Nuclear obscuration in LINERs

Clues from Spitzer/IRS spectra on the Compton thickness and the existence of the dusty torus^⋆

¹ Instituto de Astrofísica de Canarias (IAC), C/vía Láctea, s/n, 38205 La Laguna, Spain
e-mail: o.gonzalez@crya.unam.mx
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Spain
³ Centro de Radioastronomía y Astrofísica (CRyA-UNAM), 3-72 (Xangari), 8701 Morelia, Mexico
⁴ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 México DF, Mexico
⁶ Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, 9500 Bento Goncalves, 91501-970, Porto Alegre, Brazil
⁷ Instituto de Física de Cantabria, CSIC-UC, 39005 Santander, Spain
⁸ Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA

Received: 31 October 2014
Accepted: 15 January 2015

Abstract

Context. Most of the optically classified low-ionisation, narrow emission-line regions (LINERs) nuclei host an active galactic nucleus (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 N_H> 1.5 × 10²⁴ 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 (SB) nuclei. We studied the AGN compared to the starburst content in our sample using different indicators: the equivalent width of the polycyclic aromatic hydrocarbon at 6.2 μm, the strength of the silicate feature at 9.7 μm, and the steepness of the mid-IR spectra. We classified the spectra as SB-dominated and AGN-dominated, according to these diagnostics and compared the average mid-IR spectra of the various classes. Moreover, we studied the correlation between the 12 μm luminosity, νL_ν(12 μm), and the 2−10 keV energy band X-ray luminosity, L_X(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 different from the average spectra of the other optical classes, showing a rather flat spectrum at 6−28 μm. We find that the average spectrum of AGN-dominated LINERs with X-ray luminosities L_X(2−10 keV) > 10⁴¹ erg/s is similar to the average mid-IR spectrum of AGN-dominated S2s. However, faint LINERs (i.e. L_X(2−10 keV) < 10⁴¹ erg/s) show flat spectra different from any of the other optical classes. The correlation between νL_ν(12 μm) and L_X(2−10 keV) for AGN nicely extends towards low luminosities only if SB-dominated LINERs are excluded and if the 2−10 keV band X-ray luminosity is corrected in Compton-thick LINER candidates.

Conclusions. We find 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 that the dusty-torus disappear when their bolometric luminosity is below L_bol ≃ 10⁴² 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. To reconcile the Compton-thick nature of a large number of LINERs without dusty-torus signatures, we suggest that the material producing the Compton-thick X-ray obscuration is free of dust.