The nature and evolution of ultraluminous infrared galaxies: A mid-infrared spectroscopic survey

We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous inf rared galaxies (ULIRGs) (L-IR > 10(12) L.) using ISOPHOT-S on board the Inf rared Space Observatory (ISO). We use the ratio of the 7.7 mu m "polycyclic aromatic hydrocarbon" (PAH) emission feature to the local continuum as a d iscriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star forma tion, but the fraction of AGN-powered objects increases with luminosity. Wh ereas only about 15% of ULIRGs at luminosities below 2 x 10(12) L. are AGN powered, this fraction increases to about half at higher luminosity. Observ ed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extin ction and/or different physical conditions in the interstellar medium of UL IRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio o f feature-free 5.9 mu m continuum to the IRAS 60 mu m continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. T he location of starburst-dominated ULIRGs in such a diagram is consistent w ith previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indic ations that ULIRGs that are advanced mergers might be more AGN-Like, as pos tulated by the classical evolutionary scenario. No such trend has been foun d among those objects for which near-infrared images are available to asses s their likely merger status.