Molecular hydrogen excitation in ultraluminous infrared galaxies

R. I. Davies*, A. Sternberg, M. Lehnert, L. E. Tacconi-Garman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We report medium-resolution Very Large Telescope ISAAC K-band spectroscopy of the nuclei of seven ultraluminous infrared galaxies. After accounting for stellar absorption features, we have detected several molecular hydrogen (H 2) v = 1-0, 2-1, and 3-2 vibrational emission lines, as well as the H I, Brγ, and He I 21 P-21 S recombination lines. The relative H2 line intensities show little variation between the objects, suggesting that the H2 excitation mechanisms in the nuclei are similar in all the objects. The 1-0 emissions appear thermalized at temperatures T ∼ 1000 K. However, the 2-1 and 3-2 emissions show evidence of being radiatively excited by far-ultraviolet (FUV) photons, suggesting that the H2 excitation in the ultraluminous infrared galaxies (ULIRGs) might arise in dense photon-dominated regions (PDRs). We show that the line ratios in the nuclei are consistent with PDRs with cloud densities between 104 and 105 cm-3, exposed to FUV radiation fields at least 103 times more intense than the ambient FUV intensity in the local interstellar medium. We have constructed starburst models for the ULIRGs based on their H2 properties as well as on the intensities of the recombination lines. Our models provide a consistent picture of young 1-5 Myr star clusters surrounded by relatively dense PDRs that are irradiated by intense FUV fluxes. Comparison to the inner few hundred parsecs of the Milky Way indicates that the star formation efficiency in ULIRGs is 10-100 times higher than in the Galactic center.

Original languageEnglish
Pages (from-to)907-922
Number of pages16
JournalAstrophysical Journal
Issue number2 I
StatePublished - 10 Nov 2003


  • Galaxies: ISM
  • Galaxies: nuclei
  • Galaxies: starburst
  • Infrared: galaxies
  • Line: formation
  • Molecular processes


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