The origin of pseudo-bulges in cosmological simulations of galaxy formation

Abstract

More than half of nearby disc galaxies have pseudo-bulges, instead of classical bulges that are thought to be end-products of galaxy mergers. Pseudo-bulges are presumed to develop over time as a result of the secular evolution of galaxy discs. We report on simulations of galaxy formation, in which two disc galaxies with discy pseudo-bulges have formed. Based on the profile decomposition, the bulge-to-total mass ratio of the simulated galaxies is 0.6 for one galaxy and 0.3 for the other. We find that the main formation mechanism of the pseudobulges in our simulations is not the secular evolution of discs but high-redshift starbursts. The progenitors of the pseudo-bulges form as high-redshift discs with small scalelengths by the rapid supply of low angular momentum gas. By redshift 2, before the main disc formation, pseudo-bulge formation has largely completed in terms of mass. The secular evolution, such as bar instability, accounts for about 30 per cent of the bulge mass for one galaxy and only ~13 per cent for the other. However, it does affect the final shape and kinematic properties of the pseudo-bulges. © 2012 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.