Composite stellar population model, based upon the population synthesis code of Bruzual & Charlot (2003). Each model track has a single value for the age gradient (old pop.: 7 Gyr, Z = 0.004; composite pop.: 95% old and 5% young; age of young pop. in the label) and is marked at different metallicity gradients (metallicity of the young pop. From left to right: Z = 0.1, Z = 0.05, Z = 0.02, Z = 0.008, and Z = 0.004). The black solid line denotes solar metallicity for the young population. Blue-cored early-type dwarf galaxies are displayed as asterisks.