Dwarf Galaxies with Gentle Star Formation and the Counts of Galaxies from the Hubble Deep Field

Abstract

In this paper the counts and colors of the faint galaxies observed in the Hubble Deep Field are fitted by means of simple luminosity evolution models which incorporate a numerous population of fading dwarfs. The observed color distribution of the very faint galaxies allows now to put constrains on the star formation history in dwarfs. It is shown that the star-forming activity in these small systems has to proceed in a gentle way, i.e. through episodes each one lasting much longer than a simple instantaneous burst of star formation. By allowing dwarfs to form stars in this gentle way the number of predicted red remnants is severely reduced, in good agreement with the observations. Then, if the faint counts are to be fitted by means of dwarfs, the simple model for dwarfs forming stars in single, very short episodes is challenged, and a more complex star formation history has to be invoked. Recent observational evidence supporting this new dwarf models are also discussed.