Probing the epoch of reionization with Milky Way satellites

While the connection between high-redshift star formation and the local Universe has recently been used to understand the observed population of faint dwarf galaxies in the Milky Way (MW) halo, we explore how well these nearby objects can probe the epoch of first light. We construct a detailed, physically motivated model for the MW satellites based on the state-of-the-art Via Lactea II dark-matter simulations. Our model incorporates molecular hydrogen (H 2) cooling in low-mass systems and inhomogeneous photoheating feedback during the internal reionization of our own Galaxy. We find that the existence of MW satellites fainter than MV ≈ - 5 is strong evidence for H2 cooling in low-mass haloes, while satellites with -5 > MV > -9 were affected by hydrogen cooling and photoheating feedback. The age of stars in very low-luminosity systems and the minimum luminosity of these satellites are key predictions of our model. Most of the stars populating the brightest MW satellites could have formed after the epoch of reionization. Our models also predict a significantly larger dispersion in M300 values than observed and a number of luminous satellites with M300 as low as 106 M⊙.