Star Formation Histories in Andromeda
Thomas Brown
As the nearest spiral galaxy to our own, Andromeda offers an ideal
laboratory for the study of galaxy formation. We are investigating
the star formation history in Andromeda's halo, outer disk, and tidal
stream via a series of extremely deep observations using the Advanced
Camera for Surveys on the Hubble Space Telescope. These are the first
observations to directly characterize the full age distribution in
Andromeda, using the of colors and luminosities of the main sequence
stars. Unlike the ancient, metal-poor halo of the Milky Way, the
Andromeda halo is populated by stars with a surprisingly wide range of
age and metallicity. More than half of the stars are less than 11
billion years old, and nearly a third of the stars are 6-8 billion
years old. These findings suggest a more violent merger history in
Andromeda than that in the Milky Way. I review these results and
others enabled by these rich datasets.