The build-up of pseudo-bulges in a hierarchical universe

We study the cosmological build-up of pseudo-bulges using the L-Galaxies semi-analytical model for galaxy formation with a new approach for following separately the assembly of classical bulges and pseudo-bulges. Classical bulges are assumed to be the result of violent processes (i.e. mergers and starbursts), while the formation of pseudo-bulges is connected to the secular growth of discs. We apply the model to both the Millennium and the Millennium II simulations, in order to study our results across a wide range of stellar masses (107–1011.5 M). We find that z = 0 pseudo-bulges mainly reside in galaxies of Mstellar ∼ 1010–1010.5 M (Mhalo ∼ 1011.5–1012 M) and we recover structural properties of these objects (e.g. sizes and bulge-to-total ratios) that are in good agreement with observational results. Tracing their formation history, we find that pseudo-bulges assembled in galaxies with a very quiet merger history, as opposed to the host galaxies of classical bulges. Regarding the bulge structure, we find that ∼ 30 per cent of the galaxies with a predominant pseudo-bulge feature a composite structure, hosting both a pseudo- and a classical bulge component. The classical component typically constitutes ∼ 10 per cent of the total bulge galaxy mass. When looking at the properties of the host galaxies, we find that z = 0 pseudo-bulges are hosted by main-sequence galaxies, characterized by a stellar population which is generally younger compared to the one of the hosts of classical bulges.