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Session 21 - Luminous Infrared Galaxies.
Display session, Wednesday, January 07
Exhibit Hall,
We report ISO SWS and ISOPHOT-S mid-infrared spectroscopy of ultra-luminous infrared galaxies (ULIRGs). Due to their very high extinction previous (mostly optical and near-infrared) studies have not been able to determine whether ULIRGs are mainly powered by massive starbursts or active nuclei. ISO spectroscopy is much less affected by extinction and offers two new diagnostic tools to tackle this question: The ratios of high excitation to low excitation lines and the strength of the dust (PAH) features. By comparison of the mid-infrared characteristics of ULIRGs to those of starburst and AGN template galaxies, in combination with model calculations such as photoionization codes and starburst models, we try to determine the nature and physical conditions of the energy sources of these types of galaxies.
We find that the starburst component dominates the total luminosity in the majority of the ULIRGs observed so far. Possible correlations of the contribution of the AGN component with the total luminosity and the evolutionary state of the galaxy interaction are discussed.
