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

4.3. Extranuclear Ionized Gas

A wealth of extranuclear emission-line regions (ELR) have been studied in the past five years. The relevance of much or all of this information to the causes of galactic activity is uncertain, however. Recombination and cooling times in these nebulae are short (typically 10⁷ yr) compared to dynamical time-scales; this suggests that the agents that ionize and heat the gas must be continuously supplied, presumably directly or indirectly by the active nucleus. Energy estimates (10⁵² - - 10⁵⁵ erg in the ionized nebulae, 10⁵⁸ ergs in the nucleus or associated lobes of radio galaxies) add credence to this suggestion. Thus we must view the ionized nebulae with caution because clues to the origin of galactic activity implanted in the gas may be obliterated by the energy sources that power the ionized component.

ACTIVE GALAXIES     At the risk of adding credence to speculation, we have chosen to organize the following discussion by the mechanisms proposed to explain the ELRs and the origin of nuclear activity.

Accretion flows in dominant cluster galaxies     Such flows are unique, at least in a quantitative sense, to giant cluster-centered ellipticals (or cDs), as discussed in Sections 2.1, 3.1, and 3.3. Ionized gas is observed in nearby cDs, including M87 and NGC 1275, and more than half of thirteen cDs surveyed by Heckman (1981) (see also Fabian et al. 1981). Heckman's limits on the nondetected galaxies do not exclude emission at the intrinsic level of M87. Ford & Butcher (1979), Kent & Sargent (1979), Stauffer & Spinrad (1979), and others find that the filaments in M87 and NGC 1275 are probably collisionally ionized. The galaxies detected by Heckman (1981) have spectra similar to these two closer cDs; moreover, their luminosity correlates with X-ray luminosity and cluster richness, as might be expected for an accretion flow (e.g. Binney & Cowie 1981, Mushotsky et al. 1981; see also Section 2.1). The ELRs probably arise in cool (10⁴ K) thermal instabilities, perhaps triggered by the radio source pushing outward through the accretion flow (Ford & Butcher 1979).

In any event, the ELRs are associated with material involved in the flow to the nucleus, and useful abundance and kinematic information can be obtained. In M87 and NGC 1275, abundances are comparable to the respective nuclear (and solar) abundances, and point-to-point velocity differences and line widths are on the order of the sound speed in the adjacent X-ray-emitting gas (~ 10^2.5 km s^-1; Rubin et al. 1977, 1978, De Young et al. 1980, and other references above). Thus the ELRs and X-ray gas seem to be in close contact.

NGC 1275 is unique in its morphology, multiple redshift systems, and large emission-line luminosity among nearby cD systems. Only the lower of the two redshift systems appears to arise from an accretion flow (e.g. Kent & Sargent 1979).

Encounters and mergers     Explanations of the ELRs seen in some active early-type galaxies in terms of a recent encounter or merger have recently become very popular. Fosbury (1981) has suggested that the ELRs in the active ellipticals 2158-380 and 0349-27 are the partially relaxed remnants of disrupted galaxies. The same type of scenario can be (and often has been) applied to active galaxies such as 3C 120 (Baldwin et al. 1980), Cen A (Dufour et al. 1979, Tubbs 1980; in addition see Graham & Price 1981, Phillips 1981), Fornax A (Schweizer 1980), 3C 305 (Heckman et al. 1981a), IC 5063 (Caldwell & Phillips 1981, Danziger et al. 1981), IC 1182 = Mrk 298 (Bothun et al. 1981b), NGC 3310 (Balick & Heckman 1981), NGC 6240 (Fosbury & Wall 1979), NGC 1275 (Rubin et al. 1977, 1978, Kent & Sargent 1979), and the class of NGC 1052 / NGC 4278-type emission-line ellipticals (e.g. Fosbury et al. 1978, Raimond et al. 1981, Ulrich et al. 1980).

The higher velocity ELR in the multiple redshift system in NGC 1275 is of particular interest. It has been identified by Rubin et al. (1977, 1978) and Kent & Sargent (1979) as a rotating spiral galaxy, probably a nearly edge-on late-type system, seen projected onto the elliptical. The peculiar properties of the putative spiral (disturbed appearance, blue colors, abundance of giant H II regions) have been cited (e.g. van den Bergh 1977) as evidence against this hypothesis. These peculiar properties cannot be attributed to a dynamical interaction between the spiral and elliptical because the velocity difference between the two galaxies (~ 3000 km s^-1) is so large relative to characteristic galaxian dynamical velocities (e.g. Toomre & Toomre 1972). However, a hydrodynamical interaction between the spiral and the dense X-ray gas centered on NGC 1275 could severely alter the appearance of a gas-rich interloper (e.g. ram-pressure-driven star formation and ablation of the interstellar medium) and might perturb gas near the nucleus of the elliptical so as to facilitate its infall into the nucleus.

Noninteracting active spiral galaxies     The mechanism proposed to explain activity in such systems is related to global asymmetries in the gravitational potential, which can deliver gas into the nuclear vicinity (Section 3.4). Observations of gas motions in barred systems by Meaburn et al. (1981) and by Peterson, Huntley, and their collaborators confirm the existence of radial flows near the nucleus. It is thus surprising that barred systems do not show enhanced incidence or luminosity of nuclear activity.

Observations neither confirm nor reject the notion that radial streams induced by nonaxisymmetric potentials fuel nuclear activity. However, few detailed studies of near-nuclear gas motions have been made. Limited kinematical data are available for NGC 3516, an SBO galaxy (Ulrich & Pequignot 1980). Hydrodynamical models by Sanders & Tubbs (1980) of gas flows in ordinary barred galaxies with "slow" bars qualitatively explain the kinematics and the morphology of extranuclear gas in this galaxy. The motions of gas near the nucleus of NGC 1068 (an Sc galaxy with an ovally distorted disk and bright outer ring - e.g. Hodge 1968) are very chaotic (Walker 1968). Some speeds are well in excess of the galaxy's escape velocity (see also Pelat & Alloin 1980). Neither galaxy offers much insight into the origins of nuclear activity.