The growth of galactic bulges through mergers in Λ cold dark matter haloes revisited - II. Morphological mix evolution

The mass aggregation and merger histories of present-day distinct haloes selected from the cosmological Millennium Simulations I and II are mapped into stellar mass aggregation and galaxy merger histories of central galaxies by using empirical stellar-to-halo and stellar-to-gas mass relations. The growth of bulges driven by the galaxy mergers/interactions is calculated with dynamical prescriptions. The predicted bulge demographics at redshift z similar to 0 is consistent with observations as shown in Paper I. Here, we present the evolution of the morphological mix (traced by the bulge-to-total mass ratio, B/T) as a function of mass up to z = 13. This mix remains qualitatively the same up to z similar to 1: B/T <= 0.1 galaxies dominate at low masses, 0.1 < B/T <= 0.45 at intermediate masses and B/T > 0.45 at large masses. At z > 1, the fractions of disc-dominated and bulgeless galaxies increase strongly, and by z similar to 2 the era of pure disc galaxies is reached. Bulge-dominated galaxies acquire such a morphology, and most of their mass, following a downsizing trend. Since our results are consistent with most of the recent observational studies of the morphological mix at different redshifts, a Lambda CDM-based scenario of merger-driven bulge assembly does not seem to face critical issues. However, if the stellar-to-halo mass relation evolves too little with redshift, then some tension with observations appear.