|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
3.5. Stellar Populations and Birthrates
Biermann et al. (1979), De Young (1981), and others have suggested that star formation might result from nuclear activity or that star formation and nuclear activity are different manifestations of a common cause (in which case either process may occur independently). M82 is an example of an inactive galaxy undergoing a rapid burst of star formation, Mrk 335 is a luminous active galaxy with no signs of extranuclear stellar anomalies, and NGC 1068 has both an active nucleus and many young stars. In this section we explore whether nuclear activity and the presence of young stars are related and if so, how.
EMISSION LINE GALAXIES Spectroscopy of the nuclei of Seyfert, BL Lac, and radio galaxies (e.g. Costero & Osterbrock 1977, Koski 1978, Miller et al. 1978, Yee & Oke 1978, Wilkinson et al. 1981) have shown that the underlying continuum energy distribution is usually an admixture of featureless power-law and old stellar components; in such systems young stars are inconspicuous. Furthermore, although both young stars and low-level activity are not infrequent at the centers of early-type galaxies, the presence of the two phenomena is not correlated (Heckman 1980a, b). There are some important individual exceptions. In addition to those pointed out by the authors above, there are NGC 1275 (Rubin et al. 1977), 3C 305 (Heckman et al. 1981a), NGC 7582 (Clavel et al. 1980), Mrk 231 (Boksenberg et al. 1977, Kodaira et al. 1979), and Mrk 11 (Ulrich 1978).
Data on the stellar populations in extranuclear regions of active galaxies are sparse. Yee (1981) confirmed and extended the earlier results of de Vaucouleurs & de Vaucouleurs (1972), Smith et al. (1972), Weedman (1973), Penston et al. (1974), and Huchra (1980): The disks of Seyfert galaxies have colors that are normal for their morphological types. Thus far, the only Seyfert with abnormally blue colors outside the nucleus is NGC 1068 (Smith et al. 1972). NGC 1068 exhibits many other signs of recent star formation, such as an extended 20µ emission region (Telesco et al. 1980), bright H emission in a ring surrounding the nucleus, presumably from numerous H II regions (Alloin et al. 1981), and an anomalously large abundance of molecular gas (cf. Bieging et al. 1981).
Balick & Heckman (unpublished) have constructed H images of some 35 nearby active galaxies (mostly Seyferts) with the starlight removed. Of these, only 4 galaxies show evidence of abnormal numbers of H II regions for their morphological types. and it is not always obvious that nuclear activity bears any direct, let alone causal, relationship.
Sanders & Bania (1976) show how a nuclear explosion can cause transient rings in which star formation should be active. Large-amplitude resonances in density waves can achieve the same sort of result without help from the nucleus (van der Kruit 1976a). It is worth noting that galaxies with interior rings do not generally show enhanced levels of nuclear activity.
RADIO GALAXIES There is now evidence (Windhorst et al. 1981, Katgert et al. 1979, Spinrad et al. 1981) that star formation in the giant elliptical parent galaxies of radio sources has continued to relatively recent times. Models in which star formation has declined exponentially since galaxy formation (Bruzual 1981, Bruzual & Kron 1980) are successful at reproducing the available photometric (Windhorst et al. 1981) and spectroscopic (Spinrad et al. 1981) data. It is not yet known whether this continuing star formation in giant ellipticals is peculiar to radio galaxies (cf. Butcher & Oemler 1978, Turner 1980a). Heckman (in preparation) has examined the question of the stellar metallicity of active and inactive elliptical galaxies. He finds that active ellipticals tend to have stronger stellar metallic absorption lines than inactive ellipticals of the same galaxy luminosity. It is not yet possible to interpret this unambiguously in terms of a direct causal link between high metallicity and nuclear activity (e.g. it may be that both stellar metal abundance and activity are tied more directly to the galaxy mass than to luminosity).