First extragalactic direct detection of large scale molecular hydrogen in the disk of NGC 891

Edwin A. Valentijn ² & Paul P. van der Werf ²

¹ Space Research Organization of the Netherlands, and Kapteyn Astronomical Institute, P.O.Box 800, NL-9700 AV Groningen, The Netherlands

² Leiden Observatory, P.O.Box 9513, NL-2300 RA Leiden, The Netherlands

With ISO's Short Wavelength Spectrometer it has been possible, for the first time, to observe the lowest pure rotational lines of Molecular Hydrogen at 28.2 (S(0)) and 17.0  (S(1)) of relatively cool (T<300K) H₂ in the centre of the galaxy NGC 6946.

Here we present the first extragalactic, non-central, detection of both lines at 8 positions throughout the stellar disk of NGC 891, a nearby edge-on spiral galaxy very similar to the Milky Way. Special data reduction techniques have been developed to remove very small glitches due to cosmic ray events, decreasing the effective noise level by a factor 3-4. H₂ has been detected out to 12 kpc from the center of the galaxy, a H₂ rotation curve has been obtained, and the lines strength is compared to CO and HI data. The data indicate relatively warm (T = 150 - 230) molecular clouds scattered throughout the disk, in addition to a massive cooler (T=80-90K) component which dominates the signal in the outer regions. For 'normal' H₂ ortho-para ratio's of 2-3 the cool component has typical edge-on surface densities of about 3000 solar mass/pc**2, in which case it outweighs the atomic hydrogen by a factor 10. This factor matches well the mass required to resolve the problem of the missing matter of spiral galaxies, within at least the optical disk.