2010ApJ...710..289B

We present an analysis of Spitzer IRS spectroscopy of 83 active galaxies from the extended 12 µm sample. We find rank correlations between several tracers of star formation which suggest that (1) the polycyclic aromatic hydrocarbon feature is a reliable tracer of star formation, (2) there is a significant contribution to the heating of the cool dust by stars, and (3) the H₂ emission is also primarily excited by star formation. The 55-90 versus 20-30 spectral index plot is also a diagnostic of the relative contribution of starburst to active galactic nuclei (AGNs). We see there is a large change in spectral index across the sample: Δα ∼ 3 for both indices. Thus, the contribution to the IR spectrum from the AGN and starburst components can be comparable in magnitude but the relative contribution also varies widely across the sample. We find rank correlations between several AGN tracers. We find correlations of the ratios [O III]λ5007/[O IV] 26 µm and [O III]λ5007/[Ne V] 14 µm with the silicate strength which we adopt as an orientation indicator. This suggests that some of the [O III]λ5007 emission in these Seyferts is subject to orientation dependent obscuration as found by Haas et al. for radio galaxies and quasars. There is no correlation of [Ne V] equivalent width with the silicate 10 µm strength, indicating that the [Ne V] emission is not strongly orientation dependent. This suggests that the obscuring material (e.g., torus) is not very optically thick at 14 µm consistent with the results of Buchanan et al. We search for correlations between AGN and starburst tracers and we conclude that the AGN and starburst tracers are not correlated. This is consistent with our conclusion that the relative strength of the AGN and starburst components varies widely across the sample. Thus, there is no simple link between AGN fueling and black hole growth and star formation in these galaxies. The density diagnostic [Ne V] 14/24 µm and [S III] 18/33 µm line ratios are consistent with the gas being near the low density limit, i.e., ∼10³/cm3 for [Ne V] and n_e ∼ few hundred/cm3 for [S III]. The distribution of silicate 10 µm and 18 µm strengths is consistent with the clumpy torus models of Sirocky et al. We find a rank correlation between the [Ne V] 14 µm line and the 6.7 µm continuum which may be due to an extended component of hot dust. The Sy 2's with a hidden broad-line region (HBLR) have a higher ratio of AGN-to-starburst contribution to the spectral energy distribution than Sy 2's without an HBLR. This may contribute to the detection of the HBLR in polarized light. The Sy 2's with an HBLR are more similar to the Sy 1's than they are to the Sy 2's without an HBLR.