Colour gradients in normal and compact early-type galaxies at 1<z<2
  We have derived colour gradients for a sample of 20 early-type galaxies
(ETGs) at 1 < z_spec < 2 selected from the GOODS-South field. The sample
includes both normal ETGs (13) having effective radii comparable to the mean
radius of local ones and compact ETGs (7) having effective radii from two to
six times smaller. Colour gradients have been derived in the F606W-F850LP bands
(UV-U rest-frame) taking advantage of the ultradeep HST-ACS observations
covering this field and providing a spatial resolution of about 0.8 kpc at the
redshift of the galaxies. Despite of the narrow wavelength baseline covered
(1000 Angstrom), sampling approximatively the emission dominated by the same
stellar population, we detect significant radial colour variations in 50 per
cent of our sample. In particular, we find five ETGs with positive colour
gradients (cores bluer than the external regions), and five galaxies with
negative colour gradients (cores redder than the external regions), as commonly
observed in the local Universe. These results show that at 1 < z < 2, when the
Universe was only 3-4 Gyr old, ETGs constituted a composite population of
galaxies whose different assembly histories have generated different stellar
distributions with the bluest stellar population either in the center or in the
outskirts as well as throughout the whole galaxy. Moreover, we find that
compact galaxies seem to preferentially show a blue cores while moving towards
normal galaxies, central stellar populations become progressively redder.
Nonetheless, the narrow baseline covered together with the low statistics still
prevent us to be conclusive about a possible physical connection between colour
gradients and the degree of compactness of high-z ETGs.
