WHAT POWERS LUMINOUS INFRARED GALAXIES

Based on the initial data sets taken with the ISO short wavelength spe ctrometer (SWS) we present a first discussion of the source of luminos ity of (ultra-)luminous infrared galaxies (ULIRGs). By comparison of o bservations of 2.5-45 mu m lines to classical starbursts and active ga lactic nuclei and by modelling of the line emission we show that three key representatives of this class, Arp 220, NGC 6240 and NGC 3256 are likely powered mainly by recently formed, massive stars. While an act ive nucleus may well be present in anyone of these sources, ratios of fine structure lines in different stages of ionization show that most of the luminosity is in a relatively soft, 'stellar' ultraviolet radia tion field. Starburst models with stars of masses up to 50-100 M. succ essfully account for the observations with an extended burst phase of 1 to 2x10(7) years and burst ages between 2 and 7x10(7) years. Our ana lysis indicates that previous optical and near-infrared analyses were strongly hampered by the very large extinctions in these galaxies.