On the Origin of Stars in Bulges and Elliptical Galaxies

We investigate the stellar composition of bulges and elliptical galaxies as predicted by the CDM paradigm using semi-analytical modelling. We argue that spheroid stars are built up of two main components, merger and quiescent, according to the origin of the stars. The merger component is formed during major mergers by gas driven to the centre, while the quiescent component is formed in gaseous discs and added later to the spheroid during major mergers. Galaxies more massive than MC = 3 × 10 10 M⊙ have on average only a 15% merger component in their spheroids, while smaller galaxies can have up to 30%. The merger component increases with redshift due to mergers involving more gas. However we do not find mergers with gas fraction above ∼ 40% of the remnants mass. Generally the gas fraction is a decreasing function of the redshift at which the merger occurs and the mass of the remnant, with more massive remnants having smaller gas fraction and hence smaller merger components. This trend is independent of the environment of the galaxy with the only impact of the environment being that galaxies less massive than MC have slightly larger merger components in dense environments. The fraction of stars in bulges for galaxies more massive than MC is larger than 50%. We find that the majority of stars in galaxies more massive than MC reside within bulges and ellipticals independent of redshift and that the fraction increases with redshift. The most massive galaxies at each redshift are elliptical galaxies.