1998ApJ...505L.103L

We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous infrared galaxies (ULIRGs) (L_IR>10¹² L_☉) using ISOPHOT-S on board the Infrared Space Observatory (ISO). We use the ratio of the 7.7 µm ``polycyclic aromatic hydrocarbon'' (PAH) emission feature to the local continuum as a discriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star formation, but the fraction of AGN-powered objects increases with luminosity. Whereas only about 15% of ULIRGs at luminosities below 2x10¹² L_☉ are AGN powered, this fraction increases to about half at higher luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 µm continuum to the IRAS 60 µm continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs that are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found among those objects for which near-infrared images are available to assess their likely merger status.