The role of star formation in the Tully-Fisher law

We investigate the influence of the star formation rate on the Tully-Fisher relation. We find that a simple model which combines the empirically-determined star-formation rate with the expected properties of galaxy halos provides a remarkably good fit to the absolute magnitude-rotation speed correlation. We find that the power-law nature, its slope, normalisation and scatter, are all readily accounted for if the Universe has a low density parameter, with or without a cosmological constant and disks are assembled at $z \sim 1 - 1.5$. Moreover, this agreement is found simultaneously in 4 wavebands. An Einstein-de Sitter Universe produces disks which are too faint unless the disks are assembled at $z\sim 0.5$. The scatter in the relation is due to a combination of the expected range of spin parameters of the halos and the range of formation redshifts. The source of the scatter opens up possibilities of a better galaxy distance indicator, if spectroscopic observations of globular clusters can be used to determine the halo rotation.