On the LINER nuclear obscuration, Compton-thickness and the existence of the dusty torus

Context. 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. Aims. The aims of this work are to study at mid-infrared (mid-IR) (1) the Compton-thick nature of LINERs (i.e hydrogen column densities of NH > 1:5 10 24 cm 2 ); and (2) the disappearance of the dusty torus in LINERs predicted from theoretical arguments. Methods. We have compiled all the available low spectral resolution mid-IR spectra of LINERs from the InfraRed Spectrograph (IRS) onboard Spitzer. The sample contains 40 LINERs. We have complemented the LINER sample with Spitzer/IRS spectra of PG QSOs, Type-1 Seyferts (S1s), Type-2 Seyferts (S2s), and Starburst (SBs) nuclei. We have studied the AGN versus the starburst content in our sample using di erent indicators: the equivalent width (EW) of the polycyclic aromatic hydrocarbon (PAH) at 6:2 m, the strength of the silicate feature at 9.7 m, and the steepness of the mid-IR spectra. We have classified the spectra as SB-dominated and AGNdominated, according to these diagnostics. We have compared the average mid-IR spectra of the various classes. Moreover, we have studied the correlation between the 12 m luminosity, L (12 m), and the 2-10 keV energy band X-ray luminosity, LX(2 10 keV). Results. 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. The average spectrum of AGN-dominated LINERs is di erent from the average spectra of the other optical classes, showing a rather flat spectrum at 6 28 m. We have found that the average spectrum of AGN-dominated LINERs with X-ray luminosities LX(2 10 keV) > 10 41 erg=s is similar to the average mid-IR spectrum of AGN-dominated S2s. However, faint LINERs (i.e. LX(2 10 keV) < 10 41 erg=s) show flat spectra di erent from any of the other optical classes. The correlation between L (12 m) and LX(2 10 keV) for AGN nicely extends toward low luminosities only if SB-dominated LINERs are excluded and the 2-10 keV band X-ray luminosity is corrected in Compton-thick LINER candidates. Conclusions. We have found that LINERs proposed as Compton-thick candidates at X-ray wavelengths may be confirmed according to the X-ray to mid-IR luminosity relation. We show evidence in favour of the dusty-torus disappearance when their bolometric luminosity is below Lbol’ 10 42 erg=s. We suggest that the dominant emission at mid-IR of faint LINERs might be a combination of an elliptical galaxy host (characterised by the lack of gas), a starburst, a jet, and/or ADAF emission. Alternatively, the mid-IR emission of some of these faint LINERs could be a combination of elliptical galaxy plus carbon-rich planetary nebulae. In order to reconcile the Compton-thick nature of a large fraction of LINERs with the lack of dusty-torus signatures, we suggest that the material producing the Compton-thick X-ray obscuration is free of dust.