For disk galaxies ( spirals and irregulars ) , the inner circular-velocity gradient d _ { R } V _ { 0 } ( inner steepness of the rotation curve ) correlates with the central surface brightness \Sigma _ { *, 0 } with a slope of \sim 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 _ { R } ( ranging from 1 to 3 in the R -band ) and in the characteristic thickness of the central stellar component \Delta z ( 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 _ { R } and \Delta z .
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 \Lambda CDM cosmology .