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Astron. Astrophys. 315, L133-L136 (1996)

ISO-SWS spectroscopy of Arp 220: a highly obscured starburst galaxy*

E. Sturm1, D. Lutz1, R. Genzel1, A. Sternberg2, E. Egami1, D. Kunze1, D. Rigopoulou1, O.H. Bauer1, H. Feuchtgruber1,3, A.F.M. Moorwood4, and Th. de Graauw5

1 Max-Planck-Institut fuer Extraterrestrische Physik (MPE), 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 20 August 1996

Abstract. We report the first 2.4 - 45 img1.gifm spectroscopic study of Arp 220 obtained with the Short Wavelength Spectrometer onboard ISO. Observations of mid infrared lines penetrate deep into the highly obscured regions where the luminosity originates and give direct clues to their sources of excitation. From the observed Brackett line ratios and from the [SIII] 18.7 img1.gifm/33.5 img1.gifm line ratio we derive a mean extinction of Aimg2.gif 50 mag, significantly higher than most earlier values. No AGN indicators such as the high excitation [Ne V] or [O IV] lines are detected to an upper limit significantly less than in bona fide AGN galaxies, indicating that a central AGN could only be a minor contributor to the total luminosity. The intensities of the low excitation [Ne II] and [S III] lines, and the extinction corrected ratio of the Lyman continuum and bolometric luminosity imply that massive star-formation is the dominant source of luminosity in Arp 220. The spectra include bright emission of the pure rotational S(1) and S(5) lines of molecular hydrogen. If the extinction to the emitting region is the same as to the ionized medium, as much as 10% of the ISM of Arp 220 could be in the warm phase sampled by these lines.

Key words: galaxies: individual: Arp 220 - infrared: galaxies

* ISO is 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

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