Rotating disk galaxies - Yet another case for dark matter

We devise a method to probe the systematic variations of the dark matter mass fraction (DM) in the optical region of disk galaxies, based on studying the correlation between galaxy luminosity and the normalized radius where the rotation frequency exceeds a given multiple of the rotation frequency as measured at the optical disk radius. The results we find strongly suggest the ubiquitous existence of DM throughout the luminosity sequence of galaxies, thereby challenging the possibility that luminous matter alone can be responsible for the observed disk dynamics. By modeling the distribution of dark matter by means of a pseudoisothermal halo, we show that the disk-to-halo mass ratio at the optical edge, Adisk/Mhalo, has not the same value in all galaxies, but actually varies with luminosity. Remarkably, galaxies at the knee of the luminosity function have Mdisk/Mhalo ∼ 1.