A 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 μm for 77 sources with 0.02 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