Spitzer-IRS High-Resolution Spectroscopy of the 12 μm Seyfert Galaxies. II. Results for the Complete Data Set

We present our Spitzer-Infrared Spectrometer (IRS) spectroscopic survey from 10 μm to 37 μm of the Seyfert galaxies of the 12 μm Galaxy Sample, collected in a high-resolution mode (R ~ 600). The new spectra of 61 galaxies, together with the data we already published, give us a total of 91 12 μm Seyfert galaxies observed, out of 112. We discuss the mid-IR emission lines and features of the Seyfert galaxies, using an improved active galactic nucleus (AGN) classification scheme: instead of adopting the usual classes of Seyfert 1's and Seyfert 2's, we use the spectropolarimetric data from the literature to divide the objects into categories "AGN 1" and "AGN 2," where AGN 1's include all broad-line objects, including the Seyfert 2's showing hidden broad lines in polarized light. The remaining category, AGN 2's, contains only Seyferts with no detectable broad lines in either direct or polarized spectroscopy. We present various mid-IR observables, such as ionization-sensitive and density-sensitive line ratios, the polycyclic aromatic hydrocarbon (PAH) 11.25 μm feature and the H₂ S(1) rotational line equivalent widths (EWs), the (60-25 μm) spectral index, and the source extendedness at 19 μm, to characterize similarities and differences in the AGN populations, in terms of AGN dominance versus star formation dominance. We find that the mid-IR emission properties characterize all the AGN 1's objects as a single family, with strongly AGN-dominated spectra. In contrast, the AGN 2's can be divided into two groups, the first one with properties similar to the AGN 1's except without detected broad lines, and the second with properties similar to the non-Seyfert galaxies, such as LINERs or starburst galaxies. We computed a semianalytical model to estimate the AGN and the starburst contributions to the mid-IR galaxy emission at 19 μm. For 59 galaxies with appropriate data, we can separate the 19 μm emission into AGN and starburst components using the measured mid-IR spectral features. We use these to quantify the brightness thresholds that an AGN must meet to satisfy our classifications: AGN 1's have an AGN contribution >=73% and AGN 2 >= 45% of their total emission at 19 μm. The detection of [Ne V] lines turns out to be an almost perfect signature of energy production by an AGN. Only four (~7.5%) of 55 AGN 1's and two (10%) out of 20 AGN 2's do not have [Ne V] 14.3 μm down to a flux limit of ~4 × 10^-15 erg s^-1 cm^-2. We present mean spectra of the various AGN categories. Passing from AGN-dominated to starburst-dominated objects, the continuum steepens, especially at wavelengths shorter than 20 μm, while the PAH feature increases in its EW and the high ionization lines decrease. We estimate H₂ mass and excitation temperature through the measurement of the S(1) rotational line of this molecule. Finally, we derive the first local luminosity functions for the brightest mid-IR lines and the PAH feature at 11.25 μm. No statistical difference is apparent in the space densities for Seyfert 1's and 2's of a given line luminosity, or for the new classes of AGN 1's and 2's. We use the correlation between [Ne V] line and nonstellar IR continuum luminosity to derive the global output of accretion-powered galactic nuclei in the local universe.