The Low Mass Stellar Initial Mass Function of the Ultra Faint Dwarf Spheroidal Galaxy Boötes I

The stellar initial mass function (IMF) underpins our understanding of the baryonic content, star formation history, and chemical evolution of galaxies in the universe. Studies of the IMF in various environments in the Milky Way indicate that the IMF within the Galaxy is consistent with being invariant. When determining the stellar contributions to the total mass in the nearby ultra faint dwarf galaxies (UFDs), an IMF of the same form as the Milky Way's is typically assumed and integrated. However, it is unclear whether the IMF varies outside of the Milky Way. Should the IMF vary, what is the true stellar mass of these systems? In this work, we present a deep color magnitude diagram and new results on the low mass end of the IMF for Boötes I UFD to answer this question. These results were obtained utilizing new, deep Hubble Space Telescope Advanced Camera for Surveys observations in the F606W and F814W bands, with photometry reaching down to I = 27.4 and extending to a lower mass regime than previous investigations of Boötes I. The stellar population of Boötes I is universally old and very metal-poor (age > 12 Gyr, mean [Fe/H] ~ -2.5) and it likely underwent only a short epoch of star formation completed early in cosmic history, making Boötes I a fossil of star formation from an earlier era. Boötes I is the most luminous ultra faint dwarf galaxy (M_V ~ -6) and relatively nearby (distance ~ 60 kpc), and thus is an ideal target for the study of the faint end of the stellar luminosity function. At the depth of photometry reached by this work, the present day mass function is very close to the initial mass function, and thus the IMF can be modeled from direct star counts after accounting for the presence of unresolved binary stars. Through this study of the low-mass population of Boötes I, we gain better understanding of star formation at high redshift.