Spitzer Space Telescope infrared 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 mu m for 77 sources with 0.02 < z < 0.93. Observations of the pure rotational H-2 lines S(3) 9.67 mu m, S(2) 12.28 mu m, and S(1) 17.04 mu m 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 mu m > 2 Jy. The average warm molecular gas mass is similar to 2 x 10(8) M-circle dot. High extinction, inferred from the 9.7 mu m 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 mu m/F-60 mu m, which has been used to infer either starburst or AGN dominance. Similarly, the molecular mass does not scale with the 25 or 60 mu m 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 mu m emission line in a few ULIRGs. Including this line in the model fits tends to lower the temperature by similar to 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 mu m line fluxes in six ULIRGs implies similar to 3 x 10(6) M-circle dot of hot (similar to 1400 K) H-2. The warm gas mass is typically less than 1% of the cold gas mass derived from (CO)-C-12 observations.