A signature of the internal reionization of the Milky Way?

We present a new semi-analytical model of the population of satellite galaxies of the Milky Way, aimed at estimating the effect of the geometry of reionization at galaxy scale on the properties of the satellites. In this model, reionization can be either (A) externally driven and uniform, or (B) internally driven by the most massive progenitor of the Milky Way. In the latter scenario, the propagation of the ionization front and photon dilution introduce a delay in the photoevaporation of the outer satellites' gas with respect to the inner satellites. As a consequence, outer satellites experience a longer period of star formation than those in the inner halo. We use simple models to account for star formation, the propagation of the ionization front, photoevaporation and observational biases. Both scenarios yield a model satellite population that matches the observed luminosity function and mass-to-light ratios. However, the predicted population for scenario (B) is significantly more extended spatially than for scenario (A), by about 0.3 dex in distance, resulting in a much better match to the observations. The survival of the signature left by the local ultraviolet field during reionization on the radial distribution of satellites makes it a promising tool for studying the reionization epoch at galaxy scale in the Milky Way and nearby galaxies resolved in stars with forthcoming large surveys. However, more work is needed to determine how the effect reported here can be disentangled from that of cosmic variance between different realizations of Milky Way haloes.