The hierarchical build-up of bulges in CDM

Abstract

We investigate the hierarchical build-up of stars in bulges within the standard $Λ$-cold dark matter scenario. By separating the population into stars born during starbursts that accompany the formation of spheroids in major mergers ({\it starburst} component), and stars that are previously formed in discs of progenitor galaxies ({\it quiescent} component) and added to the spheroid by dynamical interaction. Our results are summarised as follows: bulges that form early have larger starburst fraction and hence should be smaller than their counter parts that form later. The quiescent fraction in bulges is an increasing function of bulge mass, becoming constant at $M_{\rm{q}}/M_{\rm{bul}} \sim 0.8$, mainly due to the infall of satellite galaxies that contribute disc stars to the bulge. Minor mergers are an order of magnitude more frequent than major mergers and must play a significant role in the evolution of bulges. Above the critical mass $M_{\rm{c}}\sim 3 \times 10^{10}$ M$_{\odot}$ most of the stars in the universe are in spheroids, which at high redshift are exclusively elliptical galaxies and at low redshifts partly bulges. Due to the enhanced evolution of galaxies ending up in high density environments, the starburst fraction and the surface mass densities of bulges below $M_{\rm{c}}$ should be enhanced with respect to field galaxies. Dissipation during the formation of massive bulges in present day early-type spirals is less important than for the formation of present day elliptical galaxies of the same mass thereby explaining the possible difference in phase-space densities between spiral galaxies and elliptical galaxies.