ASpitzer Space TelescopeInfrared Spectrograph Survey of Warm Molecular Hydrogen in Ultraluminous Infrared Galaxies

We have conducted a survey of ultraluminous infrared galaxies (ULIRGs) with the Infrared Spectrograph on the Spitzer Space Telescope , obtaining spectra from 5.0 to 38.5 μm for 77 sources with 0.02 < z < 0.93. Observations of the pure rotational H 2 lines S (3) 9.67 μm, S (2) 12.28 μm, and S (1) 17.04 μm are used to derive the temperature and mass of the warm molecular gas. We detect H 2 in 77% of the sample, and all ULIRGs with F 60 μm > 2 Jy. The average warm molecular gas mass is ~2 × 10 8 M ☉ . High extinction, inferred from the 9.7 μm silicate absorption depth, is not observed along the line of sight to the molecular gas. The derived H 2 mass does not depend on F 25 μm / F 60 μm , which has been used to infer either starburst or AGN dominance. Similarly, the molecular mass does not scale with the 25 or 60 μm luminosities. In general, the H 2 emission is consistent with an origin in photodissociation regions associated with star formation. We detect the S (0) 28.22 μm emission line in a few ULIRGs. Including this line in the model fits tends to lower the temperature by ~50-100 K, resulting in a significant increase in the gas mass. The presence of a cooler component cannot be ruled out in the remainder of our sample, for which we do not detect the S (0) line. The measured S (7) 5.51 μm line fluxes in six ULIRGs implies ~3 × 10 6 M ☉ of hot (~1400 K) H 2 . The warm gas mass is typically less than 1% of the cold gas mass derived from 12 CO observations.