The cosmological evolution of colour gradients in spheroids

The analysis of the four-colour maps of galaxies in the Hubble Deep Field has revealed in the sample of 0.4<z<1 early-type field galaxies the existence of ellipticals with a predominantly old coeval stellar population. However, there is another, unexpected, category of HDF early-type galaxies, in which the galaxy core is significantly bluer than the outer regions. We demonstrate that these colour gradients are predicted by the multi-zone chemodynamical model for the evolution of elliptical galaxies. We suggest that the colour gradient could be used as a chronometer of the evolution of elliptical galaxies: galaxies younger than a few Gyr exhibit cores bluer than the surrounding galaxy, due to ongoing star formation, while more evolved galaxies have redder cores, due to metallicity gradients increasing towards the centre.