Molecular hydrogen excitation in ultraluminous infrared galaxies

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 , Brgamma, and He I 2(1)P-2(1)S recombination lines. The relative H-2 line intensities show little variation between the objects, suggesting that the H-2 excitation mechanisms in the nuclei are similar in all the objects. The 1-0 emissions appear thermalized at temperatures Tsimilar to1000 K. However, the 2-1 and 3-2 emissions show evidence of being radiatively excited by far-ultraviolet (FUV) photons, suggesting that the H-2 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 10(4) and 10(5) cm(-3), exposed to FUV radiation fields at least 10(3) times more intense than the ambient FUV intensity in the local interstellar medium. We have constructed starburst models for the ULIRGs based on their H-2 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.