The ages of dwarf elliptical galaxies

We present narrowband photometry of 91 dwarf elliptical galaxies in the Coma and Fornax Clusters taken with the Stromgren (uvby) filter system. Dividing the sample by dwarf morphology into nucleated (dEN) and nonnucleated (dE) dwarfs reveals two distinct populations of early-type systems based on integrated colors. The class of dEN galaxies are redder in their continuum colors as compared to bright cluster ellipticals and dE type dwarfs, and their position in multicolor diagrams can only be explained by an older mean age for their underlying stellar populations. By comparison with the narrowband photometry of the M87 globular cluster system, we find that dEN's are a higher metallicity continuation of the old, metal-poor color sequence of galactic globular clusters and the blue population of M87 globular clusters. Bright ellipticals and dE dwarfs, on the other hand, follow the color sequence of the metal-rich, red population of M87 globular clusters. A comparison to SED models, convolved to a simple metallicity model, finds that dEN's and blue globular clusters are 3 to 4 Gyr older than cluster ellipticals and 5 Gyr older than dE type galaxies. The implication is that globular clusters and dEN galaxies are primordial and have metallicities set by external constraints such as the enrichment of their formation clouds. Bright ellipticals and dE galaxies have metallicities and ages that suggest an extended phase of initial star formation that produces a younger mean age, even if their formation epoch is similar to that of dEN's and blue globular clusters, and an internally driven chemical evolutionary history.