Dwarf elliptical galaxies: structure, star formation and colour-magnitude diagrams

The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N-body/hydro-dynamical simulations that include star formation, feed-back and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the color-magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present color-magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the red-shift of galaxy formation zfor = 5 and varying the cosmological parameters H0 and q0. The resulting color-magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group.