Bibcode
González-Martín, O.; Masegosa, J.; Márquez, I.; Rodríguez-Espinosa, J. M.; Acosta-Pulido, J. A.; Ramos Almeida, C.; Dultzin, D.; Hernández-García, L.; Ruschel-Dutra, D.; Alonso-Herrero, A.
Bibliographical reference
Astronomy and Astrophysics, Volume 578, id.A74, 19 pp.
Advertised on:
6
2015
Journal
Citations
46
Refereed citations
43
Description
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 NH> 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, νLν(12
μm), 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 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 LX(2-10 keV)
> 1041 erg/s is similar to the average mid-IR spectrum of
AGN-dominated S2s. However, faint LINERs (i.e. LX(2-10 keV)
< 1041 erg/s) show flat spectra different from any of the
other optical classes. The correlation between νLν(12
μm) and LX(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
Lbol ≃ 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.
Table 1 is available in electronic form at http://www.aanda.org
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