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Astron. Astrophys. 315, L269-L272 (1996)

SWS observations of the Galactic center*

D.Lutz1, H.Feuchtgruber1,2, R.Genzel1, D.Kunze1, D.Rigopoulou1, H.W.W.Spoon1, C.M.Wright1, E.Egami1, R.Katterloher1, E.Sturm1, E.Wieprecht1, A.Sternberg3, A.F.M.Moorwood4, and Th.deGraauw5

1 Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
2 ISO Science Operations Center, Astrophysics Division of ESA, P.O. Box 50727, E-28080 Villafranca/Madrid, Spain
3 School of Physics and Astronomy and The Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
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. We present a 2.4-45 img1.gifm spectrum of the center of our Galaxy obtained with the Short Wavelength Spectrometer (SWS) on board ISO. The wide range of ionic fine structure lines observed yields an average effective temperature for the ionizing stars of 35000K, with a small contribution of significantly hotter stars, consistent with ageing of an active period of massive star formation that took place a few million years ago. Several absorption features from the foreground cold ISM are detected for the first time. The extinction law towards the Galactic center lacks the expected deep minimum in the 4-8 img1.gifm range. From the detection of OH 34.6 img1.gifm absorption, we infer that radiative pumping is likely the major excitation mechanism for OH emission from the Galactic center. We discuss the rich spectrum of iron and nickel and conclude that the [FeII] spectrum is inconsistent with pure collisional excitation. This calls for caution in interpretation of [FeII] emission in sources, such as starburst galaxies, which contain intense radiation fields.

Key words: Galaxy: center - ISM: general - dust: extinction - Infrared: ISM: lines and bands

* 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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