High-resolution mid-infrared spectroscopy of ultraluminous infrared galaxies

We present R similar to 600, 10-37 mu m 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 mu m silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only similar to 20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of [Ne v] lambda 14.32 in just under half the sample, however, implies that an AGN contributes significantly to the mid-IR flux in similar to 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-circle dot, although the NLR gas in ULIRGs may be more dense. We show that the [Ne II] lambda 12.81 + [Ne III] lambda 15.56 luminosity correlates with both infrared luminosity and the luminosity of the 6.2 and 11.2 mu m 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.