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Astron. Astrophys. 315, L145-L148 (1996)

The SWS survey for rotational H2 lines in late-type galaxies*

Observations of the central regions of NGC6946

Edwin A. Valentijn1, Paul P. van der Werf2, Th. de Graauw1, and T. de Jong1,3

1 Space Research Organization of the Netherlands, P.O. Box 800, 9700 AV Groningen, The Netherlands
(e-mail: valentyn@sron.rug.nl; thijsdg@sron.rug.nl; teije@sron.rug.nl)
2 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands (e-mail: pvdwerf@strw.leidenuniv.nl)
3 Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 15 July 1996 / Accepted 6 August 1996

Abstract. We report the first extragalactic detection of the pure rotational H img6.gif S(0) and S(1) lines at 28.2 and 17.0 img7.gifm in the central 130pc molecular complex of the nearby Scd galaxy NGC6946. We deduce a temperature of this gas component of 170K and a mass of img8.gif Mimg9.gif for an ortho/para ratio of 3, corresponding to the ratio of statistical weights. The warm gas is confined to a img10.gif ( img11.gif pc) diameter region where a moderate nuclear starburst accounts for the elevated temperature. Depending on the assumed CO/H img6.gif conversion factor, the warm component contains of the order of 5-10% (with formal lower and upper limits of 2 and 20%) of the total molecular mass in this region. CO data had indicated the presence of this warm gas, but did not provide a mass measurement. The ISO results can fully account for the warm component indicated by the CO data. The CO/H img6.gif conversion factor for the 12CO img13.gif line is not strongly affected by the presence of the warm component.

Key words: galaxies: individual: NGC6946; galaxies: ISM

* Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the P.I. countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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