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Astron. Astrophys. 315, L137-L140 (1996)

What powers luminous infrared galaxies?*

D. Lutz1, R. Genzel1, A. Sternberg2, H. Netzer2, D. Kunze1, D. Rigopoulou1, E. Sturm1, E. Egami1, H. Feuchtgruber1,3, A.F.M. Moorwood4, and Th. de Graauw5

1 Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
2 School of Physics and Astronomy and The Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
3 ISO Science Operations Center, Astrophysics Division of ESA, P.O. Box 50727, E-28080 Villafranca/Madrid, Spain
4 European Southern Observatory, Karl-Schwarzschildstraße 2, D-85748 Garching, Germany
5 Space Research Organization of the Netherlands and Kapteyn Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 17 July 1996 / Accepted 28 August 1996

Abstract. 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 2.5-45 img1.gifm lines to classical starbursts and active galactic nuclei and by modelling of the line emission we show that three key representatives of this class, Arp220, NGC6240 and NGC3256 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 Mimg2.gif successfully account for the observations with an extended burst phase of 1 to 2 img3.gif107 years and burst ages between 2 and 7 img3.gif107 years. Our analysis indicates that previous optical and near-infrared analyses were strongly hampered by the very large extinctions in these galaxies.

Key words: galaxies: Seyfert - galaxies: starburst - galaxies: stellar content - infrared: galaxies

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE

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