MID-infrared spectral diagnostics of nuclear and extranuclear regions in nearby galaxies

D. A. Dale, J. D T Smith, L. Armus, B. A. Buckalew, G. Helou, R. C. Kennicutt, J. Moustakas, H. Roussel, K. Sheth, G. J. Bendo, D. Calzetti, B. T. Draine, C. W. Engelbracht, K. D. Gordon, D. J. Hollenbach, T. H. Jarrett, L. J. Kewley, C. Leitherer, A. Li, S. MalhotraE. J. Murphy, F. Walter

Research output: Contribution to journalArticlepeer-review

110 Scopus citations


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 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 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 μm 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 (AV ∼ 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.

Original languageEnglish (US)
Pages (from-to)161-173
Number of pages13
JournalAstrophysical Journal
Issue numberI
StatePublished - Jul 20 2006


  • Galaxies: active
  • Galaxies: nuclei - H ii regions
  • Infrared: ISM
  • Infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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