Papers › The buildup of galaxies and their spheroids: the contributions of mergers, disc...
The buildup of galaxies and their spheroids: the contributions of mergers, disc instabilities and star formation
Filip Huško, Cedric G. Lacey, Carlton M. Baugh
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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} (fₑₓ) increases sharply from M_*=10¹¹ M_⊙ upwards, reaching 80% at M_*=10^(11.3) M_⊙. 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 ≈25%, 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 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_(*,sph)=10^(9.5) M_⊙) and the latter for higher-mass ones (peak at M_(*,sph)=10^(10.5) M_⊙).
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