Colour gradients in cluster ellipticals at z~1.4: the hidden content of
  the galaxy central regions
  We present F775W-F850LP (rest-frame UV-U) and F850LP-F160W (rest-frame U-R)
colour gradients for a sample of 17 elliptical galaxies morphologically
selected in the cluster XMMU J2235.3-2557 at $z$=1.39. We detected significant
negative (redder inwards) U-R colour gradients in $\sim$70 per cent of the
galaxies and flat gradients for the remaining ones. On the other hand, the UV-U
gradients are significant positive (bluer inwards) for $\sim$80 per cent of the
galaxies and flat for the remaining ones. Using stellar population synthesis
models, we found that the behaviour of the two colour gradients cannot be
simultaneously explained by a radial variation of age, metallicity and/or dust.
The observed U-R gradients are consistent with a metallicity gradient (mean
value $\nabla_{Z} =-0.4$) in agreement with the one observed in the local
elliptical galaxies. The positive UV-U gradients cannot be explained with age
or metallicity variations and imply an excess of UV emission towards the
galaxies' central regions.This excess calls into question mechanisms able to
efficiently produce UV emission. The data require either steady weak star
formation (< 1 M$_\odot$ yr$^{-1}$) or an He-rich population in the cores of
these galaxies in order to simultaneously reproduce both the colour gradients.
On the contrary, the presence of a QSO cannot account for the observed UV
excess on its own. We discuss these hypotheses on the basis of current
observations and available models.
