Key points are not available for this paper at this time.
We have obtained spectroscopy of planetary nebulae in the dwarf irregular galaxy NGC 6822 and the diffuse elliptical galaxies NGC 185 and NGC 205. We have also introduced a new dwarf irregular metallicity- luminosity relation based upon a more homogeneous treatment of distances and abundances. Supplementing our best estimates of the mean oxygen abundance for NGC 185 and NGC 205 with existing data for the planetary nebula in Fornax, we find that the oxygen abundances in diffuse ellipticals exceed those in comparably luminous dwarf irregulars by at least 1.5 times the dispersion in our new metallicity-luminosity relation. When we combine our oxygen abundances with iron abundances from the literature, we find that NGC 185, NGC 205, and Fornax have O/Fe ratios that are roughly solar or higher, on average. These O/Fe ratios are larger than those in dwarf irregulars if the O/Fe ratios observed in the Magellanic Clouds are typical of those in other dwarf irregulars, which indicates that the star formation timescale in diffuse ellipticals is much shorter than that in dwarf irregulars. Therefore, while oxygen abundances allow diffuse ellipticals to be the evolved remnants of dwarf irregulars, O/Fe ratios indicate that an evolutionary connection is unlikely because they have fundamentally different star formation histories.
Richer et al. (Thu,) studied this question.