What powers luminous infrared galaxies?

Based on the initial data sets taken with the ISO short wavelength spectrometer (SWS) we present a first discussion of the source of luminosity of (ultra-)luminous infrared galaxies (ULIRGs). By comparison of observations of 2.5-45μm lines to classical starbursts and active galactic 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 active 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 radiation field. Starburst models with stars of masses up to 50-100 M_⊙ successfully account for the observations with an extended burst phase of 1 to 2×10⁷ years and burst ages between 2 and 7×10⁷ years. Our analysis indicates that previous optical and near-infrared analyses were strongly hampered by the very large extinctions in these galaxies.