The SWS survey for H2 in galaxies

ISO SWS has made the first extragalactic direct detection of the H2 molecule in pure rotational lines in the nucleus of NGC 6946. Here we focus on the interstellar medium in the disks of spiral galaxies. Our detection (Valentijn & van der Werf 1999, hereafter VW99) of the two lowest pure rotational lines of molecular hydrogen at 28.2 (S(0)) and 17.0 μm (S(1)) at 8 positions throughout the stellar disk of NGC 891 provides new insight in atomic to molecular hydrogen ratios and the physical state of the interstellar medium in the disks of late-type galaxies. H2 has been detected out to 12 kpc from the center of NGC 891. The data indicate relatively warm (T = 150 - 230 K) molecular clouds scattered throughout the disk, in addition to a massive cooler (T=80-90 K) component which dominates in the outer regions. For H2 ortho-para ratios of 2-3 the cool component has typical edge-on surface densities of about 3000 Modot/pc2, in which case it outweighs the atomic hydrogen by a factor 10. This factor raises the gas masses within the optical disks sufficiently to account for the observed rotation curves in these regions.