High Signal-to-Noise Ratio Mid-Infrared Quasar Spectral Templates
  Mid-infrared (MIR) quasar spectra exhibit a suite of emission features
including high ionization coronal lines from the narrow line region (NLR)
illuminated by the ionizing continuum, and hot dust features from grains, as
well as polycyclic aromatic hydrocarbons (PAH) features from star formation in
the host galaxy. Few features are detected in most spectra because of typically
low signal-to-noise ratio (S/N) data. By generating spectral composites in
three different luminosity bins from over 180 Spitzer Infrared Spectrograph
(IRS) observations, we boost the S/N and reveal important features in the
complex spectra. We detect high-ionization, forbidden emission lines in all
templates, PAH features in all but the most luminous objects, and broad
silicate and graphite features in emission whose strength increases relative to
the continuum with luminosity. We find that the intrinsic quasar spectrum for
all luminosity templates is consistent, and the differences in the spectra can
be explained by host galaxy contamination in the lower luminosity templates. We
also posit that star formation may be active in most quasar host galaxies, but
the spectral features of star formation are only detectable if the quasar is
faint.
