The evolution of the mass-size relation for early-type galaxies from z ∼ 1 to the present: dependence on environment, mass range and detailed morphology

We study the dependence of the galaxy size evolution on morphology, stellar mass and large-scale environment for a sample of 298 group and 384 field quiescent early-type galaxies from the COSMOS survey, selected from z similar to 1 to the present, and with masses log(M/M-circle dot) > 10.5. From a detailed morphological analysis we infer that similar to 80 per cent of passive galaxies with mass log(M/M-circle dot) > 10.5 have an early-type morphology and that this fraction does not evolve over the last 6 Gyr. However, the relative abundance of lenticular and elliptical galaxies depends on stellar mass. Elliptical galaxies dominate only at the very high mass end -log(M/M-circle dot) > 11 - while S0 galaxies dominate at lower stellar masses -10.5 < log(M/M-circle dot) < 11. The galaxy size growth depends on galaxy mass range and early-type galaxy morphology, e.g. elliptical galaxies evolve differently than lenticular galaxies. At the low-mass end -10.5 < log(M/M-circle dot) < 11 - ellipticals do not show strong size growth from z similar to 1 to the present (10 to 30 per cent depending on the morphological classification). On the other end, massive ellipticals -log(M/M-circle dot) > 11.2 - approximately doubled their size. Interestingly, lenticular galaxies display different behaviour: they appear more compact on average and they do show a size growth of similar to 60 per cent since z = 1 independent of stellar mass range. We compare our results with state-of-the art semi-analytic models. While major and minor mergers can account for most of the galaxy size growth, we find that with present data and the theoretical uncertainties in the modelling we cannot state clear evidence favouring either merger or mass-loss via quasar and/or stellar winds as the primary mechanism driving the evolution. The galaxy mass-size relation and size growth do not depend on environment in the halo mass range explored in this work (field to group mass log(M-h/M-circle dot) < 14), i.e. group and field galaxies follow the same trends. At low redshift, where we examine both Sloan Digital Sky Survey and COSMOS groups, this result is at variance with predictions from some current hierarchical models that show a clear dependence of size growth on halo mass for massive ellipticals (log(M-*/M-circle dot) > 11.2). In future work, we will analyse in detail if this result is specific of the observations and model used in this work. Brightest Cluster Galaxies (BCG) and satellite galaxies lie on the same mass-size relation, at variance with predictions from hierarchical models, which predict that BCGs should have larger sizes than satellites because they experience more mergers in groups over the halo mass range probed.