The Mid-Infrared Continua of Seyfert Galaxies
  An analysis of archival mid-infrared (mid-IR) spectra of Seyfert galaxies
from the Spitzer Space Telescope observations is presented. We characterize the
nature of the mid-IR active nuclear continuum by subtracting a template
starburst spectrum from the Seyfert spectra. The long wavelength part of the
spectrum contains a strong contribution from the starburst-heated cool dust;
this is used to effectively separate starburst-dominated Seyferts from those
dominated by the active nuclear continuum. Within the latter category, the
strength of the active nuclear continuum drops rapidly beyond ~ 20 micron. On
average, type 2 Seyferts have weaker short-wavelength active nuclear continua
as compared to type 1 Seyferts. Type 2 Seyferts can be divided into two types,
those with strong poly-cyclic aromatic hydrocarbon (PAH) bands and those
without. The latter type show polarized broad emission lines in their optical
spectra. The PAH-dominated type 2 Seyferts and Seyfert 1.8/1.9s show very
similar mid-IR spectra. However, after the subtraction of the starburst
component, there is a striking similarity in the active nuclear continuum of
all Seyfert optical types. PAH-dominated Seyfert 2s and Seyfert 1.8/1.9s tend
to show weak active nuclear continua in general. A few type 2 Seyferts with
weak/absent PAH bands show a bump in the spectrum between 15 and 20 micron. We
suggest that this bump is the peak of a warm (~200 K) blackbody dust emission,
which becomes clearly visible when the short-wavelength continuum is weaker.
This warm blackbody emission is also observed in other Seyfert optical
subtypes, suggesting a common origin in these active galactic nuclei.
