Metals and ionizing photons from dwarf galaxies

Abstract

Abstract We estimate the potential contribution of M < 109 M⊙ 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 (≳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 the Milky Way environment. Our fiducial model shows that the gaseous environment can be 50 per cent reionized at z ≈ 8 by galaxies with 107 M⊙ ≤ M < 108 M⊙. At later times, radiative feedback stops the star formation in these small systems, and reionization is completed by more massive dwarf galaxies by zrei = 6.4 ± 0.5. The metals ejected by supernova-driven outflows from M < 109 M⊙ dwarf galaxies almost uniformly fill the Milky Way environment by z ≈ 5, enriching it to Z ≈ 2 × 10−2 Z⊙. At z ≈ 2, these early metals are still found to represent the ≈50 per cent of the total mass of heavy elements in the circum-Galactic medium.