Observations of Ultraluminous Infrared Galaxies with the IRS on Spitzer

Abstract

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5−38μm region of the ten Ultraluminous Infrared Galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample. Among the BGS ULIRGs, we find a factor of 50 spread in the spectral slope from 5.5−60μm in the rest frame. There is evidence for water ice and hydrocarbon absorption (from 5.5−6.5μm) in 7/10 BGS ULIRGs, as well as absorption features of C2H2 and HCN in four and possibly six of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [NeV] emission at 14.3μm and 24.3μm in three out of the 10 BGS ULIRGs. The AGN fractions implied by either the [NeV]/[NeII] or [OIV]/[NeII] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2μm PAH EQW feature.