Abstract
The slope and scatter in the correlation of luminosity and line width (the ``Tully-Fisher'' relation [TFR]) of rotationally-supported galaxies hold important clues about the processes relevant to their formation. The Tully-Fisher (TF) residuals are discussed here in the context of the relative distribution of baryonic to dark matter fraction inside the optical disk. The residuals in the line width-luminosity and size-luminosity relations are perfectly anti-correlated and suggest that the TFR is independent of surface brightness. The same TF calibration describes spiral galaxies of low and high surface brightness, whether they are barred or not. We can reproduce the basic observed scaling relations for bright galaxies to a reasonable accuracy based on the simplest possible physical assumptions, such as the virial theorem for spherical cold-dark-matter halos and the notion that most of the gas, or a fixed fraction of it, makes stars in a disk such that the size of the stellar system is determined by angular momentum.
