Radial Distribution of the Mass-to-Luminosity Ratio in Spiral Galaxies and Massive Dark Cores

We derive radial profiles of the surface-mass-density for 19 spiral galaxies directly from their high-resolution rotation curves. Using the corresponding luminosity profiles, we obtain the radial distribution of the mass-to-luminosity ratios (M/L) from the inner bulge (∼ a few 100 pc) to the outer disk (≥ 2−10 kpc) for 11 galaxies (with inclination < 70 in order to reduce the influence of the interstellar extinction. The M/Ls in the bulges of two galaxies with sufficient resolution, NGC 4527 and NGC 6946, are found to increase steeply toward the center at radii ∼ 100−500 pc at rates of 15±3 and 7±2 times per kpc, respectively. Some other galaxies with fairly high resolution also show signs of an increase toward the center. Such an increase may indicate the existence of a new component, a “massive dark core”, which may be an object linking the bulge and a central black hole. Based on radial variations of the M/L, we further discuss the variation of the dark-mass fraction in spiral galaxies. Subject headings: Galaxies: kinematics and dynamics — Galaxies: photometry — Galaxies: spiral — Galaxies: structure