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

The main aim of my Ph.D. thesis is to study how the colour of a sample of cluster ellipticals radially changes and to study its origin in term of variation of the stellar population properties in order to put constraints on the mechanisms of the stellar mass assembly in elliptical galaxies. To this aim, I jointly studied the rest-frame UV-U and UR colour gradients for a sample of 17 elliptical galaxies morphologically selected in the cluster XMMU J2235.3-2557 at z=1.39. The analysis shows that the U-R colour gradients are systematically negative (∼70%) or null (~30%), never positive. On the other hand, the UV-U gradients are systematically positive (∼80%) or null (~20%), never negative. Using stellar population synthesis models, we found that the behavior of the two colour gradients cannot be simultaneously explained by a radial variation of age, metallicity and/or dust. In particular, the negative observed U-R gradients can be produced by an age/metallicity gradient. On the contrary, 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 without altering the spectrum at longer wavelengths. The data require either steady weak star formation (< 1 M⊙ yr−1) or an He-rich population in the cores of these galaxies in order to simultaneously reproduce both the colour gradients. We have then investigated whether colour gradients depend on the environment, comparing our results with colour gradients in field ellipticals at a comparable redshift present in literature. We found that both cluster and field ellipticals present negative U-R gradients mutually consistent. Conversely to cluster ellipticals, field ellipticals show both positive and negative UV-U gradients. Although the small statistic, this could suggest that the environment could have influenced the evolution of elliptical galaxies. Finally, through the evolution of colour gradients, we investigated how these cluster ellipticals have evolved from z=1.39 to z=0. We found that the evolution of the studied galaxies is consistent with a passive evolution.