  In this work we update the L-Galaxies semi-analytic model (SAM) to better
follow the physical processes responsible for the growth of bulges via disc
instabilities (leading to pseudo-bulges) and mergers (leading to classical
bulges). We address the former by considering the contribution of both stellar
and gaseous discs in the stability of the galaxy, and we update the latter by
including dissipation of energy in gas-rich mergers. Furthermore, we introduce
angular momentum losses during cooling and find that an accurate match to the
observed correlation between stellar disc scale length and mass at z ~ 0.0
requires that the gas loses 20% of its initial specific angular momentum to the
corresponding dark matter halo during the formation of the cold gas disc. We
reproduce the observed trends between the stellar mass and specific angular
momentum for both disc- and bulge-dominated galaxies, with the former rotating
faster than the latter of the same mass. We conclude that a two-component
instability recipe provides a morphologically diverse galaxy sample which
matches the observed fractional breakdown of galaxies into different
morphological types. This recipe also enables us to obtain an excellent fit to
the morphology-mass relation and stellar mass function of different galactic
types. Finally, we find that energy dissipation during mergers reduces the
merger remnant sizes and allows us to match the observed mass-size relation for
bulge-dominated systems.
