less dense clouds ("Narrow Line Region" or NLR) found support from photoionization modes (Shields 1974).
Early workers (e.g., Seyfert 1943) had noted that the narrow line intensities resembled those of planetary nebulae, and photoionization was an obvious candidate for the energy input to the emitting gas for both the broad and narrow lines. For 3C 273, Shklovskii (1964) noted that the kinetic energy of the emission- line gas could power the line emission only for a very short time, whereas the extrapolated power in ionizing ultraviolet radiation was in rough agreement with the emission line luminosities. Osterbrock and Parker (1965) argued against photoionization because of the observed weakness of the Bowen O III fluorescence lines. Also eliminating thermal collisional ionization because of the observed wide range of ionization stages, they proposed ionization and heating by fast protons resulting from high velocity cloud collisions. Souffrin (1969b) rejected this on the basis of thermal equilibrium considerations, and argued along with Williams and Weymann (1968) that thermal collisional ionization was inconsistent with observed temperatures. Noting that an optical-ultraviolet continuum of roughly the needed power is observed, and that the thermal equilibrium gives roughly the observed temperature, Souffrin concluded that a nonthermal ultraviolet continuum was "the only important source of ionization". Searle and Sargent (1968) likewise noted that the equivalent widths of the broad H emission lines were similar among AGN over a wide range of luminosity and were consistent with an extrapolation of the observed "nonthermal" continuum as a power law to ionizing frequencies. Detailed models of gas clouds photoionized by a power-law continuum were calculated with the aid of electronic computers, with application to the Crab nebula, binary X-ray sources, and AGN (Williams 1967; Tarter and Salpeter 1969; Davidson 1972; MacAlpine 1972). Such models showed that photoionization can account for the intensities of the strongest optical and ultraviolet emission lines. In particular, the penetrating high frequency photons can explain the simultaneous presence of very high ionization stages and strong emission from low ionization stages, in the context of a "nebula" that is optically thick to the ionizing continuum. Photoionization quickly became accepted as the main source of heating and ionization in the emission-line gas.
Attention then focussed on improving photoionization models and understanding the geometry and dynamics of the gas emitting the broad lines. It was clear that the emitting gas had only a tiny volume filling factor, and one possible possible geometry was the traditional nebular picture of clouds or "filaments" scattered through the BLR volume. Photoionization models typically assumed a slab geometry representing the ionized face of a cloud that was optically thick to the Lyman continuum. Model parameters included the density and chemical composition of the gas and the intensity and energy distribution of the incident ionizing continuum. Various line ratios, such as C III] / C IV, were used to constrain the "ionization parameter", i.e., the ratio of ionizing photon density to gas density. Chemical abundances were assumed to be approximately solar but were hard to determine because the high densities prevented a direct measurement of the electron temperature from available line ratios.
A challenge for photoionization models was the discovery that the L / H ratio was an order-of-magnitude smaller than the value ~ 50 predicted by photoionization models at the time (Baldwin 1977a; Davidsen, Hartig, and Fastie 1977). This stimulated models with an improved treatment of radiative transfer in optically thick hydrogen lines (e.g., Kwan and Krolik 1979). These models found strong Balmer line emission from a "partially ionized zone" deep in the cloud, heated by penetrating X-rays, from which Lyman line emission was unable to escape. The models still did not do a perfect job of explaining the observed ratios (e.g., Lacy et al. 1982) of the Paschen, Balmer, and Lyman lines. Models by Collin-Souffrin, Dumont, and Tully (1982) and Wills, Netzer, and Wills (1985) suggested the need for densities as high as Ne 1011 cm-3 to explain the H / H ratio.
The X-ray heated region also was important for the formation of the strong Fe II multiplet blends observed in the optical and ultraviolet. Theoretical efforts by several authors culminated in models involving thousands of Fe lines, with allowance for the fluorescent interlocking of different lines (Wills et al. 1985). These models enjoyed some success in explaining the relative line intensities, but the total energy in the Fe II emission was less than observed. Although some of this discrepancy might involve the iron abundance, Collin-Souffrin et al. (1980) proposed a separate Fe II emitting region with a high density (Ne 1011 cm-3) heated by some means other than photoionization. This region might be associated with an accretion disk. The Fe II emission and the Balmer continuum emission that combined to form the 3000 Å "little bump" still are not fully explained, nor is the tendency for radio loud AGN to have weaker Fe II and steeper Balmer decrements than radio quiet objects (Osterbrock 1977).
A tendency for the equivalent width of the C IV emission line to decrease with increasing luminosity was found by Baldwin (1977b). Explanations of this involved a possible decrease, with increasing luminosity, in the ionization parameter and in the "covering factor", i.e., the fraction ( / 4) of the ionizing continuum intercepted by the BLR gas (Mushotzky and Ferland 1984). The ionization parameter was also the leading candidate to explain the difference in ionization level between classical Seyfert galaxies and the "low ionization nuclear emission regions" or "LINERs" (Heckman 1980; Ferland and Netzer 1983; Halpern and Steiner 1983).
The geometry and state of motion of the BLR gas has been a surprisingly stubborn problem. If the BLR was a swarm of clouds, they might be falling in (possibly related to the accretion supply), orbiting, or flying out. Alternatively, the gas might be associated with an accretion disk irradiated by the ionizing continuum (e.g., Shields 1977; Collin-Souffrin 1987). Except for the BAL QSOs, there was little evidence for blueshifted absorption analogous to the P Cygni type line profiles of stars undergoing vigorous mass loss. The approximate symmetry of optically thick lines such as L and H suggested that the motion was circular or random rather than predominantly radial (e.g., Ferland, Netzer, and Shields 1979). However, for orbiting (or infalling) gas, the line widths implied rather large masses for the central object, given prevailing estimates of the BLR radius. In addition, gas in Keplerian orbit seemed likely to give a double peaked line profile or to have other problems (Shields 1978a). In the face of these conflicting indications, the most common assumption was that the gas took the form of clouds flying outward from the central object. The individual clouds would disperse quickly unless confined by some intercloud medium, and a possible physical model was provided by the two-phase medium discussed by Krolik, McKee, and Tarter (1981). Radiation pressure of the ionizing continuum, acting on the bound-free opacity of the gas, seemed capable of producing the observed velocities and giving a natural explanation of the "logarithmic" shape of the observed line profiles (Mathews 1974; Blumenthal and Mathews 1975). Interpretation of the line profiles was complicated by the recognition of systematic offsets in velocity between the high and low ionization lines (Gaskell 1982; Wilkes and Carswell 1982; Wilkes 1984)
A powerful new tool was provided by the use of "echo mapping" or "reverberation mapping" of the BLR. Echo mapping relies on the time delays between the continuum and line variations caused by the light travel time across the BLR (Blandford and McKee 1982). Early results showed that the BLR is smaller and denser than most photoionization models had indicated (Ulrich et al. 1984; Peterson et al. 1985). Masses of the central object, by this time assumed to be a black hole, could be derived with increased confidence. The smaller radii implied smaller masses that seemed reasonable in the light of other considerations, and the idea of gravitational motions for the BLR gained in popularity. This was supported by the rough tendency of the line profiles to vary symmetrically, consistent with "chaotic" or circular motions (e.g., Ulrich et al. 1984).