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We present F775W-F850LP (rest-frame UV-U) and F850LP-F160W (rest-frame U-R)colour gradients for a sample of 17 elliptical galaxies morphologicallyselected in the cluster XMMU J2235.3-2557 at $z$=1.39. We detected significantnegative (redder inwards) U-R colour gradients in $\sim$70 per cent of thegalaxies and flat gradients for the remaining ones. On the other hand, the UV-Ugradients are significant positive (bluer inwards) for $\sim$80 per cent of thegalaxies and flat for the remaining ones. Using stellar population synthesismodels, we found that the behaviour of the two colour gradients cannot besimultaneously explained by a radial variation of age, metallicity and/or dust.The observed U-R gradients are consistent with a metallicity gradient (meanvalue $\nabla_{Z} =-0.4$) in agreement with the one observed in the localelliptical galaxies. The positive UV-U gradients cannot be explained with ageor metallicity variations and imply an excess of UV emission towards thegalaxies' central regions.This excess calls into question mechanisms able toefficiently produce UV emission. The data require either steady weak starformation (< 1 M$_\odot$ yr$^{-1}$) or an He-rich population in the cores ofthese galaxies in order to simultaneously reproduce both the colour gradients.On the contrary, the presence of a QSO cannot account for the observed UVexcess on its own. We discuss these hypotheses on the basis of currentobservations and available models.

Colour gradients in cluster ellipticals at z~1.4: the hidden content of the galaxy central regions