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7.7. dE's as Debris

Of the other proposed mechanisms for forming dwarf galaxies, the most popular involve debris from giant-galaxy collisions (Gerola et al. 1983; Barnes and Hernquist 1992; Mirabel et al. 1992; Athanassoula 1994). It is no doubt true that galaxy interactions can produce low mass gas clouds, which can subsequently cool and form stars. The question is whether they will look anything like dE galaxies after star formation ceases. Massive dark halos are not expected in this scenario. The strong observational evidence for high M / L in some of the local dE's therefore argues against interactions as the sole formation mechanism. However, direct evidence for dark matter is lacking for most dE's. More indirectly, if Dekel and Silk (1986) are correct in their argument that the metallicity-luminosity-surface-brightness relations require dark matter halos, then collisions could not have produced most of the dwarfs. However, it is not obvious that dark halos are required if metal-enriched winds can develop (Vader 1987; Burkert et al. 1994). If so, there is essentially nothing to rule out formation of most dE galaxies from tidal debris. Indeed, such a possibility is somewhat attractive in that it may help account for the high dE content of clusters, where presumably many gaseous encounters occured during E galaxy formation (when relative velocities were presumably low). However, while hierarchical models provide detailed (albeit perhaps already falsified) predictions for the dwarf-galaxy luminosity function, tidal debris models have not yet been developed to the point where such predictions can be made.

It is possible that (some) cluster dE galaxies arise as debris of another sort: remnants from the destruction of spiral galaxies in the centers of clusters (Whitmore et al. 1993). The similarity of the bulges of late-type galaxies and dE's suggests that an Sc that has lost its disk might look like a dE. However, assuming the morphological mix in the field represents the initial proportion of giant spirals to giant ellipticals, only a small fraction of the dE's could have been produced this way as the reservoir of spirals is too small.