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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.

Abstract:

Spectroscopy from the Infrared Space Observatory ISO has for the first time provided the sensitivity to exploit the diagnostic power of mid-infrared fine structure lines and PAH features for the study of ultraluminous infrared galaxies (L-IR > 10(12)L.). We report on observations obtained with SWS, ISOPHOT-S, and the CVF option of ISOCAM. From both fine structure lines and PAH features, we find that the majority of ULIRGs is predominantly powered by star formation. Our total sample of about 75 ULIRGs allows to search for trends within the class of ULIRGs: The fraction of AGNs increases with luminosity above similar to 3 x 10(12)L. but there is no obvious trend for ULIRGs to be more AGN-like with more advanced merger phase.

Abstract:

We present spectroscopic mid-IR observations of prominent starburst galaxies obtained with the short wavelength spectrometer onboard the Infrared Space Observatory. The wavelength range accessible by ISO-sws (2.5-45 pm) contains a large number of emission lines of atomic and molecular hydrogen and several other atomic species. In this paper we discuss the interpretation of the [NeIII] 15.5 mum/[NeII] 12.8 mum line-ratio, the faint [OIV] 25.9 mum line and the pure rotational lines of molecular hydrogen observed in our target galaxies.