Annu. Rev. Astron. Astrophys. 1992. 30: 705-742
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Observationally, it has long been known that there may be a link between interactions and unusual forms of energy generation in galaxies. Baade & Minkowski (1954) argued that the radio source in Cygnus A consists of ``. . . two galaxies in actual collision.'' However, it was widely believed that objects like Cygnus A were anomalous and that most galactic activity was produced by explosions or other violent events within isolated galaxies. Only during the past decade have observations challenged this long-standing prejudice (for a discussion, see Balick & Heckman 1982).

Attempts to establish a definitive causal relationship between galactic activity and interactions have evoked considerable controversy. Statistical analyses are typically plagued by the lack of reliable ``control samples'' against which the active galaxies should be compared. This problem is especially acute for cosmologically distant sources since it is not easy to identify the morphological types of the galaxies (e.g. Stockton 1990). Detailed studies of individual objects, on the other hand, provide direct evidence as to the import of interactions in select cases, but provide little basis for generalization. Even so, such ``case'' studies seem more persuasive than statistical analyses in view of the difficulties associated with establishing definitive control samples.

Global Starbursts

In a classic paper, Larson & Tinsley (1978) demonstrated that the peculiar galaxies in the Arp (1966) atlas tend to be bluer, on average, than their isolated counterparts. The best fits to their data invoke intense bursts of star formation throughout the peculiar galaxies to account for their anomalous colors. These findings have been corroborated by many subsequents studies, including those of Joseph et al. (1984), Kennicutt et al. (1987), Bushouse (1987), Telesco et al. (1988), and Laurikainen & Moles (1989). There is even evidence for induced star formation between colliding galaxies (e.g. Thronson et al. 1989). However, it should be emphasized that not all interacting pairs display this behavior (Keel et al. 1985; Bushouse 1986; Kennicutt et al. 1987; Bushouse et al. 1988; Solomon & Sage 1988).

Thus, the observational link between encounters and enhanced star formation seems quite compelling. A likely explanation is that interactions accelerate star formation as gas is compressed in shocks and cloud-cloud collisions (e.g. Young et al. 1986). However, the severity of these effects undoubtedly depends on the orientations of the disks and the orbital geometry, so a starburst is not guaranteed in all collisions.

Theoretical work on this issue remains ambiguous. Using numerical simulation, Noguchi & Ishibashi (1986) and Olson & Kwan (1990) argue that star formation is enhanced during galaxy interactions through increased collisions between molecular clouds in disks; a finding disputed by Mihos et al. (1991). In reality, the connection between cloud-cloud collisions and star formation is poorly understood. While the numerical simulations do demonstrate that large-scale ``shocks'' develop in the gas as it is compressed by tidal perturbations, no existing method possesses the dynamic range needed to translate the measured effect into a reliable star formation rate.

Infrared-Luminous Galaxies

The most extreme examples of starbursting objects are those where the peculiar emission is generated mainly in the nucleus (for a summary of the observations, see Soifer et al. 1987). Spectacular examples of this phenomenon were revealed by the IRAS survey: Sources having infrared luminosities up to 1013 Lsmsun , the brightest of which invariably possess features typical of merging galaxies (e.g. Soifer et al. 1984a, b; Allen et al. 1985; Joseph & Wright 1985; Sanders et al. 1986; Armus et al. 1987; Sanders et al. 1988a, b; Kleinmann et al. 1988). CO observations of these objects indicate that they often contain large quantities of gas in their nuclei. For example, Mrk 231, Arp 220, and NGC 3256 appear to contain nuclear gas masses in excess of 1010 Msmsun , confined to their inner few hundred parsecs (e.g. Scoville et al. 1986; Sargent et al. 1987; Sargent et al. 1989).

There seems little doubt that mergers are somehow responsible for the activity in at least the brightest IRAS objects (e.g. Sanders et al. 1988a, b; Sanders 1992). This hypothesis is supported by numerical experiments which indicate that mergers between comparable-mass disks can create substantial nuclear gas concentrations (Negroponte & White 1983; Noguchi 1991; Barnes & Hernquist 1991). Gas accumulates in the center of each galaxy prior to the merger and eventually the two gas concentrations sink to the center of the ensuing remnant by dynamical friction. An example may be Arp 220, which appears to contain a double-Seyfert nucleus (Norris 1988; Diamond et al. 1989; Graham et al. 1990; but see Smith et al. 1988). There are indications that many and perhaps even all of the ultraluminous IRAS objects possess double nuclei and that the intensity of the emission is correlated with the proximity of the nuclei (Sanders 1992). Indeed the apparent dearth of single-nucleus luminous infrared galaxies is somewhat unexpected - numerical experiments indicate that the double-nucleus phase is rather short-lived (e.g. Barnes & Hernquist 1991) - and there is no obvious mechanism that would shut off the IR emission once the nuclei have merged.

Seyfert Galaxies

The most common AGN in the local universe are those which were first studied in detail by Seyfert (1943; for a discussion, see Osterbrock 1991). Unlike radio ellipticals, Seyferts are associated with late galaxy types but, like radio galaxies, are more common in the field than in rich clusters (e.g. Dressler et al. 1985). A variety of studies hint that Seyferts tend to be found in galaxies interacting with nearby companions (e.g. Adams 1977; Dahari 1984; Kennicutt & Keel 1984; Keel et al. 1985; MacKenty 1989), a proposal which has evoked controversy owing to difficulties with control samples (Fuentes-Williams & Stocke 1988). Nevertheless, there now appears to be a general consensus that at least some types of Seyfert activity are correlated with galaxy collisions (Osterbrock 1991).

