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High resolution mid-infrared spectra are presented for 155 nuclear andextranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS).The fluxes for nine atomic forbidden and three molecular hydrogen mid-infraredemission lines are also provided, along with upper limits in key lines forinfrared-faint targets. The SINGS sample shows a wide range in the ratio of[SIII]18.71um/[SIII]33.48um, but the average ratio of the ensemble indicates atypical interstellar electron density of 300-400 cm^{-3} on ~23"x15" scales and500-600 cm^{-3} using ~11"x9" apertures, independent of whether the regionprobed is a star-forming nuclear, a star-forming extranuclear, or an AGNenvironment. Evidence is provided that variations in gas-phase metallicity playan important role in driving variations in radiation field hardness, asindicated by [NeIII]15.56um/[NeII]12.81um, for regions powered by starformation. Conversely, the radiation hardness for galaxy nuclei powered byaccretion around a massive black hole is independent of metal abundance.Furthermore, for metal-rich environments AGN are distinguishable fromstar-forming regions by significantly larger [NeIII]15.56um/[NeII]12.81umratios. Finally, [FeII]25.99um/[NeII]12.81um versus [SiII]34.82um/[SIII]33.48umalso provides an empirical method for discerning AGN from normal star-formingsources. However, similar to [NeIII]15.56um/[NeII]12.81um, these mid-infraredline ratios lose their AGN/star-formation diagnostic powers for very lowmetallicity star-forming systems with hard radiation fields.

The Spitzer Infrared Nearby Galaxies Survey: A High-Resolution Spectroscopy Anthology