Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

4.5. Seyfert Galaxies

The discovery of H₂ emission at 2µ from the nucleus of NGC 1068 (Thompson et al. 1978) directly connected the study of molecular emission from galactic nuclei with that of the physical conditions in active galactic nuclei. The molecular hydrogen appears to result from shock-heating of the gas (Shull & Beckwith 1982) to relatively high temperatures (T_K ~ 2000 K). Several thousand solar masses of H₂ are required to account for the H₂ emission in NGC 1068 (Hall et al. 1981); this is a tiny fraction of the molecular mass implied by the CO observations (Rickard et al. 1977a). Even if the observed CO emission arises throughout the disk, there is likely to be a relatively massive central concentration in which the regions of H₂ line emission are embedded.

Thompson et al. (1978) suggest that the H₂ exists under conditions similar to those in planetary nebulae (Beckwith et al. 1978), but this similarity is questioned by Hall et al. (1981), who argue that the source is a collection of molecular clouds like the Orion Molecular Cloud, and the H₂ emission is thus a manifestation of star formation. The large IR luminosity of the nucleus of NGC 1068 then results, as in Orion, from active star formation. Although suggested before (Adams & Weedman 1975, Telesco & Harper 1980, Telesco et al. 1980), this point is somewhat controversial; other studies have ascribed the heating of the dust to the influence of the compact nuclear ultraviolet source (e.g. Rieke & Lebofsky 1979).

Another investigation of the H₂ emission in NGC 1068 was carried out by Carlson & Foltz (1979), who assumed that the emission is generically related to the Seyfert phenomenon. Their ionization equilibrium calculations are similar to those used in models that attempt to reproduce the optical emission lines from atomic gas in the presence of an intense uv field (e.g. Shields & Oke 1975). Carlson & Foltz found that H₂ can he shielded by carbon, and therefore that enough H₂ can be formed from H^- to account for the observed emission. Indeed, in their model, photoionization can even provide the necessary heating if shock-heating is insufficient.

The CO observations show that NGC 1068 has an unusually large total H₂ mass [(5-36) x 10⁹ M; Rickard et al. 1977a, Lo et al. 1980], which is consistent with the unusually large far-infrared luminosity (Telesco & Harper 1980). This fact might not be directly related to the Seyfert character of the galaxy, however. Of 14 other Seyferts that have been searched (Rickard et al. 1977a, Wilson et al. 1979, Bieging et al. 1981, Blitz & Mathieu, private communication), only NGC 3227, 4051, and 6814 exhibit detectable CO emission. In addition, OH absorption is seen in NGC 7469 (Turner et al., in preparation) indicating a central H₂ concentration. Bieging et al. (1981) suggest that the CO emission of NGC 3227 and 4051 arises chiefly from the disks and is not related to the Seyfert activity. On the other hand, Rickard et al. (1982) contend that the OH absorption in NGC 3227 arises within a few hundred parsecs of the nucleus, so that the molecular clouds could conceivably be related to the nuclear activity. In any case, the H₂ masses for the Seyferts (other than NGC 1068) appear to be comparable to those that would be found in a collection of normal spirals.