CAN THE TULLY-FISHER RELATION BE THE RESULT OF INITIAL CONDITIONS

We use Monte Carlo realizations of halo formation histories and a sphe rical accretion model to calculate the expected scatter in the velocit y dispersions of galactic halos of a given mass due to differences in their formation times, Assuming that the rotational velocity of a spir al galaxy is determined by the velocity dispersion of its halo and tha t its luminosity is related to its total baryonic mass, this scatter t ranslates to a minimum intrinsic scatter in the Tully-Fisher relation. For popular cosmological models we find that the scatter due to varia tions in formation histories is by itself greater than allowed by obse rvations. Unless halos of spiral galaxies formed at high redshift (z g reater than or similar to 1) and did not later accrete any significant amount of mass, the Tully-Fisher relation is not likely to be the dir ect result of cosmological initial conditions but rather a consequence of a subsequent feedback process.