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(O) and S(1) lines at 28.2 and 17.0 mu m in the central 130 pc molec ular complex of the nearby Scd galaxy NGC 6946. We deduce a temperatur e of this gas component of 170 K and a mass of 5 . 10(6) M(.) for an o rtho/para ratio of 3, corresponding to the ratio of statistical weight s. The warm gas is confined to a similar to 5 '' (135 PC) diameter reg ion where a moderate nuclear starburst accounts for the elevated tempe rature. Depending on the assumed CO/H-2 conversion factor, the warm co mponent contains of the order of 5-10% (with formal lower and upper li mits 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 ma ss measurement. The ISO results can fully account for the warm compone nt indicated by the CO data. The CO/H-2 conversion factor for the (CO) -C-12 J = 1-->0 line is not strongly affected by the presence of the w arm component.