Characterizing Quasars in the Mid-infrared: High Signal-to-Noise
  Spectral Templates
  Mid-infrared (MIR) quasar spectra exhibit a suite of emission features
including high ionization coronal lines from the narrow line region illuminated
by the ionizing continuum, broad dust bumps from silicates and graphites, and
polycyclic aromatic hydrocarbon (PAH) features from star formation in the host
galaxy. However, in Spitzer Infared Spectrograph (IRS) data, few features are
detected in most individual spectra because of typically low signal-to-noise
ratios (S/N). By generating spectral composites from over 180 IRS observations
of Sloan Digital Sky Survey broad-line quasars, we boost the S/N and reveal
features in the complex spectra that are otherwise lost in the noise. In
addition to an overall composite, we generate composites in three different
luminosity bins that span the range of 5.6 micron luminosities of
10$^{40}$--10$^{46}$ (erg~s$^{-1}$). We detect the high-ionization, forbidden
emission lines of [SIV], [OIV], and [NeV] 14 micron in all templates and PAH
features in all but the most luminous template. Ratios of lines with a range of
ionization potentials show no evidence for a strong difference in the shape of
the 41--97 eV ionizing continuum over this range of luminosities. The scaling
of the emission-line luminosities as a function of continuum luminosity is
consistent with what is expected, and shows no indication of a ``disappearing
narrow-line region.'' The broad 10 and 18 micron silicate features in emission
increase in strength with increasing luminosity, and a broad 3--5 micron black
body consistent with graphite emission at 750 K is evident in the highest
luminosity template. We find that the intrinsic quasar continua for all
luminosity templates are consistent; apparent differences arise primarily from
host galaxy contamination most evident at low luminosity.
