Nuclear Infrared Spectral Energy Distribution of Type II Active Galactic Nuclei

Abstract

We present near and mid--IR observations of a sample of Seyfert II galaxies drawn from the $12\mu$m Galaxy Sample. The sample was observed in the J, H, K, L, M and N bands. Galaxy Surface Brightness Profiles are modeled using nuclear, bulge, bar (when necessary) and disk components. To check the reliability of our findings the procedure was tested using {\em Spitzer\/} observations of M\,31. Nuclear Spectral Energy Distributions (SEDs) are determined for 34 objects, and optical spectra are presented for 38, including analysis of their stellar populations using the STARLIGHT spectral synthesis code. Emission line diagnostic-diagrams are used to discriminate between genuine AGN and HII nuclei. Combining our observations with those found in the literature, we have a total of 40 SEDs. It is found that about 40\%\ of the SEDs are characterized by an upturn in the near-IR, which we have quantified as a NIR slope $\alpha <1$ for an SED characterized as $\lambda f_{\lambda }\propto {\lambda }^{\alpha }$. Three objects with an HII nucleus and two Seyfert nuclei with strong contamination from a circumnuclear starburst, also show an upturn. For genuine AGN this component could be explained as emission from the accretion disk, a jet, or from a very hot dust component leaking from the central region through a clumpy obscuring structure. The presence of a very compact nuclear starburst as the origin for this NIR excess emission is not favored by our spectroscopic data for these objects.

9 tables, 7 figures, ApJS in press

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