The Ages of Dwarf Elliptical Galaxies

We present narrow band photometry of 91 dwarf ellipticals in the Coma andFornax clusters taken through the Stromgren (uvby) filter system. Dividing thesample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfsreveals two distinct populations of early-type systems based on integratedcolors. The class of dEN galaxies are redder in their continuum colors ascompared to bright cluster ellipticals and dE type dwarfs, and their positionin multi-color diagrams can only be explained by an older mean age for theirunderlying stellar populations. By comparison with the narrow band photometryof the M87 globular cluster system (Jordan et al. 2002), we find that dENs area higher metallicity continuation of the old, metal-poor color sequence ofgalactic globulars and the blue population of M87 globulars. Bright ellipticalsand dE dwarfs, on the other hand, follow the color sequence of the metal-rich,red population of M87 globulars. A comparison to SED models, convolved to asimple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrsolder than cluster ellipticals and 5 Gyrs older than dE type galaxies. Theimplication is that globulars and dEN galaxies are primordial and havemetallicities set by external constraints such as the enrichment of theirformation clouds. Bright ellipticals and dE galaxies have metallicities andages that suggest an extended phase of initial star formation to produce ayounger mean age, even if their formation epoch is similar to that of dENs andblue globulars, and an internally driven chemical evolutionary history.Comment: 13 pages AAS LaTeX, 6 figures, accepted for publication in A