Metals and ionizing photons from dwarf galaxies

We estimate the potential contribution of M < 10(9)M(circle dot) 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 (<(>)under tilde>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 per cent) required to reionize theMilkyWay environment. Our fiducial model shows that the gaseous environment can be 50 per cent reionized at z approximate to 8 by galaxies with 10(7) M-circle dot <= M < 10(8) M-circle dot. 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 +/- 0.5. The metals ejected by supernova-driven outflows from M < 10(9) M-circle dot dwarf galaxies almost uniformly fill the Milky Way environment by z approximate to 5, enriching it to Z approximate to 2 x 10(-2) Z(circle dot). At z approximate to 2, these early metals are still found to represent the approximate to 50 per cent of the total mass of heavy elements in the circum-Galactic medium.