CO images of the central regions of 20 nearby spiral galaxies

We report a CO(J = 1-0) aperture-synthesis survey of the central regions of 20 nearby spiral galaxies. The sample is selected on the basis of inclinat ion, single-dish CO flux, and lack of significant dynamical perturbation. N o selection is made on the basis of their nuclear activity, starburst, or i nfrared luminosity. The observations have been made using the Nobeyama and Owens Valley millimeter arrays, with typical resolutions of 4 " (300 pc) an d 20 km s(-1), and sensitivities of similar to 30 mJy beam(-1) for a 20 km s(-1) channel. In this paper, we present integrated intensity and mean velo city maps, azimuthally averaged radial distributions, and position-velocity plots. Most galaxies in the sample have scale lengths for the radial CO di stribution that are much smaller in the nuclear regions than in the outer d isks. Typically, the CO emission falls to lie of the central peak value at radii of similar to 500 pc. This is significantly smaller than the scale le ngths for the global CO distributions in the galactic disks. CO position-ve locity diagrams exhibit a steep rise in rotation velocity, dV/dr similar to 1 km s(-1) pc(-1), in the central regions of most galaxies. The CO line wi dth typically reaches similar to 95% of the overall H I line width within a radius of I kpc. This steep rise in the rotation velocity mainly reflects the highly concentrated mass distributions in the galactic centers, though it may well be partly due to noncircular motions in barred galaxies. In a n umber of galaxies, the position-velocity diagrams show a small central hole (d less than or similar to 100 pc) in CO emission. In many galaxies, we de tect CO clumps of subkiloparsec size which may be giant molecular associati ons (GMAs). In the Appendix, we demonstrate that small holes at the galacti c center can be readily detected in position-velocity diagrams even when th ey are smaller than the spatial resolution of the data.