The cosmological evolution of colour gradients in spheroids

The analysis of the four-colour maps of galaxies in the Hubble Deep Field ( HDF) has revealed, in the sample of 0.4 < z < 1 early-type field galaxies, the existence of ellipticals with a predominantly old coeval stellar popula tion. However, there is another, unexpected, category of HDF early-type gal axies, in which the galaxy core is significantly bluer than the outer regio ns. We demonstrate that these colour gradients are predicted by the multizo ne chemodynamical model for the evolution of elliptical galaxies. We sugges t that the colour gradient could be used as a chronometer for the evolution of elliptical galaxies: galaxies younger than a few Gyr exhibit cores blue r than the surrounding galaxy as a result of ongoing star formation, while more evolved galaxies have redder cores because of metallicity gradients in creasing toward the centre.