The size evolution of elliptical galaxies

Abstract

Recent work has suggested that the amplitude of the size-mass relation of massive early-type galaxies (ETGs) evolves with redshift. Here we use a semi-analytical galaxy formation model to study the size evolution of massive ETGs. We find this model is able to reproduce the amplitude and slope of the relation between size and stellar mass for these galaxies, as well as its evolution. The amplitude of this relation reflects the typical compactness of dark haloes at the time when most of the stars are formed. This link between size and star formation epoch is propagated in galaxy mergers. Mergers of high or moderate mass ratio (less than 1:3) become increasingly important with increasing present day stellar mass for galaxies more massive than 1011.4M. At lower masses, low mass ratio mergers play a more important role. In situ star formation contributes more to the size growth than it does to stellar mass growth.We also find that, for ETGs identified at z = 2, minor mergers dominate subsequent growth both for stellar mass and in size, consistent with earlier theoretical results.