SpitzerIRS Spectra of a Large Sample of Seyfert Galaxies: A Variety of Infrared Spectral Energy Distributions in the Local Active Galactic Nucleus Population

We are conducting a large observing program with the Spitzer Space Telescopeto determine the mid-to-far infrared spectral energy distributions of awell-defined sample of 87 nearby, 12 micron-selected Seyfert galaxies. In thispaper we present the results of IRS low-resolution spectroscopy of astatistically representative subsample of 51 of the galaxies (59%), with ananalysis of the continuum shapes and a comparison of the Seyfert types. We findthat the spectra clearly divide into groups based on their continuum shapes andspectral features. Some spectral features are clearly related to a starburstcontribution to the IR spectrum, while the mechanisms producing observedpower-law continuum shapes, attributed to an AGN component, may be dust ornon-thermal emission. The infrared spectral types appear to be related to theSeyfert types. Principal component analysis results suggest that the relativecontribution of starburst emission may be the dominant cause of variance in theobserved spectra. We find that Sy 1's have higher ratios of IR/radio emissionthan Sy 2's, as predicted by the unified model if the torus is optically thickin the mid-IR. However, smooth-density torus models predict a much largerdifference between type 1's and 2's than observed in our sample. Ourobservations may be consistent with clumpy torus models containing a steepradial distribution of optically thick dense clumps. (Abridged)