| Spitzer IRS spectra of a large sample of Seyfert galaxies: A Variety of infrared SEDs in the local AGN population (2006) | |||||||||||||
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| Accepted to the Astronomical Journal (July issue). Also archived in: arXiv: astro-ph/0604222 v1 10 Apr 2006. We are conducting a large observing program with the Spitzer Space Telescope to determine the mid-to-far infrared spectral energy distributions of a well-defined sample of 87 nearby, 12 µm-selected Seyfert galaxies. In this paper we present the results of IRS low-resolution spectroscopy of a statistically representative subsample of 51 of the galaxies (59%), with an analysis of the continuum shapes and a comparison of the Seyfert types. We find that the spectra clearly divide into groups based on their continuum shapes and spectral features. The largest group (47% of the sample of 51) shows very red continuum suggestive of cool dust and strong emission features attributed to PAHs. Sixteen objects (31%) have a power-law continuum with spectral indices 5−20µm =-2.3 – -0.9 that flatten to 20−35µm =-1.1 – 0.0 at 20 µm. Clear silicate emission features at 10 and 18 µm are found in two of these objects (Mrk 6 and Mrk 335). A further 16% of the sample show power-law continua with unchanging slopes of 5−35µm =-1.7 – -1.1. Two objects are dominated by a broad silicate absorption feature. One object in the sample shows an unusual spectrum dominated by emission features, that is unlike any of the other spectra. Some spectral features are clearly related to a starburst contribution to the IR spectrum, while the mechanisms producing observed power-law continuum shapes, attributed to an AGN component, may be dust or non-thermal emission. The infrared spectral types appear to be related to the Seyfert types. Principal component analysis results suggest that the relative contribution of starburst emission may be the dominant cause of variance in the observed spectra. The derived starburst component of each spectrum, however, contributes . We thank the referee for a careful reading of the manuscript, which resulted in significant improvements to the paper. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and Ames Research Center. SMART was developed by the IRS Team at Cornell University and is available through the Spitzer Science Center at Caltech. This research has made use of NASA’s Astrophysics Data System. | |||||||||||||
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