Dark and visible matter in spiral galaxies

Exploiting relevant information from the profiles of rotation curves, we calculate the dark-to-luminous mass ratio within the disc size for a sample of 43 spiral galaxies. The values we find, while proving the ubiquitous presence of dark matter, vary with luminosity. Faint and bright galaxies are found to be respectively halo-and disc-dominated in the disc regions. The luminosity sequence turns out to be a dark-to-luminous sequence. The Tully–Fisher correlation is justified as connected with the equilibrium condition of a thin disc embedded in a spherical halo. The dynamical effect of dark matter does not disrupt such a centrifugal equilibrium, because the dark-matter mass fraction is a smooth function of luminosity. By removing the dark-matter contribution from the velocity at the disc edge, the dispersion affecting the luminosity–kinematics relation is dramatically decreased as compared with the conventional Tully–Fisher correlation (⁠|0. 3| mag). Comparison with stellar evolution models shows that the dynamically computed (ℳ/LB) disc ratios are able to explain the colours of spiral galaxies in a scenario involving a 10-Gyr star-formation phase with H0=75 km s−1 Mpc−1.