While some of the differences in the HI content and distribution observed in galaxies of separate morphological classes undoubtedly result from the circumstances of the era of galaxy formation, the present capacity of a galaxy for star formation may be influenced to some extent by its current environment. The content and distribution of the neutral component of a galaxy's interstellar gas serves then as a useful probe of possible external influences on galaxy evolution, as the neutral gas represents a long-lived but vital component for star formation. A recent review of this subject is given by Haynes et al. (1984).
As illustrated in the tidal disruption model of the Leo triplet presented in Figure 12.5, close encounters between neighboring galaxies in loose groups can result in the removal of a significant portion-observed to be as high as 50% - of a galaxy's initial HI mass. Tidal models such as those presented by Toomre and Toomre (1972) illustrate two significant consequences of tidal encounters, consequences that may dramatically alter galaxy evolution. First, if the relative orbital angular momentum vector is at least roughly aligned with the rotational angular momentum of the target disk, then particle capture is enhanced by the prolonged tidal acceleration so that mass transfer from one galaxy to another occurs. Additionally, after the collision, gas either captured from the companion or raised to large z-heights above the plane may be gravitationally pulled back toward the bottom of the target's potential well. The removal of angular momentum from the gaseous component may even cause such gas to fall into the center of the galaxy. This gas influx may fuel nuclear activity; it is well known that many (although by no means all) active galaxies have close companions. Excess infrared and radio emission, indicative of a recent burst of star formation, is observed in obviously interacting galaxies. Such activity may be relatively short-lived. HI observations of active galaxies are discussed in Section 12.7.3.
Because the outer portions of a galaxy are those most likely to be disrupted in an encounter, HI makes an excellent tracer of tidal events. While ongoing tidal interaction can be inferred from observations of disturbed optical morphology, strong optical emission lines, and large infrared, radio, or X-ray flux, the details of an encounter can be more readily deduced from the HI spatial distribution and its velocity field. Cottrell (1978) has claimed that the coexistence of early-type stellar spectra implying recent star formation with colors characteristic of an underlying older population in Irr II galaxies is likely produced by binary collisions in which mass transfer occurs if at least one of the progenitor galaxies contains a significant amount of interstellar gas. A number of nearby Irr II systems such as NGC 3077 and NGC 4747 show HI tidal tails. As mentioned in Section 12.5.2, mass transfer may play an important role in providing the HI presently seen in some elliptical and lenticular galaxies.
Although the density of galaxies is much higher in a rich cluster, the potential for dramatic tidal effects may be greater in loose groups. Since the tidal force varies as 1 / rp3 and the effective duration of the encounter roughly as rp / vr, the "disruption damage" is just 1 / rp2 vr, yr in the impulse approximation, where rp is the perigalactic distance and vp is the relative velocity of the two galaxies. Because the typical velocity dispersion in a small group is much lower than that found in a rich cluster, a collision in a dense cluster will produce only about one-tenth the damage done by one with the same impact parameter and mass ratio in a small group.