Seyfert 2 Mid-infrared Spectral Signatures, With And Without Starbursts

Astronomy and Astrophysics – Astronomy

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Scientific paper

Dusty material in active galactic nuclei (AGNs) obscures the central engines and reprocesses the intrinsic short wavelength radiation to emerge at longer wavelengths. In particular, mid-infrared observations usefully probe the surroundings of these supermassive black holes. We examine nearby Seyfert 2 galaxies in order to isolate the emission and environment of the AGN alone (in "pure'' Seyfert 2 galaxies) and to identify the effects of nuclear starbursts that may be simultaneously present (in Seyfert 2/starburst composite galaxies). Optical and UV observations classify the fifteen sample members as pure or composite galaxies, and we obtained MIR spectra of them with the Infrared Spectrograph on board the Spitzer Space Telescope.
The strengths of the silicate dust absorption and emission features at 10 and 18 microns together reveal the geometry of obscuring material surrounding the centers of AGNs. We find that composites tend to show smooth distributions of obscuring material while pure Seyfert 2s tend to have clumpy surroundings. However, the presence or absence of a starburst does not significantly affect the strengths of either the 10 or 18 microns feature separately. The composite galaxies exhibit significantly steeper continuum slopes between 14 and 30 microns than the pure Seyfert 2s. Thus, star formation, when present, dominates the long-wavelength emission.
The AGNs of both subsamples are not different, as the [O IV] 25.9 micron line luminosity traces. Characteristic emission of star-forming regions, including [Ne II] 12.8 micron and [Ne III] 15.5 micron line luminosity and equivalent widths of polycyclic aromatic hydrocarbons near 6.2 and 7.7 microns significantly stronger in the composite galaxies, as expected. However, the large stellar contribution in these cases is not apparent in the line ratio [O IV]/[Ne II], which is indistinguishable in the two classes.
We acknowledge work supported by the NSF under grant number 0237291.

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