1999MNRAS.307..553D

New moderate-resolution mid-infrared spectroscopy at 10µm 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 L_IR(8-1000µm)≥5x10¹¹L_☉. 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 L_IR(8-1000µm)≥1.6x10¹¹L_☉, of 25 galaxies is defined. Within this subsample, it is found that the dominant power source of infrared galaxies in the luminosity range 1.6x10¹¹<L_IR(8-1000µm)<10¹²L_☉ 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 L_IR(8-1000µm)≥10¹²L_☉] 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.