New 8-13  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 catalogues of galaxies; 17 of these sources have LIR(8–1000 μm)≥5×1011 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(8–1000 μm)≥1.6×1011 L⊙, of 25 galaxies is defined. Within this subsample, it is found that the dominant power source of infrared galaxies in the luminosity range 1.6×1011<LIR(8–1000 μm)<1012 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(8–1000 μm)≥1012 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 anticorrelation between the 11.3-μm feature equivalent width and infrared light to molecular gas mass ratios is discussed.