  Although mergers and starbursts are often invoked in the discussion of QSO
activity in the context of galaxy evolution, several studies have questioned
their importance or even their presence in QSO host galaxies. Accordingly, we
are conducting a study of z~0.2 QSO host galaxies previously classified as
passively evolving elliptical galaxies. We present deep Keck LRIS spectroscopy
of a sample of 15 hosts and model their stellar absorption spectra using
stellar synthesis models. The high S/N of our spectra allow us to break various
degeneracies that arise from different combinations of models, varying
metallicities, and contamination from QSO light. We find that none of the host
spectra can be modeled by purely old stellar populations and that the majority
of the hosts (14/15) have a substantial contribution from intermediate-age
populations with ages ranging from 0.7 to 2.4 Gyr. An average host spectrum is
strikingly well fit by a combination of an old population and a 2.1 (+0.5,
-0.7) Gyr population. The morphologies of the host galaxies suggest that these
aging starbursts were induced during the early stages of the mergers that
resulted in the elliptical-shaped galaxies that we observe. The current AGN
activity likely corresponds to the late episodes of accretion predicted by
numerical simulations, which occur near the end of the mergers, whereas earlier
episodes may be more difficult to observe due to obscuration. Our off-axis
observations prevent us from detecting any current star formation or young
stellar populations that may be present in the central few kiloparsecs.
