2009ApJ...693.1821D

High-resolution mid-infrared spectra are presented for 155 nuclear and extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS). The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared emission lines are also provided, along with upper limits in key lines for infrared-faint targets. The SINGS sample shows a wide range in the ratio of [S III] 18.71 µm/[S III] 33.48 µm, but the average ratio of the ensemble indicates a typical interstellar electron density of 300-400/cm3 on ∼23''x15'' scales and 500-600/cm3 using ∼11''x9'' apertures, independent of whether the region probed is a star-forming nuclear, a star-forming extranuclear, or an active galactic nuclei (AGN) environment. Evidence is provided that variations in gas-phase metallicity play an important role in driving variations in radiation field hardness, as indicated by [Ne III] 15.56 µm/[Ne II] 12.81 µm, for regions powered by star formation. Conversely, the radiation hardness for galaxy nuclei powered by accretion around a massive black hole is independent of metal abundance. Furthermore, for metal-rich environments AGN are distinguishable from star-forming regions by significantly larger [Ne III] 15.56 µm/[Ne II] 12.81 µm ratios. Finally, [Fe II] 25.99 µm/[Ne II] 12.81 µm versus [Si II] 34.82 µm/[S III] 33.48 µm also provides an empirical method for discerning AGN from normal star-forming sources. However, similar to [Ne III] 15.56 µm/[Ne II] 12.81 µm, these mid-infrared line ratios lose their AGN/star-formation diagnostic powers for very low metallicity star-forming systems with hard radiation fields.