A Spitzer Space Telescope Infrared Spectrograph Survey of Warm Molecular Hydrogen in Ultraluminous Infrared Galaxies

September 2006 • 2006ApJ...648..323H

Authors • Higdon, S. J. U. • Armus, L. • Higdon, J. L. • Soifer, B. T. • Spoon, H. W. W.

Abstract • 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 ~2×10⁸ M_solar. 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 ~3×10⁶ M_solar of hot (~1400 K) H₂. The warm gas mass is typically less than 1% of the cold gas mass derived from ¹²CO observations.

Based on observations obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, for the National Aeronautics and Space Administration.

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Lee Armus

Senior Scientist