Mid-infrared Spectral Properties of Post-Starburst Quasars
  We present Spitzer InfraRed Spectrograph (IRS) low-resolution spectra of 16
spectroscopically selected post-starburst quasars (PSQs) at z ~ 0.3. The
optical spectra of these broad-lined active galactic nuclei (AGNs)
simultaneously show spectral signatures of massive intermediate-aged stellar
populations making them good candidates for studying the connections between
AGNs and their hosts. The resulting spectra show relatively strong polycyclic
aromatic hydrocarbon (PAH) emission features at 6.2 and 11.3\micron and a very
weak silicate feature, indicative of ongoing star formation and low dust
obscuration levels for the AGNs. We find that the mid-infrared composite
spectrum of PSQs has spectral properties between ULIRGs and QSOs, suggesting
that PSQs are hybrid AGN and starburst systems as also seen in their optical
spectra. We also find that PSQs in early-type host galaxies tend to have
relatively strong AGN activities, while those in spiral hosts have stronger PAH
emission, indicating more star formation.
