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Abstract:

We review the results of a major spectroscopic survey of nearby Starbursts and moderate redshift Ultraluminous IRAS Galaxies (ULIRGs) using ISO-SWS, ISOPHOT-S and ISOCAM (CVF) on board ISO. Detailed modeling reveals that massive stars, M-upper greater than or equal to 100 M-. are formed in most Starbursts in relatively short bursts (one or a few O star lifetimes). In the area of ULIRGs we address the main question of the dominant energy source as well as their evolution. We have found that the combination of mid-infrared fine structure line ratio with the strength of the PAH feature provides an excellent tool for discriminating between starburst and AGN activity. Our studies indicate that, on average, ULIRGs are mainly powered by Starbursts. The strength of the PAH feature is also used to probe for evolutionary effects. We find no evidence that ULIRGs found in advanced stages of merging are more AGN-like, as postulated by the classical evolutionary scenario. We suggest that local conditions (related to the energetics of the AGN) determine the dominance of the starburst or AGN.

Abstract:

We have obtained ISOPHOT-S low resolution mid-infrared spectra of a sample of 60 Ultraluminous Infrared Galaxies (ULIRGs). We use the strength of the 'PAH' mid-infrared features as a discriminator between starburst and AGN activity, and to probe for evolutionary effects. Observed ratios of PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. We suggest that such PAH ratio changes relate to the conditions in the interstellar medium in these galaxies, and in particular to extinction. (C)1999 COSPAR. Published by Elsevier Science Ltd.