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

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 R(e) and larger velocity dispersions sigma than younger ones of the same stellar mass M(star). We use the age-corrected luminosity L(r)(corr) as a proxy for Mstar to minimize biases: below L(r)(corr) similar to 10(11) L(circle dot), galaxies with age similar to 11 Gyrs have R(e) smaller by 40 per cent and sigma 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 R(e) - L(r)(corr) and sigma - L(r)(corr) relations at high L(r)(corr). 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 M(dyn)/M(star) 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.