Radial distribution of the mass-to-luminosity ratio in spiral galaxies andmassive dark cores

We derive radial profiles of the surface mass density for 19 spiral galaxie s directly from their high-resolution rotation curves. Using the correspond ing luminosity profiles, we obtain the radial distribution of the mass-to-l uminosity ratios (M/Ls) from the inner bulge (around a few hundred parsecs) to the outer disk (greater than or equal to 2-10 kpc) for 11 galaxies (wit h inclination < 70 degrees in order to reduce the influence of the interste llar extinction). The M/Ls in the bulges of two galaxies with sufficient re solution, NGC 4527 and NGC 6946, are found to increase steeply toward the c enter at radii similar to 100-500 pc at rates of 15 +/- 3 and 7 +/- 2 times per kpc, respectively. Some other galaxies with fairly high resolution als o show signs of an increase toward the center. Such an increase may indicat e 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 variatio ns of the M/Ls, we further discuss the variation of the dark mass fraction in spiral galaxies.