Mid-Infrared Properties of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. I. Emission-Line Diagnostics

We compare mid-infrared emission-line properties from high-resolution Spitzer spectra of a hard X-ray (14–195 keV) selected sample of nearby ( z < 0.05) active galactic nuclei (AGNs) detected by the Burst Alert Telescope (BAT) aboard Swift . The luminosity distribution for the mid-infrared emission lines, [O iv ] 25.89 μm, [Ne ii ] 12.81 μm, [Ne iii ] 15.56 μm, and [Ne v ] 14.32/24.32 μm, and hard X-ray continuum show no differences between Seyfert 1 and Seyfert 2 populations; however, six newly discovered BAT AGNs are under-luminous in [O iv ], most likely the result of dust extinction in the host galaxy. The overall tightness of the mid-infrared correlations and BAT fluxes and luminosities suggests that the emission lines primarily arise in gas ionized by the AGNs. We also compare the mid-infrared emission lines in the BAT AGNs with those from published studies of ULIRGs, Palomar-Green quasars, star-forming galaxies, and LINERs. We find that the BAT AGN sample falls into a distinctive region when comparing the [Ne iii ]/[Ne ii ] and the [O iv ]/[Ne iii ] ratios. These line ratios are lower in sources that have been previously classified in the mid-infrared/optical as AGNs than those found for the BAT AGNs, suggesting that, in our X-ray selected sample, the AGNs represent the main contribution to the observed line emission. These ratios represent a new emission line diagnostic for distinguishing between AGNs and star-forming galaxies.