Ages and Metallicities of Fornax Dwarf Elliptical Galaxies

Narrowband photometry is presented on 27 dwarf ellipticals in the Fornax cluster. Calibrated with Galactic globular cluster data and spectrophotometric population models, the colors indicated that dwarf ellipticals have a mean [Fe/H] of -1.00 ± 0.28 ranging from -1.6 to -0.4. The mean age of dwarf ellipticals, also determined photometrically, is estimated at 10 ± 1 Gyr compared with 13 Gyr for bright Fornax ellipticals. Comparison of our metallicity color and Mg2 indices demonstrates that the [Mg/Fe] ratio is lower in dwarf ellipticals than their more massive cousins, which is consistent with a longer duration of initial star formation to explain their younger ages. There is a increase in dwarf metallicity with distance from the Fornax cluster center, where core galaxies are on average 0.5 dex more metal-poor than halo dwarfs. In addition, we find the halo dwarfs are younger in mean age compared with core dwarfs. One possible explanation is that the intracluster medium ram pressure strips the gas from dwarf ellipticals, halting star formation (old age) and stopping enrichment (low metallicity) as they enter the core.