Superdense galaxies and the mass-size relation at low redshift
  We search for massive and compact galaxies (superdense galaxies, hereafter
SDGs) at z=0.03-0.11 in the Padova-Millennium Galaxy and Group Catalogue, a
spectroscopically complete sample representative of the local Universe general
field population. We find that compact galaxies with radii and mass densities
comparable to high-z massive and passive galaxies represent 4.4% of all
galaxies with stellar masses above 3 X 10^10 M_sun, yielding a number density
of 4.3 X 10^-4 h^3 Mpc^-3. Most of them are S0s (70%) or ellipticals (23%), are
red and have intermediate-to-old stellar populations, with a median
luminosity-weighted age of 5.4 Gyr and a median mass-weighted age of 9.2 Gyr.
Their velocity dispersions and dynamical masses are consistent with the small
radii and high stellar mass estimates. Comparing with the WINGS sample of
cluster galaxies at similar redshifts, the fraction of superdense galaxies is
three times smaller in the field than in clusters, and cluster SDGs are on
average 4 Gyr older than field SDGs. We confirm the existence of a universal
trend of smaller radii for older luminosity-weighted ages at fixed galaxy mass.
On top of the well known dependence of stellar age on galaxy mass, the
luminosity-weighted age of galaxies depends on galaxy compactness at fixed
mass, and, for a fixed mass and radius, on environment. This effect needs to be
taken into account in order not to overestimate the evolution of galaxy sizes
from high- to low-z. Our results and hierarchical simulations suggest that a
significant fraction of the massive compact galaxies at high-z have evolved
into compact galaxies in galaxy clusters today. When stellar age and
environmental effects are taken into account, the average amount of size
evolution of individual galaxies between high- and low-z is mild, a factor
~1.6. (abridged)
