Resolving the AGN and host emission in the mid-infrared using a
  model-independent spectral decomposition
  We present results on the spectral decomposition of 118 Spitzer Infrared
Spectrograph (IRS) spectra from local active galactic nuclei (AGN) using a
large set of Spitzer/IRS spectra as templates. The templates are themselves IRS
spectra from extreme cases where a single physical component (stellar,
interstellar, or AGN) completely dominates the integrated mid-infrared
emission. We show that a linear combination of one template for each physical
component reproduces the observed IRS spectra of AGN hosts with unprecedented
fidelity for a template fitting method, with no need to model extinction
separately. We use full probability distribution functions to estimate
expectation values and uncertainties for observables, and find that the
decomposition results are robust against degeneracies. Furthermore, we compare
the AGN spectra derived from the spectral decomposition with sub-arcsecond
resolution nuclear photometry and spectroscopy from ground-based observations.
We find that the AGN component derived from the decomposition closely matches
the nuclear spectrum, with a 1-sigma dispersion of 0.12 dex in luminosity and
typical uncertainties of ~0.19 in the spectral index and ~0.1 in the silicate
strength. We conclude that the emission from the host galaxy can be reliably
removed from the IRS spectra of AGN. This allows for unbiased studies of the
AGN emission in intermediate and high redshift galaxies -currently inaccesible
to ground-based observations- with archival Spitzer/IRS data and in the future
with the Mid-InfraRed Instrument of the James Webb Space Telescope. The
decomposition code and templates are available at
http://www.denebola.org/ahc/deblendIRS.
