We present results of high resolution hydrodynamical simulations of the formation and evolution of dwarf galaxies .
Our simulations start from cosmological initial conditions at high redshift .
They include metal-dependent cooling , star formation , feedback from type II and type Ia supernovae and UV background radiation , with physical recipes identical to those applied in a previous study of Milky Way type galaxies .
We find that a combination of feedback and the cosmic UV background results in the formation of galaxies with properties similar to the Local Group dwarf spheroidals , and that their effect is strongly moderated by the depth of the gravitational potential .
Taking this into account , our models naturally reproduce the observed luminosities and metallicities .
The final objects have halo masses between 2.3Â \times~ { } 10 ^ { 8 } and 1.1Â \times~ { } 10 ^ { 9 } ~ { } \mathrm { M } _ { \odot } , mean velocity dispersions between 6.5 and 9.7Â kms ^ { -1 } , stellar masses ranging from 5Â \times~ { } 10 ^ { 5 } to 1.2Â \times 10 ^ { 7 } ~ { } \mathrm { M } _ { \odot } , median metallicities between [ Fe/H ] Â = -1.8 and -1.1 , and half-light radii of the order of 200 to 300 pc , all comparable with Local Group dwarf spheroidals .
Our simulations also indicate that the dwarf spheroidal galaxies observed today lie near a halo mass threshold around 10 ^ { 9 } ~ { } \mathrm { M } _ { \odot } , in agreement with stellar kinematic data , where supernova feedback not only suffices to completely expel the interstellar medium and leave the residual gas-free , but where the combination of feedback , UV radiation and self-shielding establishes a dichotomy of age distributions similar to that observed in the Milky Way and M31 satellites .