2007ApJ...667..149F

We present R∼600, 10-37 µm spectra of 53 ultraluminous infrared galaxies (ULIRGs), taken using the Infrared Spectrograph on board Spitzer. The spectra show fine-structure emission lines of neon, oxygen, sulfur, silicon, argon, chlorine, iron, and phosphorous; molecular hydrogen lines, and C₂H₂, HCN, and OH^– absorption features. We employ diagnostics based on the fine-structure lines, the polycyclic aromatic hydrocarbon (PAH) features and the 9.7 µm silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only ∼20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of [Ne V] λ14.32 in just under half the sample, however, implies that an AGN contributes significantly to the mid-IR flux in ∼42% of ULIRGs. The starbursts and AGNs in ULIRGs appear more extincted, and for the starbursts more compact than those in lower luminosity systems. The excitations and electron densities in the narrow-line regions of ULIRGs appear comparable to those of starbursts with L≲10^11.5 L_☉, although the NLR gas in ULIRGs may be more dense. We show that the [Ne II] λ12.81 + [Ne III] λ15.56 luminosity correlates with both infrared luminosity and the luminosity of the 6.2 and 11.2 µm PAH features, and derive a calibration between PAH luminosity and star formation rate. Finally, we show that ULIRGs with silicate absorption strengths S_sil of 0.8≲S_sil≲2.4 are likely to be powered mainly by star formation, but that ULIRGs with S_sil≲0.8, and possibly those with S_sil≳2.4, contain an IR-luminous AGN.