8 – 13 μ m spectroscopy of luminous and ultraluminous infrared galaxies

New moderate-resolution mid-infrared spectroscopy at 10 μm of 27 infrared galaxies is presented. The galaxies have been chosen from three 60 μm selected and one 12 μm selected complete flux-limited catalogs of galaxies; 17 of these sources have LIR(8– 1000μm) ≥ 5 × 10 L⊙. A high-resolution spectrum of the source Arp 299B1 is also presented. Combining these new results with previously published results, a nearly complete 60 μm selected flux limited subsample, with LIR(8–1000μm) ≥ 1.6 × 10 11 L⊙ of 25 galaxies is defined. Within this subsample, it is found that the dominant power source of infrared galaxies in the luminosity range 1.6 × 10 < LIR(8– 1000μm) < 10 L⊙ is massive star formation based on the clear presence of the 11.3 μm aromatic hydrocarbon emission feature in the majority of the spectra. Three of the five ultraluminous infrared galaxies (LIR(8–1000μm) ≥ 10 12 L⊙) within this subsample show evidence that an active galactic nucleus provides an energetically important power source based on the detection of silicate absorption in their mid-infrared spectra. The physical basis of a possible anti-correlation between the 11.3 μm feature equivalent width and infrared light to molecular gas mass ratios is discussed.