Disc instabilities and semi-analytic modelling of galaxy formation
E. Athanassoula

This paper points out that the method to form spheroids in semi-analytic models of galaxy formation are wrong. A criterium, based on bar instability for given disk parameters is often used in these models and either the whole disk, or a fraction of the disk than suddenly transforms into a spheroid. This is a necessary ingredient to match the observed near-IR luminosity functions.

In reality, this criterium does not hold. It was derived from 2D Nbody simulation long ago. If you take into account that the halo is non-static and that there are random motions in the disk and in the halo, then the situation is different: disks are much more stable, and form at best small pseudo-bulges. A simple creterium for bulge formation from disk instability is not easily possible, and should not be used in semi-analytic models.