On the H2 line emission from NGC 6240 and other starburst galaxies

B. T. Draine, D. T. Woods

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We examine several possibilities for the production of the extremely powerful H2 line emission from the starburst galaxy NGC 6240. We review observations of NGC 6240 and argue that neither UV pumping by starlight nor collisional excitation in shock waves appears to be capable of producing the observed emission. We propose instead that most of the H2 line emission originates in molecular gas which is heated by transient X-ray irradiation; only ∼20%-30% of the H2 υ = 1 → OS(1) emission is attributed to UV pumping in photodissociation fronts. We deduce the physical conditions under which reprocessing of X-rays can account for the observed H2 line emission. Under suitable conditions, the observed H2 line emission could be produced if-∼ 1 × 1010 L of soft X-rays is being absorbed by molecular material in NGC 6240. The required X-rays could be produced by either supernova explosions (at a rate ∼1.5 yr-1) or high-velocity shock waves associated with the galaxy merger. The required supernova rate does not appear to be incompatible with the likely stellar content of NGC 6240, if it underwent a major burst of star formation ∼ 1-2 × 107 yr ago. Our scenario appears able to account naturally for the reported weakness of the υ= 2 → 1S(3) line and may provide a favorable environment for the pumping of H2O masers. Our model predicts strong emission in unobserved H2 lines such as υ = 0 → OS(9) λ4.6946 μm, and in fine-structure lines such as [O I] 63 μm. Rotational and vibrational transitions of H3+ provide a unique spectral signature of X-ray-heated molecular gas; we predict that some of these transitions, including lines at 16.33 μm and 4.35 μm, may be detectable.

Original languageEnglish (US)
Pages (from-to)464-479
Number of pages16
JournalAstrophysical Journal
Issue number2
StatePublished - Nov 10 1990

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Galaxies: individual (NGC 6240)
  • Galaxies: interstellar matter
  • Infrared: spectra
  • Interstellar: molecules
  • Radiation mechanisms


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