The origin of pseudobulges in cosmological simulations of galaxy formation

Abstract

More than half of nearby disc galaxies have pseudobulges instead of classical bulges that are though to be end-products of galaxy mergers. Pseudobulges are presumed to develop overtime as a result of secular evolution of galaxy discs. We report simulations of galaxy formation, in which two disc galaxies with disky pseudobulges 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 pseudobulges form as high-redshift discs with small scale lengths by rapid supply of low angular momentum gas. By redshift 2, before the main disc formation, pseudobulge formation has largely completed in terms of mass. The secular evolution such as bar instability accounts for about 30 % of the bulge mass for one galaxy and only 13 % for the other but does affect the final shape and kinematic properties of the pseudobulges.