2003ApJ...597..907D

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 (H₂) v=1-0, 2-1, and 3-2 vibrational emission lines, as well as the H I, Brγ, and He I 2¹P-2¹S recombination lines. The relative H₂line intensities show little variation between the objects, suggesting that the H₂excitation 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 H₂excitation 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/cm3, 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 H₂properties 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.