The evolution of dwarf galaxies

We propose an evolutionary model for dwarf galaxies in which initially metal-poor gas-rich dwarf irregular (dI) galaxies evolve through bursting Blue Compact Dwarf (BCD) stages and eventually fade from the BCD phase to become dwarf ellipticals (dE). During the bursting phase the surface brightness of the galaxy increases rapidly due to enhanced OB star formation. The source of fuel for the intermittent bursts of star formation is assumed to be primordial gas which continues to collapse onto the already formed central structure. The dE galaxies form as a result of eventual gas depletion through star formation.With this proposed dIBCDdE evolutionary sequence we can explain the similar photometric structure of the different dwarf types and the differences in their star formation rates, surface brightnesses,Hi contents and metallicities. A final central BCD burst can account for the nucleation in brighter dEs and their residual star formation, while earlier more widespread star formation bursts would fade to give an irregular dI. Inflow of gas may allow dEs to be less flattened than dIs.Using galaxy fading and metallicity models we can reproduce the observed number ratiosN(dI)N(BCD) andN(dI)N(dE) and also the observed metallicity magnitude relation of local dwarf spheroidal galaxies.