Ultrafaint dwarfs - Star formation and chemical evolution in the smallest galaxies

In earlier work, we showed that a dark matter halo with a virial mass of 10⁷ M _⊙ can retain a major part of its baryons in the face of the pre-ionization phase and supernova (SN) explosion from a 25 M _⊙ star. Here, we expand on the results of that work, investigating the star formation and chemical evolution of the system beyond the first SN. In a galaxy with a mass M _vir = 10⁷ M _⊙, sufficient gas is retained by the potential for a second period of star formation to occur. The impact of a central explosion is found to be much stronger than that of an off-center explosion both in blowing out the gas and in enriching it, as in the off-center case most of the SN energy and metals escape into the intergalactic medium. We model the star formation and metallicity, given the assumption that stars form for 100, 200, 400, and 600 Myr, and discuss the results in the context of recent observations of very low-mass galaxies. We show that we can account for most features of the observed relationship between [α/Fe] and [Fe/H] in ultra-faint dwarf galaxies with the assumption that the systems formed at a low mass, rather than being remnants of much larger systems.