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C. L. Buchanan (RIT), J. F. Gallimore (Bucknell University), C. P. O'Dea, S. A. Baum, D. J. Axon, A. Robinson (RIT), M. Elitzur (University of Kentucky), M. Elvis (CfA Harvard)
We are conducting a large observing program with the Spitzer Space Telescope to determine the infrared spectral energy distributions of a well-defined sample of 87 nearby, 12~\micron-selected Seyfert galaxies. Here we present the results of IRS low-resolution spectroscopy of a statistically representative subsample of 51 of the galaxies (59%). We find that the spectra clearly divide into groups based on their continuum shapes and spectral features. The largest group (47% of the subsample) shows red continuum suggestive of cool dust and strong emission features attributed to PAHs. Sixteen objects (31%) have power-law continua with spectral indices of -2.3 -- -0.9 that flatten to -1.1 -- 0.0 at ~20~\micron. Silicate emission features at 10 and 18~\micron\ are present in at least one of these objects (Mrk 6). A further 16% of the sample show power-law continua with unchanging slopes of -1.7 to -1.1. Two objects are dominated by a broad silicate absorption feature. The infrared spectrum of one object is dominated by emission features and is unlike any other in the sample. Some spectral features are clearly related to a starburst contribution to the IR spectrum, while the mechanisms producing the 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 suggests that the relative contribution of starburst emission may be the dominant cause of variance in the spectra. We find that the Seyfert 2's typically show stronger starburst contributions than the Seyfert 1's, contrary to the expectations of the unified scheme for AGN. This may be due to the selection effect that only those Seyfert 2's with strong starburst contributions had high enough integrated 12~\micron\ flux densities to fall above the flux limit of the sample.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.