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

TL;DR

This paper presents high-resolution mid-infrared spectra of 155 regions in nearby galaxies, analyzing emission lines to understand interstellar medium properties, metallicity effects, and distinguishing features of active galactic nuclei versus star-forming regions.

Contribution

It provides a comprehensive spectral dataset and empirical diagnostics for differentiating AGN from star-forming regions based on mid-infrared line ratios, considering metallicity influences.

Findings

Average interstellar electron density is 300-600 cm^{-3}.

Gas-phase metallicity influences radiation field hardness in star-forming regions.

Mid-infrared line ratios can distinguish AGN from star-forming regions, except in low-metallicity environments.

Abstract

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 [SIII]18.71um/[SIII]33.48um, but the average ratio of the ensemble indicates a typical interstellar electron density of 300-400 cm^{-3} on ~23"x15" scales and 500-600 cm^{-3} using ~11"x9" apertures, independent of whether the region probed is a star-forming nuclear, a star-forming extranuclear, or an 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 [NeIII]15.56um/[NeII]12.81um, 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 [NeIII]15.56um/[NeII]12.81um ratios. Finally, [FeII]25.99um/[NeII]12.81um versus [SiII]34.82um/[SIII]33.48um also provides an empirical method for discerning AGN from normal star-forming sources. However, similar to [NeIII]15.56um/[NeII]12.81um, these mid-infrared line ratios lose their AGN/star-formation diagnostic powers for very low metallicity star-forming systems with hard radiation fields.