Subarcsecond mid-infrared observations of NGC 6240: Limitations of active galactic nucleus-starburst power diagnostics

E. Egami, G. Neugebauer, B. T. Soifer, K. Matthews, E. E. Becklin, M. E. Ressler

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


In order to examine the relative importance of powerful starbursts and Compton-thick active galactic nuclei (AGNs) in NGC 6240, we have obtained mid-infrared images and low-resolution spectra of the galaxy with sub-arcsecond spatial resolution using the Keck telescopes. Despite the high spatial resolution (∼200 pc) of our data, no signature of the hidden AGNs has been detected in the mid-infrared. The southern nucleus, which we show provides 80%-90% of the total 8-25 μm luminosity of the system, has a mid-infrared spectrum and a mid-/far-infrared spectral energy distribution consistent with starbursts. At the same time, however, it is also possible to attribute up to 60% of the bolometric luminosity to an AGN, consistent with X-ray observations, if the AGN is heavily obscured and emits mostly in the far-infrared. This ambiguity arises because the intrinsic variation of properties among a given galaxy population (e.g., starbursts) introduces at least a factor of a few uncertainty even into the most robust AGN-starburst diagnostics. We conclude that with present observations it is not possible to determine the dominant power source in galaxies when AGN and starburst luminosities are within a factor of a few of each other.

Original languageEnglish (US)
Pages (from-to)1253-1261
Number of pages9
JournalAstronomical Journal
Issue number3
StatePublished - Mar 2006
Externally publishedYes


  • Galaxies: active
  • Galaxies: individual (NGC 6240)
  • Galaxies: interactions
  • Galaxies: nuclei
  • Galaxies: starburst
  • Infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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