A metallicity-flattening relation for dwarf elliptical galaxies

It is shown that bright cluster dwarf ellipticals follow a relation between metallicity and apparent flattening. Rounder dwarfs tend to be more metal-rich. The evidence is based on colour as well as spectroscopic line-strength data from the literature. The large scatter of dEs around the mean metallicity-luminosity relation, usually ascribed to large observational errors, turns out to be an ellipticity effect. In the magnitude range −14 > ∼ MB ∼ −18 the metallicity of dEs depends more strongly on ellipticity than luminosity. A possible explanation is that galaxies with masses around 10 M⊙ suffered a partial blowout of metal-enriched gas along their minor axis, rendering ellipticity a critical parameter for metallicity (De Young & Heckman 1994).