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Astron. Astrophys. 315, L125-L128 (1996)

SWS spectroscopy of the starburst galaxy NGC 3256*

D. Rigopoulou1, D. Lutz1, R. Genzel1, E. Egami1, D. Kunze1, E. Sturm1, H. Feuchtgruber1,2, S. Schaeidt1,2, O.H. Bauer1, A. Sternberg3, H. Netzer3, A.F.M. Moorwood4, and Th. de Graauw5

1 Max-Planck-Institut fuer Extraterrestrische Physik, Postfach 1603, D-85740 Garching bei München, Germany
2 ISO Science Operations Centre, ESA, P.O. Box 50727, E-28080 Villafranca, Madrid, Spain
3 School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978 Israel
4 European Southern Observatory, Karl-Schwarzschildstrasse 2, D-85748 Garching bei München, Germany
5 SRON Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 17 July 1996 / Accepted 13 August 1996

Abstract. We present 2.5-40 img1.gifm ISO SWS spectra of the starburst galaxy NGC 3256. We have observed many fine-structure lines of various atomic species as well as three rotational transitions of molecular hydrogen. From the [SIII] 18 img1.gifm/33 img1.gifm line ratio we infer low electron densities of img2.gif 300 cm-3 in the HII regions. The observed [NeIII]/[NeII] (15 img1.gifm/12 img1.gifm), [ArIII]/[ArII] (8.9 img1.gifm/6.9 img1.gifm), and [SIV]/[SIII] (10.5 img1.gifm/18.7 img1.gifm) line ratios are consistent with an ionizing radiation field with an effective temperature of img2.gif 41000 K. Comparison of the observed fine-structure line ratios with theoretical models of nebular emission from evolving starbursts shows that stars with masses img4.gif 50 Mimg5.gif have been forming recently in NGC 3256. The H2 observations reveal the presence of warm molecular gas. We estimate that img2.gif 109 Mimg5.gif of H2 gas is present at temperatures close to 150 K. This mass corresponds to a few percent of the total cold molecular mass as estimated from CO studies.

Key words: galaxies: individual (NGC 3256) - galaxies: interactions - galaxies: starburst - 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.

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