The age dependence of the size-stellar mass relation and some implications

Abstract

We use a sample of about 48 000 Sloan Digital Sky Survey early-type galaxies to show that older galaxies have smaller half-light radii Re and larger velocity dispersions σ than younger ones of the same stellar mass Mstar. We use the age-corrected luminosity Lrcorr as a proxy for Mstar to minimize biases: below Lrcorr ~ 1011 L⊙, galaxies with age ~11 Gyrs have Re smaller by 40 per cent and σ larger by 25 per cent, compared to galaxies that are 4 Gyr younger. The sizes and velocity dispersions of more luminous galaxies vary by less than 15 per cent, whatever their age, a challenge for current galaxy formation models. A closer check reveals that the lowering in the dispersion is caused by older galaxies that show a significant departure from the Re - Urcorr and σ - Lrcorr relations at high Lrcorr. Such features might find an explanation in models where more massive galaxies undergo more minor mergers than less massive galaxies at late times, thus causing a break in the homology. In terms of the Fundamental Plane of early-type galaxies, the data indicate that all galaxies show a significant and similar increase in the dynamical-to-stellar mass ratio with increasing mass, independent of their age. However, older galaxies have smaller Mdyn/Mstar ratios than objects which formed more recently. These findings may suggest that lower mass galaxies and, at fixed stellar mass, higher redshift galaxies, formed from gas-richer progenitors, thus underwent more dissipation and contraction. © 2009 The Authors. Journal compilation © 2009 RAS.