3.4 Mergers and Interactions
It has become evident that interactions and merging between galaxies is a major driver in the evolution of galaxies, affecting the number evolution and morphological mix of the galaxy population. Mergers and interactions will also affect the star formation activity in galaxies, and in this respect be important for chemical evolution. Gas flow or accretion may lead to increased star formation (SF) activity or starbursts, whereas stripping or harassment in rich environments may inhibit SF. Gas accretion may also affect ISM abundances even if new star formation is not triggered, and may also provide a means for large scale mixing of the ISM.
Are some dwarfs ejected debris from interacting disc galaxies, and what consequences for their chemical evolution would this have? One would expect rather high metallicities, but if the process occurred long ago, before the interacting giants were enriched, this is not necessarily true. Evidence that dwarf-like objects are being formed within the gaseous tails of the encounters is now quite well established (e.g. Duc and Mirabel 1994, 1998), although their ultimate fate is unknown. Abundance analysis shows that their metal content is comparable to that of the parent galaxies re-enforcing the view that the gas from which they originate comes from the central regions of their parents. Another aspect is that many metal-poor dwarf galaxies in clustered environments may have been swallowed or torn apart by massive galaxies. The recently discovered Galactic satellite in Sagittarius (Ibata et al. 1994) may perhaps be such a case.