The Nonstellar Infrared Continuum of Seyfert Galaxies

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Abstract
JHKL'M (1–5 μm) imaging of a sample of Seyfert 2 galaxies is presented. We have performed an accurate estimate of the near-infrared nonstellar nuclear fluxes. We confirm that the near-infrared nuclear continuum between 1 and 2.2 μm of some Seyfert 2s is dominated by stellar emission, whereas the continuum emission at longer wavelengths (λ = 3–5 μm) is almost entirely nonstellar in origin. The nonstellar spectral energy distributions (SED) in the infrared (up to 15 μm) of Seyfert galaxies show a variety of shapes, and they are well reproduced with the tapered disk models of Efstathiou & Rowan-Robinson. We have used two models, one including an optically thin cone component found to fit the SED of NGC 1068 and a coneless model. Although our modeling of the SEDs does not allow us to favor either model to account for all the observed SEDs, we find that the viewing angle toward the central source is well constrained by both models. The galaxies in our sample have fitted values of the viewing angle in the range θv = 0°–64°, for the assumed model parameters. We have also investigated nonstellar color-color diagrams (L'-M vs. H-M and L'-M vs. H-L'). The colors of the Seyfert galaxies with viewing angles θv < 30° are better reproduced with the cone model. These diagrams provide a good means to separate Seyfert 2s with moderate obscuration (AV 20 mag from hard X-ray observations) from those with high obscuration. The ground-based 4.8 μm and ISO 9.6 μm luminosities are well correlated with the hard X-ray luminosities of Seyfert 1s and 2s. These continuum emissions appear as a good indicator of the AGN luminosity, at least in the cases of hard X-ray Compton-thin Seyfert galaxies (NH ≤ 1024 cm-2). We finally stress the finding that some Compton thick galaxies show bright nonstellar emission at 5 μm. This suggests that the near-infrared emission in Seyfert galaxies is produced in an extended component illuminated by the central source, that is more visible from all viewing angles, providing a good explanation for the differing NH/AV ratios found in some Seyfert 2s. We discuss possible implications of mid-infrared surveys for the search of counterparts of highly obscured hard X-ray sources.
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