Galaxy interactions can easily lead to non-axisymmetries in the gravitational potential of one or more of the galaxies involved, and as such can be implicated in angular momentum loss of inflowing material, and thus conceivably in starburst and AGN fuelling (e.g., Shlosman et al. 1989, 1990; Mihos & Hernquist 1996).
3.1. Interactions and starburst activity
It is well known that there is ample anecdotal evidence for the connection between galaxy interactions and starburst activity. This is perhaps clearest for the most extreme infrared sources, specifically the Ultra-Luminous InfraRed Galaxies (ULIRGs). They are powered mainly by starbursts (Genzel et al. 1998), and it has been known since briefly after their discovery that they occur in galaxies with disturbed morphologies, presumably as a result of recent interactions (e.g., Joseph & Wright 1985; Armus, Heckman, & Miley 1987; Sanders et al. 1988; Clements et al. 1996; Murphy et al. 1996; Sanders & Mirabel 1996). Given that the ULIRGs are both among the most extreme starbursts known, and are occurring in interacting galaxies, one can infer that such massive starbursts are in fact powered by gas which has lost angular momentum in galaxies which are undergoing a major upheaval, i.e., are merging or interacting.
More in general, and considering galaxies less extreme than those in
the ULIRG class, there is considerable evidence for a connection
between interactions and enhanced star formation in galaxies, often
measured using galaxy colours which are bluer in the case of current
star formation (see, e.g., the seminal paper by
Larson & Tinsley
1978).
But even so, a more detailed consideration can expose possible
caveats. We mention the recent paper by
Bergvall, Laurikainen,
& Aalto (2003),
who considered two matched samples of nearby interacting
(pairs and clear cases of mergers) and non-interacting galaxies, and
measured star formation indices based on UBV colours. From this
analysis, Bergvall et al. do not find evidence for significantly
enhanced star-forming activity among the interacting/merging galaxies,
although they do report a moderate increase in star formation in the
very centres of their interacting galaxies. Interesting in this
respect are also recent results from a combination of Sloan Digital
Sky Survey and 2dF Galaxy Redshift Survey data, presented by
Balogh et al. (2004).
These authors study the equivalent width of
H
emission, a measure of starburst activity, and find no correlation
between its distribution among the star-forming population of galaxies
and the environment.
So although mergers can undoubtedly lead to massive starbursts, they appear to do so only in exceptionally rare cases. Bergvall et al. (2003) estimate that only about 0.1% of a magnitude limited sample of galaxies will host massive starbursts generated by interactions and mergers. Most interactions between galaxies may not lead to any increase in the starburst activity. Those that do may be selected cases where a set of parameters, both internal to the galaxies and regarding the orbital geometry of the merger, is conducive to the occurrence of starburst activity (e.g., Mihos & Hernquist 1996). To further illustrate this point, we quote the results published by Laine et al. (2003), who find very little evidence for trends in starburst activity from detailed HST imaging of the Toomre sequence of merging galaxies.