Published in

Oxford University Press (OUP), Monthly Notices of the Royal Astronomical Society, 1(488), p. 609-632, 2019

DOI: 10.1093/mnras/stz1694

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

This paper was not found in any repository, but could be made available legally by the author.
This paper was not found in any repository, but could be made available legally by the author.

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Abstract

ABSTRACT 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 ($\rm 10^{7}\!-\!10^{11.5}\, {\rm M}_{⊙ }$). We find that z = 0 pseudo-bulges mainly reside in galaxies of $\mathit{ M}_{\rm stellar} \, {∼ }\, 10^{10}\!-\!10^{10.5}\, {\rm M}_{⊙ }$ ($\mathit{ M}_{\rm halo} \, {∼ }\, 10^{11.5}\!-\!10^{12}\, {\rm M}_{⊙ }$) 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 $\, {∼ }\, 30{{\ \rm per\ cent}}$ 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 ${∼ }\, 10{{\ \rm per\ cent}}$ 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.

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