Hints on the gradual re-sizing of the torus in AGN by decomposing
  IRS/Spitzer spectra
  Several authors have claimed that the less luminous active galactic nuclei
(AGN) are not capable of sustaining the dusty torus structure. Thus, a gradual
re-sizing of the torus is expected when the AGN luminosity decreases. Our aim
is to confront mid-infrared observations of local AGN of different luminosities
with this scenario. We decomposed about ~100 IRS/Spitzer spectra of LLAGN and
powerful Seyferts in order to decontaminate the torus component from other
contributors. We have used the affinity propagation (AP) method to cluster the
data into five groups within the sample according to torus contribution to the
5-15 um range (Ctorus) and bolometric luminosity. The AP groups show a
progressively higher torus contribution and an increase of the bolometric
luminosity, from Group 1 (Ctorus~ 0% and logLbol ~ 41) and up to Group 5
(Ctorus ~80% and log(Lbol) ~44). We have fitted the average spectra of each of
the AP groups to clumpy models. The torus is no longer present in Group 1,
supporting the disappearance at low-luminosities. We were able to fit the
average spectra for the torus component in Groups 3 (Ctorus~ 40% and log(Lbol)~
42.6), 4 (Ctorus~ 60% and log(Lbol)~ 43.7), and 5 to Clumpy torus models. We
did not find a good fitting to Clumpy torus models for Group 2 (Ctorus~ 18% and
log(Lbol)~ 42). This might suggest a different configuration and/or composition
of the clouds for Group 2, which is consistent with a different gas content
seen in Groups 1, 2, and 3, according to the detections of H2 molecular lines.
Groups 3, 4, and 5 show a trend to decrease of the width of the torus (which
yields to a likely decrease of the geometrical covering factor), although we
cannot confirm it with the present data. Finally, Groups 3, 4, and 5 show an
increase on the outer radius of the torus for higher luminosities, consistent
with a re-sizing of the torus according to the AGN luminosity.
