2009MNRAS.395..554F

The formation of massive spheroidal galaxies is studied on a visually classified sample extracted from the Advanced Camera for Surveys/Hubble Space Telescope (ACS/HST) images of the Great Observatories Origins Deep Survey north and south fields, covering a total area of 360 arcmin2. The sample size (910 galaxies brighter than i_F775W_= 24) allows us to explore in detail the evolution over a wide range of redshifts (0.4 < z < 1.5; median redshift 0.68). Three key observables are considered: comoving number density, internal colour distribution and the Kormendy relation. The comoving number density of the most massive galaxies is found not to change significantly with redshift. Extrapolation of our sample to z = 0 gives an increase in the comoving number density of M_s > 10¹¹M_☉ galaxies by a factor of 2 between z = 1 and 0, in contrast with a factor of ∼50 for lower mass galaxies (10¹⁰ < M_s/M_☉< 10¹¹). One-quarter of the whole sample of early types are photometrically classified as blue galaxies. On a volume-limited sample out to z < 0.7, the average stellar mass of the blue ellipticals is 5x10⁹M_☉ compared to 4 x 10¹⁰M_☉ for red ellipticals. On a volume-limited subsample out to z = 1.4 probing the brightest galaxies (M_V < -21), we find the median redshift of blue and red early types: 1.10 and 0.85, respectively. Blue early types only amount to 4 per cent of this sample (compared to 26 per cent in the full sample). The intrinsic colour distribution correlates overall bluer colours with blue cores (positive radial gradients of colour), suggesting an inside-out process of formation. The redshift evolution of the observed colour gradients is incompatible with a significant variation in stellar age within each galaxy. The slope of the Kormendy relation in the subsample of massive galaxies does not change over 0.4 < z < 1.4 and is compatible with z = 0 values. The `zero-point' of the Kormendy relation (i.e. the surface brightness at a fixed half-light radius) is 1 mag fainter (in the B band) for the subsample of low-mass (M_s < 3.5x10¹⁰M_☉) early types.