  We use the Millennium I and II cosmological simulations to revisit the impact
of mergers in the growth of bulges in central galaxies in the LCDM scenario. We
seed galaxies within the growing CDM haloes using semi-empirical relations to
assign stellar and gaseous masses, and an analytic treatment to estimate the
transfer of stellar mass to the bulge of the remnant after a galaxy merger. We
find that this model roughly reproduces the observed correlation between the
bulge-to-total (B/T) mass ratio and stellar mass in present-day central
galaxies as well as their observed demographics, although low-mass B/T<0.1
(bulgeless) galaxies might be scarce relative to the observed abundance. In our
merger-driven scenario, bulges have a composite population made of (i) stars
acquired from infalling satellites, (ii) stars transferred from the primary
disc due to merger-induced perturbations, and (iii) newly formed stars in
starbursts triggered by mergers. We find that (i) and (ii) are the main
channels of mass assembly, with the first being dominant for massive galaxies,
creating large bulges with different stellar populations than those of the
inner discs, while the second is dominant for intermediate/low-mass galaxies
creating small bulges with similar stellar populations to the inner discs. We
associate the dominion of the first (second) channel to classical (pseudo)
bulges, and compare the predicted fractions to observations. We remark that our
treatment does not include other mechanisms of bulge growth such as intrinsic
secular disc instabilities or misaligned gas accretion. We find that the
evolution of the stellar and gaseous contents of the satellite as it moves
towards the central galaxy is a key ingredient in setting the morphology of the
remnant, and that a good match to the observed bulge demographics occurs when
this evolution proceeds closely to that of the central galaxy.
