Prof. Dimitra Rigopoulou

Abstract:

We present a survey of molecular hydrogen line emission in nearby starburst and Seyfert galaxies. Excitation temperatures and gas masses for loci-lying energy levels of Hz are derived from the observed pure rotational lines. The H-2 (0-0) S(1) line in Seyferts is emitted by a "warm" gas at temperature of 120-140 K. This component accounts for 0.2-20 per cent of the total gas content. In starbursts the temperature of the S(1) emitting gas is somewhat higher (up to 210 K) and contributes typically a few per cent to the total gas mass (up to 9 per cent in M82). The S(5) and S(7) lines trace a "hotter" gas component at temperatures of about 1100K (up to 1700K in IC342) both in Seyfert and starburst galaxies. The excitation of the molecular hydrogen in starbursts and some of the Seyfert galaxies is dominated by UV-fluorescence. However it cannot be excluded that a part of the "warm" H-2 gas is thermally excited. For some of the Seyferts indications for shock excitation are found.

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.