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 sigma
than younger ones of the same stellar mass Mstar. We use the age-corrected
luminosity Lcorr as a proxy for Mstar to minimize biases: below Lcorr~1e11
Lsun, galaxies with age ~11 Gyrs have re smaller by 40% and sigma 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--Lcorr and sigma--Lcorr relations at high
Lcorr. 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.
