Prof. Nitya Kallivayalil: Local Group Cosmology: lessons from mapping the stellar populations of the nearby universe

I will present results from an HST Treasury program that provides deep imaging, below the oldest main-sequence turn-off, for all presently known Milky Way dwarf galaxies out to 400 kpc. The data allow us to measure proper motions and hence construct orbital histories, measure morphologies (sizes and light profiles), and obtain star formation histories for these dwarfs, thereby addressing critical issues related to galaxy formation, cosmic reionization, and the nature of dark matter. First results include testing the prediction that hierarchical evolution occurs on all mass scales, i.e., that satellites of the Milky Way should also have companions; whether sizes of dwarf galaxies agree with predictions from cosmological simulations; and tantalizing evidence that star formation histories for dwarf galaxies associated with the Magellanic Cloud system are different from dwarf galaxies associated with the Milky Way.