There are also many indications that mergers, in addition to transient encounters, are related to Seyfert activity. Some Seyferts display multiple nuclei and tidal tails, characteristic of merger events (Fricke & Kollatschny 1989; Kollatschny & Fricke 1989). It is probably also significant that a large fraction of Seyferts in MacKenty's (1990) sample are ``amorphous'' or otherwise disturbed. Numerical simulations by Hernquist (1989a, b; 1991a) demonstrate that the accretion of small satellites by gas-rich disks lead to rapid nuclear inflows of gas and leave remnants that have distorted, but mostly featureless disks. As in mergers of comparable-mass galaxies containing gas, these inflows can be driven by large-scale stellar bars, excited by the decaying satellite. Moreover, this effect can operate even if stellar bars are not generated during a merger (Hernquist 1989a, b). In some cases, the tidal field of the satellite ``squeezes'' gas orbits in the disk, leading to rapid dissipation as streamlines intersect. If the gas is self-gravitating it can fragment, and the blobs of gas left over will sink to the center of the disk by shedding angular momentum to surrounding stars via dynamical friction. It is natural to suppose that events such as these can simultaneously trigger Seyfert activity and leave remnants with global morphologies similar to those of the galaxies in MacKenty's sample. However, the fate of the central gas in the models is problematic and there is little hard observational evidence to support an evolutionary connection between starbursts and Seyfert activity (e.g. De Robertis & Shaw 1988).

A variety of models suggest that transient encounters can also drive gas to the centers of galaxies (e.g. Byrd et al. 1987; Noguchi 1987; 1988; Lin et al. 1988). However, these calculations either ignore the effects of dissipation or are not fully self-consistent. Judging from the simulations of Barnes (1988) and Barnes & Hernquist (1991) it is not inconceivable that the violent interactions needed to provoke nuclear inflows of gas would lead to mergers and complete destruction of the disks involved. The remnants of such events do not resemble Seyferts but are instead similar to the elliptical and peculiar galaxies which harbor starbursts and radio sources. These issues will not be resolved until detailed parameter surveys are available which include the full self-gravity of all components. It should also be noted that not all Seyfert activity is triggered by interactions and that some other fueling mechanism, such as bar-induced inflow (Simkin et al. 1980), may be more widespread than encounters of galaxies.

QSOs & Quasars

There is considerable indirect evidence to support the conjecture that quasar activity is triggered by galaxy collisions. Studies of the morphological structure of quasar hosts by several groups (e.g. Gehren et al. 1984; Hutchings et al. 1984; Malkan et al. 1984; Smith et al. 1986) indicate that a large fraction of these galaxies are disturbed. Among low-redshift quasars, 70% or more have nearby companions (French & Gunn 1983; Heckman et al. 1984; Vader et al. 1987; Hutchings et al. 1989), some possess features reminiscent of tidal tails (Stockton 1978; MacKenty & Stockton 1984; Stockton & Ridgway 1991), and still others appear to be linked to neighboring galaxies by bridges or display evidence for multiple nuclei (Stockton & MacKenty 1987; Hutchings et al. 1988; Block & Stockton 1991). In addition, there are compelling arguments indicating that many of the more luminous IRAS galaxies are, in fact, ``buried quasars,'' or systems which will eventually evolve into quasars (e.g. Sanders et al. 1990). As noted above, the brightest of these systems are invariably associated with mergers.

In general, theoretical studies on the relevance of mergers to quasar activity mainly comprise wishful thinking. While calculations like those by Negroponte & White (1983), Noguchi (1988), and Barnes & Hernquist (1991) demonstrate that tidal forces can drive gas into the nuclei of interacting galaxies, they do not predict the ultimate fate of the gas and so cannot determine if an AGN will develop. Moreover, it is not clear if the galaxy models used by these workers, which are analogues of present-day disks, are even reasonable caricatures of quasar hosts. Nevertheless, if collisions between galaxies and an abundant supply of gas are necessary ingredients for forming quasars, then it is expected that AGN should be more abundant at high redshifts than in the local universe since interactions were more frequent then and, presumably, more free gas was available than today (e.g. Yee & Green 1984, 1987; Roos 1985).

Radio Galaxies

The most convincing observations linking interactions with radio activity in ellipticals are studies by Heckman et al. (1986) and Vader et al. (1989) implying that many radio galaxies are the products of recent mergers. These objects display structural irregularities often associated with merger candidates, including dust lanes, tails, bridges, shells, and double nuclei (see, also, Schweizer 1980; van Albada et al. 1982; de Ruiter et al. 1988; Smith & Heckman 1989). Others display stellar disks that may be debris from a merger (e.g. Gonzalez-Serrano & Perez-Fournon 1989). Moreover, some show evidence of recent or ongoing star formation and unusually high levels of infrared emission, suggesting that they are relatively young. It is quite natural, therefore, to speculate that these galaxies formed by mergers of disk progenitors which also triggered the observed activity.

The numerical studies reviewed in Section 5 indicate that merger remnants are morphologically similar to elliptical galaxies and that if the galaxies involved are gas-rich a substantial fraction of the gas may be fall into the nuclei of merger remnants. In this regard, it is interesting to note that the kinematics of radio ellipticals are quite similar to those of normal ones (Heckman et al. 1985; Smith et al. 1990) and that many merger candidates exhibit r1/4 law luminosity profiles (e.g. Schweizer 1982; Wright et al. 1990). The calculations lack sufficient dynamic range to determine the evolution of the gas on scales much smaller than 100 pc, but a variety of phenomenological arguments suggest that the gas should continue to fragment and contract on smaller scales and may eventually form a black hole or be accreted by an existing one (Begelman et al. 1984; Norman & Scoville 1988; Scoville & Norman 1988; Shlosman et al. 1989).

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