8. VELOCITY FIELD

The nature(s) of the velocity field(s) in AGNs is one of the most important subjects of present research. Veron (1981), Pelat, Alloin and Fosbury (1981), Atwood, Baldwin and Carswell (1982), Filippenko and Halpern (1984), De Robertis and Osterbrock (1984; 1986), Vrtilek and Carleton (1985), Whittle (1984a, b, c), and others have investigated the line profiles in Seyfert 2s and the narrow-line profiles in Seyfert 1s. They are generally broader than the emission lines in HII region galaxies, but some AGNs have lines as narrow as in some HII region galaxies (FWHM 200 km s^-1). The forms of the narrow-line profiles are almost invariably asymmetric, with a longer tail to the blue than to the red. The profiles of all the different narrow lines in any given AGN are quite similar; the profiles in different AGNs are also similar to one another. They can all be described by one general form. This strongly suggests that the same physical mechanism is important in all these objects.

Radial flow plus extinction will clearly produce the observed asymmetry, and has been suggested as the mechanism in many of the papers listed above. Simple arguments, however, cannot be used to settle the question as to whether the sense of the flow is radially inward or outward, though most of the observers have assumed it to be an outwardly flowing wind. Vrtilek (1985) has calculated several series of kinematic models on this basis, and shown that they can fit the observed [O III] profiles well. Many of the specific features of the profiles can be reproduced by a simplified dynamical "catapult" model, in which the clouds move under the combined forces of gravity and the ram pressure from the central source (Mardaljevic, Raine and Smith 1986; Mobasher and Raine 1987). These dynamical, photoionization models appear promising; a crucial test is that one model reproduce the relative strengths and profiles of all the lines in an observed AGN spectrum.

All the models published to date assume spherical symmetry, largely to simplify the calculations. Likewise, discussions of the direction of the velocity field (radially inward or outward) as determined from the observed profiles, such as that to be given by De Robertis and Shaw at this symposium, have all been based on the assumption of spherical symmetry. Actually, as described above, there are many observational indications of cylindrical symmetry, with much of the dust responsible for the extinction concentrated to the plane of symmetry. Thus models, such as that schematically indicated in Figure 7, in which the velocity of the ionized gas is radially outward from the central nucleus, but there is strong extinction by dust in dense, mostly neutral gas in a plane, should also be explored.

Figure 7. Schematic model of AGN with ionized gas flowing radially outward in spherical or conical distribution, and heavy obscuration in a plane, leading to line profiles with asymmetric wings extended to blue.

In many Seyfert galaxies the widths of the emission lines are correlated either with ionization potential of the ions concerned or with critical electron density for collisional deexcitation of the ionic energy levels concerned. Correlations in which the FWHMs increase with increasing potential are more prevalent in Seyfert 1s, while correlations in which the FWHMs increase with increasing critical density are more prevalent in Seyfert 2s. However, both types of correlations also occur in some Seyfert galaxies of the other type, and in still others there are not significant correlations of either type. These results strongly suggest that the mean electron density decreases outward, away from the central source, which agrees with (and is no doubt related to, or an alternate description of) the density decrease from the BLR to the NLR. These different types of correlations indicate different types of mean density distributions; on the average more nearly constant with distance from the nucleus in Seyfert 1s, and decreasing more steeply with distance in the Seyfert 2s (De Robertis and Osterbrock 1986).

However, very high-resolution line profiles recently obtained by Veilleux, with the Hamilton coude spectrograph at Lick Observatory show that many of these narrow line profiles have irregularities or multiple components at this velocity scale, as he will report at this Symposium. Such features were previously described, particularly by Pelat and Alloin (1982).

Good signal-to-noise ratio measurements of broad-line profiles of Seyfert 1 galaxies have been obtained by Osterbrock and Shuder (1982) and by Crenshaw (1986), and of QSOs by Wilkes (1984), among others. In contrast with narrow-line profiles, the broad-line profiles have no preferred direction of asymmetry. Roughly equal numbers of Seyfert 1s have broad-line profiles that are asymmetric to the blue and to the red, and a sizable fraction have approximately symmetric profiles. Radial flow alone will not produce such profiles, unless there are large deviations from symmetry in the density distribution in the BLR, such as might be described as a relatively few, separate large clumps. Symmetric kinematical models combining outward flow along an axis or in a cone, combined with rotation in a disk can fit the observed profiles (De Robertis 1985).

However, there are severe theoretical difficulties with the concept of a Keplerian rotational of velocity field under the gravitational force of the central black hole, because of the calculated shearing and destruction of clouds (Mathews 1986). Yet, if this problem of stability could be solved, a typical average velocity v 2500 km s^-1, a typical black-hole mass M 10⁸ M, and a typical average radius R 0.035 pc agree with the gravitational-force equation v² = GM / R. Furthermore, analysis of the broad-line profiles shows that the line ratios H / H, He I 5876 / H and He II 4686 / H all increase with increasing velocity, all as predicted if the velocity and density both increase inward toward the central source, as in a rotational velocity field (Shuder 1982).

Furthermore, the radial flow has generally been assumed to be due to an outward wind of a hot, confining medium, which carries the clouds with it. However, a recent theoretical analysis shows that the temperature in any such wind T 10⁷ °K, but that at such temperatures, the wind cannot confine the clouds (Mathews and Ferland 1987). Evidently severe discrepancies still exist between observational data on the velocity field in the BLR and our theoretical understanding of this region. One recent suggestion is that the BLR may be pervaded by a magnetic field of sufficient strength to confine the observed BL clouds, and that it (with a low-density plasma) may thus replace the hot wind in the models (Rees 1987).