2006ApJ...648..323H

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 H₂ 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 H₂ in 77% of the sample, and all ULIRGs with F_60µm>2 Jy. The average warm molecular gas mass is ∼2x10⁸ 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 H₂ mass does not depend on F_25µm/F_60µm, 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 H₂ 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 ∼3x10⁶ M_☉ of hot (∼1400 K) H₂. The warm gas mass is typically less than 1% of the cold gas mass derived from ¹²CO observations.