Molecular Hydrogen Excitation in Ultraluminous Infrared Galaxies

We report medium resolution VLT ISAAC K-band spectroscopy of the nuclei ofseven ultraluminous infrared galaxies. After accounting for stellar absorptionfeatures, we have detected several molecular hydrogen (H_2) v=1-0, 2-1, and 3-2vibrational emission lines, as well as the HI Br\gamma and HeI 2^1P-2^1Srecombination lines. The relative H_2 line intensities show little variationbetween the objects, suggesting that the H_2 excitation mechanisms in thenuclei are similar in all the objects. The 1-0 emissions appear thermalised attemperatures T\sim1000K. However, the 2-1 and 3-2 emissions show evidence ofbeing radiatively excited by far-ultraviolet (FUV) photons, suggesting that theH_2 excitation in the ULIRGs may arise in dense photon dominated regions(PDRs). We show that the line ratios in the nuclei are consistent with PDRswith cloud densities between 10^4 to 10^5cm^{-3}, exposed to far ultraviolet(FUV) radiation fields at least 10^3 times more intense than the ambient FUVintensity in the local interstellar medium. We have constructed starburstmodels for the ULIRGs based on their H_2 properties, as well as on theintensities of the recombination lines. Our models provide a consistent pictureof young 1-5Myr star clusters surrounded by relatively dense PDRs which areirradiated by intense FUV fluxes. Comparison to the inner few hundred parsecsof the Milky Way indicates that the star formation efficiency in ULIRGs is10--100 times higher than in the Galactic Center.Comment: accepted by ApJ (32 pages including figures