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

We present direct observations of molecular hydrogen in the disk of the nearby edge-on spiral galaxy NGC 891. With Infrared Space Observatory's Short-Wavelength Spectrometer (SWS) it has been possible, for the first time, to observe the lowest pure rotational lines of H-2 [S(0) at 28.2 mu m and S(1) at 17.0 mu m] at eight positions throughout the stellar disk of NGC 891. Both lines have been detected at all the surveyed positions out to 11 kpc north of the center of the galaxy. An H-2 rotation curve is derived, and we compare H-2 radial profiles with CO and H I data. The observed line ratios 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 the signal in the outer regions. For H-2 ortho/para ratios of 2-3, the cool gas has typical edge-on column densities (1-3) x 10(23) cm(-2) (or similar to 3000 M. pc(-2)), in which case it outweighs the H I by a factor of 5-15. This factor matches well the mass required to resolve the problem of the missing matter of spiral galaxies within at least the optical disk. The newly discovered cool H-2 component would be less massive in the case in which its dominant ortho/para ratio is near unity. We address the thermal balance of this component by a comparison with [C II] 158 mu m data. When combining the new coolish molecular gas results with recent SCUBA cold dust observations of NGC 891, the total gas-to-dust ratio at r < 12 kpc remains around 200.