On the Orgin of Stars in Bulges and Elliptical Galaxies -I. Implication for surface mass density & size distribution

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%. Additionally, the fraction of stars in bulges for galaxies more massive than MC is larger than 50%. We argue here that the surface mass density of galaxies is connected to the fraction of the merger component in spheroids, in such a way that a higher fraction results in a higher surface mass density. Using this assumption, we are able to reproduce the scatter in the observed size distribution of elliptical galaxies and the environmental dependence of the surface mass density. The scatter in the size distribution comes as a consequence of the scatter in the time since the last major merger experienced by an elliptical galaxy. The environmental dependence is a combination of earlier mergers in high density environments and continued occurrence of major mergers for massive galaxies. Furthermore we propose, based on the size distribution, a new method to determine the epoch of the last major merger of elliptical galaxies.