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

We use a sample of about 48,000 SDSS 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 L corr r as a proxy for Mstar to minimize biases: below L corr r ∼ 10L⊙, galaxies with age ∼ 11 Gyrs have Re smaller by 40% and σ larger by 25%, compared to galaxies that are 4 Gyrs younger. The sizes and velocity dispersions of more luminous galaxies vary by less than 15%, 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–L corr r and σ–L r relations at high L r . 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 dynamicalto-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.