  For disk galaxies (spirals and irregulars), the inner circular-velocity
gradient (inner steepness of the rotation curve) correlates with the central
surface brightness with a slope of ~0.5. This implies that the central
dynamical mass density scales almost linearly with the central baryonic
density. Here I show that this empirical relation is consistent with a simple
model where the central baryonic fraction f_bar(0) is fixed to 1 (no dark
matter) and the observed scatter is due to differences in the baryonic
mass-to-light ratio M_bar/L (ranging from 1 to 3 in the R-band) and in the
characteristic thickness of the central stellar component dz (ranging from 100
to 500 pc). Models with lower baryonic fractions are possible, although they
require some fine-tuning in the values of M_bar/L and dz. Regardless of the
actual value of f_bar(0), the fact that different types of galaxies do not show
strong variations in f_bar(0) is surprising, and may represent a challenge for
models of galaxy formation in a LCDM cosmology.
