We have compiled high-resolution position-velocity diagrams observed along the major axes of nearby spiral galaxies in the CO line emission, and derived rotation curves for the inner regions of the galaxies. We have combined the inner rotation curves with the outer H I and optical rotation curves to obtain the total rotation curves. The inner rotation curves are characterized by a steep increase within a radius of a few hundred parsecs, indicating a compact massive concentration near the nucleus. We fit the obtained rotation curves for individual galaxies by a modified Miyamoto-Nagai potential by assuming existence of four mass components: a nuclear mass component with a scale radius of 100-150 pc and a mass of ∼3-5 × 10⁹ Msun a central bulge of 0.5-1 kpc radius of mass ∼10¹⁰ Msun; a disk with scale radius 5-7 kpc and thickness 0.5 kpc of mass ∼1-2 × 10¹¹ Msun and a massive halo of scale radius 15-20 kpc with a mass ∼2-3 × 10¹¹ Msun. We discuss the implication of the nuclear compact mass component for the formation mechanism of multiple structures within the central bulge of a galaxy during its formation.
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