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 × 10 10 M ⊙ , yielding a number density of 4.3 × 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 ∼ 1.6.

We compare the number density of compact (small size) massive galaxies at low and high redshift using our Padova Millennium Galaxy and Group Catalogue (PM2GC) at z = 0.03 − 0.11 and the CANDELS results from Barro et al. (2013) at z = 1 − 2. The number density of local compact galaxies with luminosity weighted (LW) ages compatible with being already passive at high redshift is compared with the density of compact passive galaxies observed at high-z. Our results place an upper limit of a factor ∼ 2 to the evolution of the number density and are inconsistent with a significant size evolution for most of the compact galaxies observed at high-z. The evolution may be instead significant (up to a factor 5) for the most extreme, ultracompact galaxies. Considering all compact galaxies, regardless of LW age and star formation activity, a minority of local compact galaxies (≤ 1/3) might have formed at z < 1. Finally, we show that the secular decrease of the galaxy stellar mass due to simple stellar evolution may in some cases be a non-negligible factor in the context of the evolution of the mass-size relation, and we caution that passive evolution in mass should be taken into account when comparing samples at different redshifts.

We present the construction and describe the properties of the Padova–Millennium Galaxy and Group Catalogue (PM2GC), a galaxy catalogue representative of the general field population in the local Universe. We characterize galaxy environments by identifying galaxy groups at 0.04 ≤z≤ 0.1 with a Friends‐of‐Friends (FoF) algorithm using a complete sample of 3210 galaxies brighter than MB=−18.7 taken from the Millennium Galaxy Catalogue (MGC, Liske et al.), a 38  photometric and spectroscopic equatorial survey. We identified 176 groups with at least three members, comprising in total 1057 galaxies and representing ∼43 per cent of the general field population in that redshift range. The median redshift and velocity dispersion of our groups are 0.0823 and 192 km s−1, respectively. 88 per cent of the groups have fewer than 10 members, and 63 per cent have fewer than five members. Non‐group galaxies were subdivided into ‘binary’ systems of two bright close companions, and ‘single’ galaxies with no companion, in order to identify different environments useful for future scientific analyses. We performed a detailed comparison with the 2PIGG catalogue to validate the effectiveness of our method and the robustness of our results. Galaxy stellar masses are computed for all PM2GC galaxies, and found to be in good agreement with Sloan Digital Survey Data Release 7 (SDSS‐DR7) mass estimates. The catalogues of PM2GC groups, group properties and galaxy properties in all environments are publicly available on the World Wide Web.

We use ∼ 2000 galaxies belonging to different environments to show how the fractions of different galaxy morphological types vary with global environment and as function of galaxy stellar mass at low redshift. Considering mass limited galaxy samples with log 10 M ⋆ /M ⊙ 10.25, we find a smooth increase/decline in the fraction of Es-S0s/late type galaxies going from single galaxies, to binaries, to groups. Considering all environments, the fractional variation is more conspicuous for S0s and late-types than for ellipticals solely due to a sharp enhancement/dearth of S0s/late-types in clusters compared to other environments. The morphological distribution of galaxies in the mass range 10.25 < log 10 M ⋆ /M ⊙ < 11 is rather independent both of galaxy stellar mass and global environment, except in clusters. The morphologies of galaxies more massive than log 10 M ⋆ /M ⊙ = 11 are instead a function of both galaxy mass and global environment. The morphology-mass relation therefore changes with global environment, showing that galaxy stellar mass cannot be the only parameter driving the morphological distribution of galaxies. The morphology-mass relations for S0 and late-type galaxies in clusters are peculiar compared to other environments, and this strongly suggests that cluster-specific effects act on these two types of galaxies, and that a significant number of S0s in clusters has a different origin with respect to S0s in other environments.

Exploiting the capabilities of four different surveys – the Padova–Millennium Galaxy and Group Catalogue (PM2GC), the WIde‐field Nearby Galaxy‐cluster Survey (WINGS), the IMACS (Inamori‐Magellan Areal Camera and Spectrograph) Cluster Building Survey (ICBS) and the ESO (European Southern Observatory) Distant Cluster Survey (EDisCS) – we analyse the galaxy stellar mass distribution as a function of local density in mass‐limited samples, in the field and in clusters from low () to high () redshift. We find that at all redshifts and in all environments, local density plays a role in shaping the mass distribution. In the field, it regulates the shape of the mass function at any mass above the mass limits. In clusters, it seems to be important only at low masses ( in WINGS and in EDisCS), otherwise it seems not to influence the mass distribution. Putting together our results with those of Calvi et al. and Vulcani et al. for the global environment, we argue that at least at local density is more important than global environment in determining the galaxy stellar mass distribution, suggesting that galaxy properties are not much dependent on halo mass, but do depend on local scale processes.