Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey ( SINGS ) sample plus archival data from the Infrared Space Observatory and the Spitzer Space Telescope .
The SINGS dataset includes low- and high-resolution spectral maps and broadband imaging in the infrared for over 160 nuclear and extranuclear regions within 75 nearby galaxies spanning a wide range of morphologies , metallicities , luminosities , and star formation rates .
Our main result is that these mid-infrared diagnostics effectively constrain a target ’ s dominant power source .
The combination of a high ionization line index and PAH strength serves as an efficient discriminant between AGN and star-forming nuclei , confirming progress made with ISO spectroscopy on starbursting and ultraluminous infrared galaxies .
The sensitivity of Spitzer allows us to probe fainter nuclear and star-forming regions within galaxy disks .
We find that both star-forming nuclei and extranuclear regions stand apart from nuclei that are powered by Seyfert or LINER activity .
In fact , we identify areas within four diagnostic diagrams containing > 90 % Seyfert/LINER nuclei or > 90 % H ii regions/H ii nuclei .
We also find that , compared to starbursting nuclei , extranuclear regions typically separate even further from AGN , especially for low-metallicity extranuclear environments .
In addition , instead of the traditional mid-infrared approach to differentiating between AGN and star-forming sources that utilizes relatively weak high-ionization lines , we show that strong low-ionization cooling lines of X-ray dominated regions like [ Si ii ] 34.82 \mu m can alternatively be used as excellent discrimants .
Finally , the typical target in this sample shows relatively modest interstellar electron density ( \sim 400 ~ { } { cm } ^ { -3 } ) and obscuration ( A _ { V } \sim 1.0 mag for a foreground screen ) , consistent with a lack of dense clumps of highly obscured gas and dust residing in the emitting regions .