Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-04-10
Astron.J.132:401-419,2006
Astronomy and Astrophysics
Astrophysics
39 pages, 17 figures, accepted to AJ, version with high-resolution figures available at http://www.cis.rit.edu/~clbsps/papers/
Scientific paper
10.1086/505022
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 micron-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. 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. We find that Sy 1's have higher ratios of IR/radio emission than Sy 2's, as predicted by the unified model if the torus is optically thick in the mid-IR. However, smooth-density torus models predict a much larger difference between type 1's and 2's than observed in our sample. Our observations may be consistent with clumpy torus models containing a steep radial distribution of optically thick dense clumps. (Abridged)
Axon David J.
Baum Stefi A.
Buchanan Catherine L.
Elitzur Moshe
Elvis Martin
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