Times, environments and channels of bulge formation in a Lambda cold dark matter cosmology

We analyse predictions from two independently developed galaxy formation models to study the mechanisms, environments and characteristic times of bulge formation in a Lambda cold dark matter (ACDM) cosmogony. For each model, we test different prescriptions for bulge formation in order to quantify the relative importance of different channels. Our results show that the strong correlation between galaxy and halo mass for central galaxies, and the richer merger history of more massive systems, naturally give rise to a strong correlation between galaxy mass and morphology, and between halo mass and morphological type of central galaxies. Large fractions of the bulge mass are acquired through major and minor mergers, but disc instability plays an important role, particularly for intermediate-mass galaxies. We find that the modelling of disc instability events, as well as of the galaxy merger times, can significantly affect the timing of bulge formation, and the relative importance of different channels. Bulge-dominated galaxies acquire their morphology through major mergers, but this can be modified by cooling of gas from the surrounding hot halo. We find that disc regrowth is a non-negligible component of the evolution of bulge-dominated galaxies, particularly for low to intermediate masses, and at high redshifts.