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 deter mined star formation rate with the expected properties of galaxy haloes pro vides a remarkably good fit to the absolute magnitude-rotation speed correl ation. We find that the power-law nature, and its slope, normalization and scatter are all readily accounted for if the Universe has a low density par ameter, with or without a cosmological constant, and discs are assembled at z similar to 1-1.5. Moreover, this agreement is found simultaneously in fo ur wavebands. An Einstein-de Sitter universe produces discs that are too fa int unless the discs are assembled at z similar to 0.5. The scatter in the relation is due to a combination of the expected range of spin parameters o f the haloes and the range of formation redshifts. The source of the scatte r opens up possibilities of a better galactic distance indicator, if spectr oscopic observations of globular clusters can be used to determine the halo rotation.