The buildup of galaxies and their spheroids: The contributions of mergers, disc instabilities, and star formation

We use the GALFORM semi-analytical model of galaxy formation and the Planck-Millennium simulation to investigate the origins of stellar mass in galaxies and their spheroids. We compare the importance of mergers and disc instabilities, as well as the starbursts that they trigger. We find that the fraction of galaxy stellar mass formed ex situ (i.e. through mergers; f_ex) increases sharply from M_* = 10¹¹ M_⊙ upwards, reaching 80 per cent at M_* = 10^11.3 M_⊙. The massive end of the f_ex-M_* relation does not evolve with redshift, in disagreement with other models. For low-mass galaxies we find larger ex situ contributions at z = 0 than in other models (7-12 per cent), with a decrease towards higher redshifts. Major mergers contribute roughly half of the ex situ mass, with minor mergers and smooth accretion of satellites both accounting for ≈25 per cent, almost independent of stellar mass and redshift. Mergers dominate in building up high-mass (M_{*, sph} > 10¹¹ M_⊙) and low-mass (M_{*, sph} < 10^8.5 M_⊙) spheroids. Disc instabilities and their associated starbursts dominate for intermediate-mass spheroids (10^8.5 < M_{*, sph} < 10¹¹ M_⊙) at z = 0. The mass regime where pseudo-bulges dominate is in agreement with observed pseudo-bulge fractions, but the peak value in the pseudo-bulge fraction predicted by GALFORM is likely too high. Starbursts induced by disc instabilities are the dominant channel for spheroid growth at all redshifts, while merger-induced starbursts are relatively negligible, except at very high redshifts (z > 5).