The Kormendy relation of massive elliptical galaxies at z ∼ 1.5:: evidence for size evolution

We present the morphological analysis based on HST-NIC2 (0.075 arcsec pixel(-1)) images in the F160W filter of a sample of nine massive field (> 10(11) M-circle dot) galaxies spectroscopically classified as early-types at 1.2 < z < 1.7. Our analysis shows that all of them are bulge-dominated systems. In particular, six of them are well fitted by a de Vaucouleurs profile (n = 4) suggesting that they can be considered pure elliptical galaxies. The remaining three galaxies are better fitted by a Sersic profile with index 1.9 < n(fit) < 2.3 suggesting that a disc-like component could contribute up to 30 per cent to the total light of these galaxies. We derived the effective radius R-e and the mean surface brightness (SB) (e) within R-e of our galaxies and we compared them with those of early-types at lower redshifts. We find that the SB (e) of our galaxies should get fainter by 2.5 mag from z similar to 1.5 to similar to 0 to match the SB of the local ellipticals with comparable R-e, that is, the local Kormendy relation. Luminosity evolution without morphological changes can only explain half of this effect, as the maximum dimming expected for an elliptical galaxy is similar to 1.6 mag in this redshift range. Thus, other parameters, possibly structural, may undergo evolution and play an important role in reconciling models and observations. Hypothesizing an evolution of the effective radius of galaxies we find that R-e should increase by a factor of 1.5 from z similar to 1.5 to similar to 0.