Life and times of dwarf spheroidal galaxies

We propose a cosmological scenario for the formation and evolution of dwarf spheroidal galaxies (dSphs), satellites of the Milky Way (MW). An improved version of the semi-analytical code gamete (GAlaxy Merger Tree and Evolution) is used to follow the dSphs evolution simultaneously with the MW formation, matching the observed properties of both. In this scenario dSph galaxies represent fossil objects virializing at z= 7.2 ± 0.7 (i.e. in the pre-reionization era z > zrei= 6) in the MW environment which at that epoch has already been pre-enriched up to [Fe/H]∼−3; their dynamical masses are in the narrow range M= (1.6 ± 0.7) × 108M⊙, although a larger spread might be introduced by a more refined treatment of reionization. Mechanical feedback effects are dramatic in such low-mass objects, causing the complete blow-away of the gas ∼100 Myr after the formation epoch: 99 per cent of the present-day stellar mass, M*= (3 ± 0.7) × 106M⊙, forms during this evolutionary phase, i.e. their age is >13 Gyr. Later on, star formation is re-ignited by returned gas from evolved stars and a second blow-away occurs. The cycle continues for about 1 Gyr during which star formation is intermittent. At z= 0 the dSph gas content is Mg= (2.68 ± 0.97) × 104M⊙. Our results match several observed properties of Sculptor, used as a template of dSphs: (i) the metallicity distribution function; (ii) the colour–magnitude diagram; (iii) the decrement of the stellar [O/Fe] abundance ratio for [Fe/H] > −1.5; (iv) the dark matter content and the light-to-mass ratio; (v) the H i gas mass content.