  We present results that suggest that disc angular momentum catastrophes
(DAMCs) plaguing hierarchical hydrodynamical simulations of galaxy formation,
not including star formation processes, are due to the inward gas transport
that follows bar disk instabilities triggered by interactions and mergers. They
also show that DAMCs can be easily avoided by including star-forming processes,
as they lead unavoidably to the formation of compact stellar bulges that
stabilize disks against bars. The formation of disks similar to those observed
demands, in addition, that not all the gas is depleted into stars at high z, so
that they can be formed at lower z.
