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 & 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 \pm 0.7 ( i.e .
in the pre-reionization era z > z _ { rei } = 6 ) in the MW environment which at that epoch has already been pre-enriched up to [ Fe/H ] \sim - 3 ; their dynamical masses are in the narrow range M = ( 1.6 \pm 0.7 ) \times 10 ^ { 8 } { M _ { \odot } } , 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 \sim 100 Myr after the formation epoch : 99 \% of the present-day stellar mass , M _ { * } = ( 3 \pm 0.7 ) \times 10 ^ { 6 } { M _ { \odot } } , 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 M _ { g } = ( 2.68 \pm 0.97 ) \times 10 ^ { 4 } { M _ { \odot } } .
Our results match several observed properties of Sculptor , used as a template of dSphs : ( i ) the Metallicity Distribution Function ; ( ii ) the Color 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 HI gas mass content .