We estimate the potential contribution of M < 10 ^ { 9 } M _ { \odot } dwarf galaxies to the reionization and early metal-enrichment of the Milky Way environment , or circum-Galactic Medium .
Our approach is to use the observed properties of ancient stars ( \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 12 Gyr old ) measured in nearby dwarf galaxies to characterize the star-formation at high- z .
We use a merger-tree model for the build-up of the Milky Way , which self-consistently accounts for feedback processes , and which is calibrated to match the present-day properties of the Galaxy and its dwarf satellites .
We show that the high- z analogues of nearby dwarf galaxies can produce the bulk of ionizing radiation ( > 80 \% ) required to reionize the Milky Way environment .
Our fiducial model shows that the gaseous environment can be 50 \% reionized at z \approx 8 by galaxies with 10 ^ { 7 } M _ { \odot } \leq { M < 10 ^ { 8 } M _ { \odot } } .
At later times , radiative feedback stops the star-formation in these small systems , and reionization is completed by more massive dwarf galaxies by z _ { rei } = 6.4 \pm 0.5 .
The metals ejected by supernova-driven outflows from M < 10 ^ { 9 } M _ { \odot } dwarf galaxies almost uniformly fill the Milky Way environment by z \approx 5 , enriching it to Z \approx 2 \times 10 ^ { -2 } Z _ { \odot } .
At z \approx 2 these early metals are still found to represent the \approx 50 \% of the total mass of heavy elements in the circum-Galactic Medium .