Diagnostic Value of Mid-Infrared Fine Structure Lines in Galaxies

Abstract

Infrared fine structure lines are commonly observed in H II regions, where they are primarily excited by collisions between electrons and ions. Widely used empirical methods combine the emission from these with hydrogen recombination lines or radio continuum measurements to deduce heavy element abundances. However, these abundance studies could have issues. In particular, they rely on the assumption that ionized hydrogen and the different ionized heavy elements are co-spatial. We present an analysis of fine structure line emission from Ar, Ne and S in the nuclei of nearby galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) observed with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, and in a sample of Galactic H II regions observed with the Short Wavelength Spectrometer (SWS) (Giveon et al. [3]). The goal is to compare the emission measure weighted filling factors deduced from each element (H, Ar, Ne and S) in order to test for consistency and improve the diagnostic value of these lines.

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