We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxiesWe combine the survey results with a detailed case study, based on arc-(L IR º 1012 L _ ). second resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near-and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We Ðnd the following :

We present new ISO-SWS data for a sample of 27 starburst galaxies, and with these data examine the issues of formation and evolution of the most massive stars in starburst galaxies. Using starburst models which incorporate time evolution, new stellar atmosphere models for massive stars, and a starburst model geometry derived from observations of the prototypical starburst M82, we model the integrated mid-infrared line ratio [Ne III](15.6µm)/[Ne II](12.8µm). This line ratio is sensitive to the hardness of the stellar energy distribution and therefore to the most massive stars present.We conclude from our models, with consideration of recent determinations of the stellar census in local, high-mass star forming regions, that the [Ne III]/[Ne II] ratios we measure are consistent with the formation of massive (∼ 50-100M ⊙ ) stars in most starbursts. In this framework, the low nebular excitation inferred from the measured line ratios can be attributed to aging effects. By including estimates of the ratio of infrared-to-Lyman continuum luminosity for the galaxies in our sample, we further find that most starbursts are relatively short-lived (10 6 -10 7 years), only a few O-star lifetimes. We discuss a possible cause of such short events: the effectiveness of stellar winds and supernovae in destroying the starburst environment.

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Abstract. We present results from a survey of molecular hydrogen emission from a sample of Starburst and Seyfert galaxies carried out with the Infrared Space Observatory (ISO). Pure rotational H2 emission has been detected in a number of extragalactic objects and a variety of environments. A number of transitions from S(7) to S(0) are detected in both starbursts and Seyferts. Using excitation diagrams we derive temperatures and masses of the "warm" molecular hydrogen. We find that the temperature of the "warm" gas is similar in starbursts and Seyferts (those Seyferts for which we have firm detections of the S(0) line) with a value of around T ∼ 150 K. This "warm" gas accounts for as much as 10% of the total galactic mass (as probed by CO molecular observations) in starbursts. The fraction of "warm" gas is overall higher in Seyferts, ranging between 2-35%. We then investigate the origin of the warm H2 emission. Comparison with published theoretical models and Galactic templates implies that although emission from photodissociation regions (PDR) alone could explain the emission from starbursts and Seyferts, most likely a combination of PDR, shock emission and gas heated by X-rays (mostly for the Seyferts) is responsible for H2 excitation in extragalactic environments. Finally, we find that although PAH and H2 line emission correlate well in starbursts and the large scale emission in AGN, H2 emission is much stronger compared to PAH emission in cases where a "pure" AGN dominates the energy output.