Spectral Energy Distributions of Seyfert Nuclei

Abstract

We present nuclear spectral energy distributions (SEDs) in the range 0.4–16 μm for an expanded CfA sample of Seyfert galaxies. The spectral indexes (fν ∝ ν) from 1 to 16 μm range from αIR ∼ 0.9 to 3.8. The shapes of the spectra are correlated with Seyfert type in the sense that steeper nuclear SEDs (νfν increasing with increasing wavelength) tend to be found in Seyfert 2's, and flatter SEDs (νfν is constant) in Seyfert 1–1.5's. The galaxies optically classified as Seyferts 1.8's and 1.9's display values of αIR as in type 1 objects, or values intermediate between those of Seyfert 1's and Seyfert 2's. The intermediate SEDs of many Seyfert 1.8–1.9's may be consistent with the presence of a pure Seyfert 1 viewed through a moderate amount (AV ≲ 5 mag) of foreground galaxy extinction. We find, however, that between 10% and 20% of galaxies with broad optical line components have steep infrared SEDs. Torus models usually adopt high equatorial opacities to reproduce the infrared properties of Seyfert 1's and 2's, resulting in a dichotomy of infrared SEDs (flat for type 1's, and steep for type 2's). Such a dichotomy, however, is not observed in our sample. The wide range of spectral indexes observed in the type 2 objects, the lack of extremely steep SEDs, and the large numbers of objects with intermediate spectral indexes cannot be reconciled with predictions from existing optically thick torus models. We discuss possible modifications to improve torus models, including low optical depth tori, clumpy dusty tori, and high optical depth tori with an extended optically thin component.