Uncovering dark fossils of the Universe

According the ΛCDM model of the universe, there should be a halo of dark matter (a hypothetical form of matter that only interacts gravitationally on normal matter) surrounding and containing every galaxy. The model has faced challenges in recent years due to observations of low mass galaxies (Mgalaxy < 106 M ), which have found significantly fewer satellite galaxies than naively predicted by assigning galaxies to dark matter only simulations of the ΛCDM model. Reionization in the early Universe — when the first stars and other luminous sources ionized the gasses in the universe — has likely played a role in this observed lack of satellites, as ionizing the gas would disrupt the cold and dense environment required for stellar formation. We simulate the effects of different timing and sources of reionization on dwarf galaxies using high resolution cosmological zoom-in simulations of dwarf galaxies (mgas = 250M ) run the FIRE-(Feedback In Realistic Environments) code. FIRE is a powerful tool for simulating dwarf galaxies because it uses a realistic stellar feedback all based from first principles. We simulate three different reionization backgrounds with the FIRE code, ’Double’ in which we double the photo-heating and photo-ionization rate, ’Late’ in which reionization occurs later than in our fiducial runs, and ’Normal’ where all parameters are as we predict. We investigate the star formation histories and stellar mass to halo mass relationship of the three reionization backgrounds to make testable predictions for their effect on the counts and properties of dwarf galaxies. In this way, we can place constraints on the sources and local timing of reionization.