CO OBSERVATIONS OF EDGE-ON GALAXIES .5. NGC-5907 - CENTRAL DEFICIENCYOF GAS IN AN SC GALAXY-MERGER IN THE BULGE

The edge-on Sc galaxy NGC 5907 has been observed in the (CO)-C-12(J = 1-0)-line emission using the Nobeyama 45-m telescope. The radial densi ty distribution at between 2 and 13 kpc is well represented by a super position of an exponential-law disk of scale radius 3.5 kpc and a ring of 7 kpc radius. However, no concentration of gas has been observed i n the central 2 kpc. The radial distribution and position-velocity dia gram are compared in detail with those obtained for H I. We found that the molecular gas dominates in the central 5 kpc region, while H I do minates in the outer region, clearly separated by a ''molecular front' ' at 5 kpc radius. This molecular front could be the place where the p hase change from H I to H-2, or vice versa, is taking place on a galac tic scale. The central deficiency of the molecular gas is exceptional for a late-type galaxy, which was found for the first time among Sc ga laxies, and the second case after the Sb galaxy M31. We argue for the possibility of destruction of the nuclear gas disk by a merger with an other galaxy, and discuss its relation to the outer warping disk.