CO-Line Rotation Curves, Deep Potential of Massive Cores, and High-density Molecular Nuclei

We review observational studies of central rotation curves of spiral galaxies using the Nobeyama mm-wave interferometer in the CO 2.6-mm line emission. The observed high-accuracy rotation curves show universal characteristics: RCs rise steeply in the nuclei, or they start at finite speed, and are flat toward the galaxy edges. Calculated mass distributions are similar to each other: spiral galaxies generally have a massive core of ~10⁹M _⊙ in the central 100 pc, bulge, disk and a dark halo. We found extremely high-density, single-peaked molecular gas nuclei in many galaxies, which are embedded in deep gravitational potential of the massive core. Although the molecular nuclei is as dense as that of a giant molecular cloud, the gas is kept gravitationally stable because of the high-velocity rotation.