Between small accretions that barely affect disk galaxies and major mergers that wreck disks there must be intermediate-strength interactions and minor mergers that significantly affect disks yet do not destroy them. This immediately suggests three questions: (1) How fragile are galaxy disks? (2) Can bulges form through minor mergers? And (3) if so, what fraction of bulges formed in this manner?
Early theoretical worries that accretions of even only a few percent in mass might disrupt disks (Tóth & Ostriker 1992) have been dispelled by N-body simulations showing that model disk galaxies do survive minor mergers with mass ratios of up to m/M 0.3, albeit tilted, warped, slightly thickened, and often with an increased bulge (Walker et al. 1996; Huang & Carlberg 1997; Velázquez & White 1999). Hence, galaxy disks are apparently less fragile than once thought, a fact also suggested by observations.
First, note that optical images are not always a reliable indicator of tidal interactions, as the case of M 81 illustrates. Even when displayed at high contrast such images of M 81 paint a rather serene scene of a symmetric grand-design spiral. Yet, the H I distribution is highly asymmetric and dominated by long tidal features whose kinks reveal a strong triple interaction between M 81, NGC 3077, and M 82 (Yun et al. 1994). M 81 has not only survived this interaction, but probably owes its beautiful spiral structure to it (TT).
Second, S0 galaxies with polar rings of gas, dust, and young stars increasingly suggest that especially gas-rich disks may well survive minor mergers occurring from near-polar orbits. Such S0 galaxies were long thought to have accreted their ring gas during a flyby or minor merger (e.g. Toomre 1977; Schweizer et al. 1983). Yet, many of the S0 bodies feature poststarburst spectra, and H I observations show that the gas contents of the polar rings tend to be large and typical of full-grown late-type spirals (Richter et al. 1994; Reshetnikov & Combes 1994; Arnaboldi et al. 1997), as illustrated in figure 2. Thus it appears that the central S0 galaxies may be remnants of disk companions having fallen into spiral galaxies - now polar rings - nearly over their poles (Bekki 1998). If so, these central S0 bodies represent failed bulges. The crucial point is that two disk systems of not too dissimilar mass apparently can survive a merger and - helped by gaseous dissipation - retain their disk identity.
Disk galaxies survive even non-polar minor mergers, as evidenced by a multitude of kinematic signatures. For example, the Sab galaxy NGC 4826 has a gas disk consisting of two nested counterrotating parts, each of nearly equal mass (Braun et al. 1994). The inner component rotates like the stellar disk and bulge, while the outer component counterrotates (Rubin 1994). The two comparable gas masses suggest that the intruder galaxy was not a mere dwarf a few percent in mass, but a more massive companion leading to a minor merger.
Whereas similar kinematic signatures are rare among Sb galaxies, they are more frequent among Sa galaxies and nearly the norm among S0 galaxies. From the statistics of counterrotating, skewedly rotating, and corotating ionized-gas disks one can conclude that at least 40%-70% of all S0 galaxies experienced minor mergers (Bertola et al. 1992). The fact that the frequency of kinematic signatures of past mergers increases with bulge size strongly suggests that at least major bulges formed through mergers.
Another powerful merger signature correlating with morphological type is the subpopulations of stars counterrotating in disk galaxies of types S0 to Sb. A well known example is the E/S0 galaxy NGC 4550, in which half of the disk stars rotate one way and the other half the opposite way (Rubin et al. 1992). In several Sa and Sb galaxies the split between normal- and counterrotating disk stars is of the order of 70/30%. Finally, a bulge rotating at right angles to the stellar disk has been observed in the Sa galaxy NGC 4698 (Bertola et al. 1999), and bulges counterrotating to the disks are seen in the Sb galaxies NGC 7331 and NGC 2841 (Prada et al. 1996, and private commun.; but see Bottema 1999). N-body simulations suggest that minor and not-so-minor mergers can indeed produce such odd rotations (Thakar & Ryden 1998; Balcells & González 1998).
In short, galactic disks - especially those rich in gas - appear not nearly as fragile as thought only a few years ago. Both observations and numerical simulations suggest that minor mergers do occur in disk galaxies and contribute to bulge building. However, we do not know the exact fraction of bulges that were built in this manner. Also unclear is how unique or varied the possible paths to, say, a present-day Sb galaxy are. Which formed first: the disk or the bulge? And did disks and bulges grow episodically, perhaps even by turns?