4.4. Environmental Effects?
Dwarf galaxies are known to exhibit a strong morphology-density relationship. Binggeli et al. (1990) have shown that dE galaxies are almost always found in cluster environments or as companions to larger galaxies, while dI galaxies are more commonly found in lower density environments (see Ferguson & Binggeli 1994 for a more comprehensive review). Given this background, it seems logical to ask whether environment can play a role in the chemical evolution of dwarf galaxies.
The single concerted effort to answer this question has been conducted by Vílchez (1995). He has assembled four different samples of dwarf galaxies: Virgo Cluster core dwarfs; Virgo "Clump" dwarfs; Void dwarfs; and Virgo Cluster peripheral dwarfs. The HII regions in these galaxies have been studied spectroscopically, and their properties (excitation, Balmer line equivalent width, oxygen abundance) have been compared. The different groups show differences in excitation, but it is not clear whether this is due to intrinsic differences in the populations or differences in the selection criterion. Thus, interpretation at this point must be a bit guarded. However, when using the metallicity-luminosity relationship as a diagnostic, the dwarfs from the low density environments appear to show a relationship which is offset from that of the dwarfs from high density environments. Unfortunately, the abundances are based on empirical, and not direct, measurements, and therefore the uncertainties are significant (see discussion in Section 2). Vílchez is careful to emphasize these uncertainties, but the possibility of a real difference between the chemical properties of low density and high density environment dwarfs exists. Follow-up work on this question is definitely to be encouraged!