Modelling the strongest silicate emission features of local type 1 AGN
  We measure the 10 and $18\mu$m silicate features in a sample of 67 local
($z<0.1$) type 1 active galactic nuclei (AGN) with available {\it Spitzer}
spectra dominated by non-stellar processes. We find that the $10\mu$m silicate
feature peaks at $10.3^{+0.7}_{-0.9}\mu$m with a strength (Si$_{p}$ = ln
f$_{p}$(spectrum)/f$_{p}$(continuum)) of $0.11^{+0.15}_{-0.36}$, while the
$18\mu$m one peaks at $17.3^{+0.4}_{-0.7}\mu$m with a strength of
$0.14^{+0.06}_{-0.06}$. We select from this sample sources with the strongest
10$\mu$m silicate strength ($\sigma_{Si_{10\mu m}}>0.28$, 10 objects). We carry
out a detailed modeling of the IRS/{\it Spitzer} spectra by comparing several
models that assume different geometries and dust composition: a smooth torus
model, two clumpy torus models, a two-phase medium torus model, and a
disk+outflow clumpy model. We find that the silicate features are well modeled
by the clumpy model of Nenkova et al. 2008, and among all models those
including outflows and complex dust composition are the best (Hoenig et al.
2017). We note that even in AGN-dominated galaxies it is usually necessary to
add stellar contributions to reproduce the emission at the shortest
wavelengths.
