Environmental mechanisms shaping the nature of dwarf spheroidal galaxies; the view of computer simulations

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We review numerical work carried out over the last decade on the role of environmental mechanisms in shaping the nature of the faintest galaxies known, dwarf spheroidals (dSphs). In particular we discuss a model in which dSphs originated from gas dominated, disky dwarfs that were accreted by massive galaxies several billions of years ago. Dwarfs accreting at z > 1, when the cosmic ultraviolet ionizing flux was much higher than today and was thus able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark matter dominated objects with truncated star formation histories, such as the Draco dSph. The low star formation efficiency expected in such low-metallicity objects prior to their infall was crucial for keeping their disks gas dominated until stripping took over. Therefore gas stripping along with inefficient star formation provides a new feedback mechanism, alternative to photoevaporation or supernovae feedback, playing a crucial role in dwarf galaxy formation and evolution. Tidally induced instabilities, termed "tidal stirring", turned the surviving stellar disk into a spheroid. Moreover, we examine the properties of the newly discovered ultra-faint dSphs in light of this scenario and argue that they likely belong to a different population of lower mass dwarf satellites. These were mostly shaped by reionization rather than by environmental mechanisms and are thus good candidates for being "reionization fossils". We discuss implications of the morphological transformation scenario on the substructure problem. (Abridged)