We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) z ¹ 0.3) using ISOPHOT-S on board the Infrared (L IR [ 1012L _ , Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 km polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We Ðnd that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, for starburst-dominated ULIRGs is very L PAH /L IR , similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable e †ect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we Ðnd that systems in the early stages of merging may well put out maximum luminosity. We also Ðnd that the total mass of interstellar gas, as estimated from the CO (1 ] 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we Ðnd that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

We present low-resolution mid-infrared (MIR) spectra of 16 ultraluminous infrared galaxies (ULIRGs) obtained with the circular variable Ðlter (CVF) spectroscopy mode of ISOCAM on board the Infrared Space Observatory (ISO). Our sample completes previous ISO spectroscopy of ultra-and hyperluminous infrared galaxies toward higher luminosities. The combined samples cover an infrared luminosity range of D1012È1013.1To discriminate active galactic nucleus (AGN) and starburst activity, we use the L _ . AGN-related MIR continuum and the starburst-related 6.2, 7.7, 8.6, and 11.3 km MIR emission bands attributed to aromatic carbonaceous material. For about half of the high-luminosity ULIRGs studied here, strong aromatic emission bands suggest starburst dominance. Other spectra are dominated by a strong AGN-related continuum with weak superposed emission features of uncertain nature. Our sample contains one unusual example, IRAS F00183[7111, of an AGN that is highly obscured even in the MIR. An improved method to characterize quantitatively the relative contribution of star formation and AGN activity to the MIR emission of ULIRGs is presented. The ULIRG spectra are Ðtted by a superposition of a starburst and an AGN spectrum, both of which may be obscured at di †erent levels. Models in which starburst and AGN obscuration di †er are signiÐcantly more successful than models with a single extinction. Previous results based on a simpler line-to-continuum measure of aromatic emission strength are conÐrmed, further supporting the robustness of the aromatic emission feature as a diagnostic of ULIRG power sources. As dominant sources of the bolometric luminosity, starbursts prevail at the lower end and AGNs at the higher end of this range. The transition between mostly starburst and mostly AGN powered occurs at D1012.4È1012.5and individual luminous starbursts are found up to L _ , D1012.65 L _ .