  We present new semi-analytical models for the formation of disk galaxies with
the purpose of investigating the origin of the near-infrared Tully-Fisher (TF)
relation. The models assume that disks are formed by cooling of the baryons
inside dark halos with realistic density profiles, and that the baryons
conserve their specific angular momentum. Only gas with densities above the
critical density given by Toomre's stability criterion is considered eligible
for star formation, and a simple recipe for supernovae feedback is included. We
emphasize the importance of extracting the proper luminosity and velocity
measures from the models, something that has often been ignored in the past.
The observed K-band TF relation has a slope that is steeper than simple
predictions based on dynamical arguments suggest. Taking the stability related
star formation threshold densities into account steepens the TF relation,
decreases its scatter, and yields gas mass fractions that are in excellent
agreement with observations. In order for the TF slope to be as steep as
observed, further physics are required. We argue that the characteristics of
the observed near-infrared TF relation do not reflect systematic variations in
stellar populations, or cosmological initial conditions, but are governed by
feedback. Finally we show that our models provide a natural explanation for the
small amount of scatter that makes the TF relation useful as a cosmological
distance indicator.
