A large mid-infrared spectroscopic and near-infrared imaging survey of ultraluminous infrared galaxies: Their nature and evolution

We present a low-resolution mid-infrared spectroscopic survey of an unbiase d sample of 62 ultraluminous infrared galaxies (ULIRGs) (L-IR > 10(12)L., Z less than or equal to 0.3) using ISOPHOT-S on board the Infrared Space Obs ervatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 ac tive galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line- to-continuum ratio of the 7.7 mu m polycyclic aromatic hydrocarbon (PAH) em ission feature is used as a discriminator between starburst and AGN activit y in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, L-PAH/L-IR, for starburst-dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extincti on has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S U LIRGs 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 measur ed from our new near-infrared imaging as well as from the literature, we ha ve examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classica l evolutionary scenario. No correlation is found between the stage of merge r in ULIRGs and their infrared luminosity. In fact, we find that systems in the early stages of merging may well put out maximum luminosity. We also f ind 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 find that the starburst dominates the luminosity output. We propose that the available ga s reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.