4.5 Tidal Dwarfs

Zwicky (1956) proposed that dwarf galaxies could be formed from debris when large galaxies collide, an idea that was further addressed by Schweizer (1978). Though not a real morphological class of its own, this group of galaxies have a common history which is likely to have a considerable impact on their chemical composition. If dwarfs can form from material originating in giant galaxies we would expect them to be comparatively metal rich for a given luminosity. Some recent progress in this field (Mirabel et al. 1992, Duc & Mirabel 1994, 1998) suggests that, even though the final fate of dwarf galaxoids forming in mergers is uncertain, they have metallicities that are systematically higher than in other dwarfs of similar luminosity. Tidal dwarfs thus appear to be a fundamentally distinct class of galaxies. Moreover, their future evolution is uncertain: are they stable, do they contain dark matter, how will they be affected by future interactions with their parent galaxy?

In rich clusters, where interactions should have been frequent, explaining the presence of cD galaxies and the overabundance of ellipticals, we would expect a larger fraction of tidal dwarfs. If they can survive and escape their progenitor, dwarfs in clusters must be more metal rich than field dwarfs. There are preliminary indications that this may be the case (as well as indications that it is not, cf. Sect. 4.1.1.). Of course, galaxies may simply evolve faster in high density environments. Interactions with other galaxies and the intercluster medium may stimulate star formation in dwarfs and make the chemical evolution clock run faster.

In view of their chemical abundances, it is not likely that the local dwarfs have formed this way. But what if this process occurred early in the Universe, at high redshifts when the parent galaxies were not yet significantly enriched?