Astronomy and Astrophysics Letters dedicated to ISO

The SWS survey for rotational H2 lines in late-type galaxies. Observations of the central regions of NGC 6946

Valentijn E.A., van der Werf P.P., de Graauw T., de Jong T.


We report the first extragalactic detection of the pure rotational H2 S(0) and S(1) lines at 28.2 and 17.0 mu m in the central 130pc molecular complex of the nearby Scd galaxy NGC 6946. We deduce a temperature of this gas component of 170K and a mass of 5x106Msolar for an ortho/para ratio of 3, corresponding to the ratio of statistical weights. The warm gas is confined to a ~5" (135pc) diameter region where a moderate nuclear starburst accounts for the elevated temperature. Depending on the assumed CO/H2 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/H2 conversion factor for the 12CO J=1-0 line is not strongly affected by the presence of the warm component.

galaxies: individual: NGC 6946 - galaxies: ISM

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