On the LINER nuclear obscuration, Compton-thickness and the existence of
  the dusty torus; Clues from Spitzer/IRS spectra
  Most of the optically classified low ionisation narrow emission-line regions
(LINERs) nuclei host an active galactic nuclei (AGN). However, how they fit
into the unified model (UM) of AGN is still an open question. The aims of this
work are to study at mid-infrared (mid-IR) (1) the Compton-thick nature of
LINERs; and (2) the disappearance of the dusty torus in LINERs predicted from
theoretical arguments. We have compiled all the available low spectral
resolution mid-IR spectra of LINERs from the IRS/Spitzer (40 LINERs). We have
complemented this sample with Spitzer/IRS spectra of PGQSOs, S1s, S2s, and SBs
nuclei. We have studied the AGN versus the starburst content in our sample
using different indicators: the EW(PAH 6.2um), the strength of the silicate
feature at 9.7um, and the steepness of the mid-IR spectra. In 25 out of the 40
LINERs (i.e., 62.5%) the mid-IR spectra are not SB-dominated, similar to the
comparison S2 sample (67.7%). The average spectra of both SB-dominated LINERs
and S2s are very similar to the average spectrum of the SB class. Moreover, the
average spectrum of AGN-dominated LINERs with Lx(2-10keV)>10E+41 erg/s is
similar to the average mid-IR spectrum of AGN-dominated S2s. However, faint
LINERs show flat spectra different from any of the other optical classes,
suggesting the disappearance of the dusty torus. The correlation between
nuLnu(12um) and Lx(2-10keV) for AGN nicely extends toward low luminosities only
if SB-dominated LINERs are excluded and Lx(2-10keV) is corrected in
Compton-thick LINER candidates. We discuss the nature of faint LINERs by
comparing it with the spectra of several emission mechanisms like jet, ADAF,
planetary nebulae, or post-AGB stars. We suggest that the material producing
the Compton-thick X-ray obscuration is free of dust, to reconcile the
Compton-thick nature of a large fraction of LINERs with the lack of dusty-torus
signatures.
