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Abstract:

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultra-luminous IRAS galaxies. We combine the survey results with a detailed case study, based on near-IR and mm imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultra-luminous galaxies to ISO and literature data of thirty starburst and active galactic nuclei (AGN), template galaxies. We find that 1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly powered by recently formed massive stars. 20-30% are powered by a central AGN. These conclusions are based on a new infrared 'diagnostic diagram' involving the ratio of high to low excitation mid-IR emission lines on the one hand, and on the strength of the 7.7um PAH feature on the other hand. 2) at least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring. 3) the mid-infrared emitting regions are highly obscured. After correction for these extinctions, we estimate that the star forming regions in ULIRGs have ages between 10^7 and 10^8 years, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4) in the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time dependent compression of the circum-nuclear interstellar gas, the accretion rate onto the central black hole and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

Abstract:

Soon after the IRAS mission, the first ever mission in exploring the Infrared world from space, it was realised that there exists a great number of galaxy populations that are extremely luminous at infrared wavelengths. Yet, their optical output was significantly smaller when compared to their infrared one. The enormous amounts of dust present in these galaxies is of course the reason for the different appearence of these galaxies at different wavelengths. And although optical spectroscopy is generally a useful tool, it can not provide much inside on these galactic nuclei as their centers suffer high extinctions. However, infrared spectroscopy although difficult to be performed from the ground, is the ideal tool for probing the obscured nuclei of galaxies. The Infrared Space Observatory (ISO) provides the best opportunity to probe these obscured nuclei, and even more, promises to open up our view of the infrared world.

Abstract:

The Infrared Space Observatory is opening the 2.5 to 200 mu m band for detailed studies of galaxies. Based an the first results on ionic emission lines obtained with the Short-Wavelength-Spectrometer me discuss the nature of ultra-luminous IR, galaxies.