Stellar Population Gradients in ULIRGs: Implications for Gas Inflow
  Timescales
  Using longslit, optical spectra of Ultraluminous Infrared Galaxies (ULIRGs),
we measure the evolution in the star-formation intensity during galactic
mergers. In individual galaxies, we resolve kpc scales allowing comparison of
the nucleus, inner disk, and outer disk. We find that the strength of the Hbeta
absorption line increases with the projected distance from the center of the
merger, typically reaching about 9 Angstrom around 10 kpc. At these radii, the
star formation intensity must have rapidly decreased about 300-400 Myr ago;
only stellar populations deficient in stars more massive than Type A produce
such strong Balmer absorption. In contrast, we find the star formation history
in the central kpc consistent with continuous star formation. Our measurements
indicate that gas depletion occurs from the outer disk inwards during major
mergers. This result is consistent with merger-induced gas inflow and
empirically constrains the gas inflow timescale. Numerical simulations
accurately calculate the total amount of infalling gas but often assume the
timescale for infall. These new measurements are therefore central to modeling
merger-induced star formation and AGN activity.
