Born to be Starless: Revisiting the Missing Satellite Problem

Abstract The two massive galaxies in the Local Group both host substantially fewer satellites than the subhalos expected from the cold dark matter paradigm, and recent investigations have highlighted the interplay between baryons and dark matter. We investigate the processes that make subhalos starless, using high-resolution cosmological simulations. We find that the number of satellites around Milky Way analogs closely aligns with observations, which accords with recent studies. In our simulations, the majority of subhalos are devoid of stars, i.e., “starless.” We first examined supernova feedback and the environmental effects associated with subhalos’ orbital motion as candidates for their origin. However, neither seems to be the main driver. Supernova feedback causes a reduction in cold gas in “starred” subhalos, but its impact is not significant. In the case of starless subhalos, supernova feedback is irrelevant because most of them do not have in situ star formation in the first place. The orbital motion in dense environments causes the removal of gas in all subhalos, but it is not enough to remove preexisting stars. The key is found to be the effect of reionization instead. Starless subhalos are initially born in regions that are less efficient in accreting matter. This makes them lack sufficiently dense gas to self-shield from UV background heating, preventing their gas from cooling below the star formation threshold. This indicates that starless subhalos are not made but born.