SIMBAD references

The photometric evolution and possible evolutionary links between the three main types of dwarf galaxies (dI, BCD, dE) are investigated with evolutionary synthesis models. The low metallicities of dwarf galaxies are accounted for by using metallicity dependent stellar evolutionary tracks and photometric calibrations (Z=0.02 and 0.001). The observed photometric properties of the three main types of dwarf galaxies are compatible with a dI -> BCD -> dE evolutionary sequence. In this scenario dIs are characterised by continuous evolution with a constant star formation rate, BCDs form an intermediate phase of active star formation described as a starburst and dEs evolve without any further star formation after the starburst. dEs must be about 10Gyr old. Alternatively, the observed colour indices of dEs and dIs can also be explained by two independent scenarios where the dIs evolve under constant star formation conditions and the dEs are described by an exponentially decreasing star formation rate. Then both types of galaxies must be about 10Gyr old and the different evolutionary paths must have been imposed at galaxy formation.