Annu. Rev. Astron. Astrophys. 1982. 20: 431-468
Copyright © 1982 by . All rights reserved

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6. PERSPECTIVES

6.1. Summary

We have now reviewed the galactic and extragalactic environment of galaxies with emphasis on how active and normal galaxies may differ in their intrinsic properties.

CLUSTER ENVIRONMENT     Location in a cluster appears to be far less important for galactic activity than previously thought. Large, cluster-dominant ellipticals are often active. However, in general, the radio luminosity as a function of absolute magnitude does not differ strongly for cluster and field galaxies, Seyferts, quasars, and emission-line galaxies are less frequently found in the centers of rich clusters than in other relatively sparser environments.

LOCAL GALAXY DENSITY     Radio and optical-line emission is enhanced in galaxy groups or pairs compared to isolated galaxies. However, for very luminous activity the situation is less clear. Mergers and interactions appear to be very important for fostering activity.

MORPHOLOGICAL TYPE     Extended radio emission and the occurrence of BL Lac-type nuclear activity is most pronounced in elliptical galaxies. Luminous radio sources are frequently identified with D galaxies, which may differ from ellipticals. Radio morphology is relatively compact - and perhaps related to bursts of near-nuclear star formation in many spirals, particularly late-type. Flat-spectrum nuclear sources are seen in early-type spirals and predominately ultracompact SOs. Seyfert and low-level emission lines indicative of activity are seen almost exclusively in early-type galaxies, but may be deficient in ellipticals.

STELLAR DYNAMICS     Extended radio sources seem to occur preferentially in elliptical (D?) galaxies, which rotate relatively rapidly. The most luminous radio galaxies exhibit the highest degree of rotation axis-radio axis alignment. Radio-loud ellipticals exhibit higher core velocity dispersions (and hence higher masses) at a fixed galaxy luminosity than radio-quiet ellipticals. Many (or perhaps all) Seyfert nuclei are found in galaxies showing some sign of a global nonaxisymmetric potential (implying noncircular motions); however, classical stellar bars do not appear preferentially in active disk galaxies.

ABSOLUTE MAGNITUDE     The propensity for radio emission correlates well with galaxy absolute magnitude (especially for radio galaxies and BL Lacs). The hosts of Seyfert nuclei (and probably quasars) are galaxies of "normal" stellar luminosity. Nuclear activity of any kind avoids dwarf galaxies. More generally, it seems that bulge luminosity, rather than total stellar luminosity, is the critical parameter regarding activity.

STELLAR POPULATIONS     Anomalously large star-formation rates are observed in some active galaxies, but cannot be linked statistically to nuclear activity. Radio-loud ellipticals have a higher metal-abundance stellar population at a fixed galaxy luminosity than radio-quiet ellipticals.

DUST     Some radio galaxies exhibit conspicuous dust lanes oriented along their minor axes and perpendicular to the radio-lobe axis. Some Seyferts are exceptionally dusty galaxies. Statistical data on the frequency of anomalous dust in active galaxies compared to normal galaxies are not available. Dust is probably ubiquitous in the emission-line regions in active nuclei.

NEUTRAL AND MOLECULAR GAS     Gas-rich ellipticals are more likely to be active than gas-poor systems. In spirals there is no clear correlation of nuclear activity and gas content. Some exceptionally gas-rich galaxies are active. HI absorption is seen in several active galaxies, often at a greater velocity than the galactic systemic velocity by about 102 km s-1. However, cases of blue-shifted H I absorption are also known, so interpretation is not vet completely clear.

IONIZED GAS     Active galaxies often exhibit extranuclear ionized gas, but this is probably more a result of activity than a cause. Study of the properties of extranuclear emission line gas lends support to the idea of galaxy mergers and of cooling accretion flow onto cluster-centered giant ellipticals (two hypothetical causes of nuclear activity). In some cases, highly unusual emission-line regions can be seen outside of active galaxies that pose challenging interpretive problems.

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