CO OBSERVATIONS OF EDGE-ON GALAXIES .3. NGC 891 - THRESHOLD RADIUS FOR A STAR-FORMATION DISK

Edge-on galaxy NGC 891 has been observed in (CO)-C-12(J = 1-0)-line em ission using the Nobeyama 45-m telescope at an angular resolution of 1 5''. We obtained an intensity distribution and a position-velocity dia gram along the major axis for a length of +/-5' about the galaxy cente r. We derived the radial distributions of the molecular gas density an d surface mass density. The results indicate the presence of (a) a mol ecular gas ring of 3.5 kpc radius associated with an extended outer di sk having a flat rotation, (b) a rigidly-rotating inner disk or a bar of radius of 3 kpc, and (c) a compact nuclear component showing a rigi d rotation and high-velocity dispersion. From the molecular ring struc ture and kinematical characteristics, we conclude that NGC 891 is an S Bb galaxy. The molecular gas within the bar part is shown to be rotati ng rigidly. We find that there exists a threshold radius for the star- formation rate in the gas disk: The star-formation efficiency is almos t constant within 2.2' (= 5.7 kpc; threshold radius), while it decreas es steeply beyond this radius. This implies that the star-formation ra te is not uniquely determined by the gas density, but is also strongly dependent on the structural conditions within the galaxy disk. We obt ained a map toward the tangential direction of the molecular ring with in the main disk, which indicates the presence of a molecular gas spur extending for a few scale heights of the disk. The map also shows the existence of an extended molecular gas disk of 2 kpc thickness, which comprises about 35% of the total molecular mass.