The SWS survey for rotational H_2_ lines in late-type galaxies: Observations of the central regions of NGC 6946

We report the first extragalactic detection of the pure rotational H_2_ S(0) and S(1) lines at 28.2 and 17.0μm 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 5x10^6^Msun_ 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/H_2_ 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_2_ conversion factor for the ^12^CO J=1-0 line is not strongly affected by the presence of the warm component.