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 \textit{ex situ} ($f_\mathrm{ex}$) increases sharply from $M_*=10^{11}$ M$_\odot$ upwards, reaching $80\%$ at $M_*=10^{11.3}$ M$_\odot$. For low-mass galaxies we find larger \textit{\textit{ex situ}} contributions at $z=0$ than in other models ($7$-$12\%$), with a decrease towards higher redshifts. The global \textit{ex situ} fraction of all stellar mass falls sharply with redshift, from $40\%$ at $z=0$ to $3\%$ at $z=10$. Major mergers contribute roughly half of the \textit{ex situ} mass, with minor mergers and smooth accretion of satellites both accounting for $\approx25\%$, almost independent of stellar mass and redshift. Mergers dominate in building up high-mass ($M_\mathrm{*,sph}>10^{11}$ M$_\odot$) and low-mass ($M_\mathrm{*,sph}<10^{8.5}$ M$_\odot$) spheroids. Disc instabilities and their associated starbursts dominate for intermediate-mass spheroids ($10^{8.5}<M_\mathrm{*,sph}<10^{11}$ M$_\odot$) at $z=0$. The mass regime where pseudobulges dominate is in agreement with observed pseudobulge fractions, but the peak value in the pseudobulge fraction predicted by GALFORM is likely too high. The total contributions of disc instabilities and their starbursts are roughly equal at $z=0$, with the former dominating for lower-mass spheroids (peak at $M_\mathrm{*,sph}=10^{9.5}$ M$_\odot$) and the latter for higher-mass ones (peak at $M_\mathrm{*,sph}=10^{10.5}$ M$_\odot$).