Mid‐Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies

Abstract

Mid-infrared diagnostics are presented for a large portion of the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from ISO and Spitzer. The SINGS data set 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 highionization line index and PAH strength serves as an efficient discriminant between AGNs 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 starforming 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 AGNs, especially for low-metallicity extranuclear environments. In addition, instead of the traditional midinfrared approach to differentiating between AGNs and star-forming sources that utilizes relatively weak highionization lines, we show that strong low-ionization cooling lines of X-ray-dominated regions like [Si ii] 34.82 m can alternatively be used as excellent discriminants. Finally, the typical target in this sample shows relatively modest interstellar electron density ($400 cm 3 ) and obscuration (A V $ 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.