The History of Star Formation in Galaxies
  If we are to develop a comprehensive and predictive theory of galaxy
formation and evolution, it is essential that we obtain an accurate assessment
of how and when galaxies assemble their stellar populations, and how this
assembly varies with environment. There is strong observational support for the
hierarchical assembly of galaxies, but by definition the dwarf galaxies we see
today are not the same as the dwarf galaxies and proto-galaxies that were
disrupted during the assembly. Our only insight into those disrupted building
blocks comes from sifting through the resolved field populations of the
surviving giant galaxies to reconstruct the star formation history, chemical
evolution, and kinematics of their various structures. To obtain the detailed
distribution of stellar ages and metallicities over the entire life of a
galaxy, one needs multi-band photometry reaching solar-luminosity main sequence
stars. The Hubble Space Telescope can obtain such data in the outskirts of
Local Group galaxies. To perform these essential studies for a fair sample of
the Local Universe will require observational capabilities that allow us to
extend the study of resolved stellar populations to much larger galaxy samples
that span the full range of galaxy morphologies, while also enabling the study
of the more crowded regions of relatively nearby galaxies. With such
capabilities in hand, we will reveal the detailed history of star formation and
chemical evolution in the universe.
