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 \times 10 ^ { 10 } M _ { \odot } , yielding a number density of 4.3 \times 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 .
As a consequence , the median mass-size relation shifts towards smaller radii for galaxies with older stars , but the effect is much more pronounced in clusters than in the field .
Our results show that , 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 \sim 1.6 .