We discuss the origin of the Tully-Fisher (TF) relation using the N-body/smoothed particle hydrodynamics method, which includes cooling, star formation, and stellar feedback of energy, mass, and metals. We consider initially rotating overdense spheres and trace formation processes of disk galaxies from z = 25 to z = 0 in the cold dark matter cosmology. To clarify the origin of the TF relation, we simulate formation of 14 isolated galaxies with different masses and spin parameters and compute observable values, such as the total magnitude and the line width. We find that the simulated galaxies reproduce the slope and scatter of the TF relation: the slope is originated in the difference of total galactic masses, and the scatter is produced by the difference of initial spin parameters. As well as the TF relation, observed features of spiral galaxies, such as the exponential light profile and the flat rotation curve, are reproduced in our simulations, which were assumed a priori in past semianalytical approaches.
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