A1 Refereed original research article in a scientific journal
Colour gradients in cluster ellipticals at z similar to 1.4: the hidden content of the galaxy central regions
Authors: Ciocca F, Saracco P, Gargiulo A, De Propris R
Publisher: OXFORD UNIV PRESS
Publication year: 2017
Journal: Monthly Notices of the Royal Astronomical Society
Journal name in source: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Journal acronym: MON NOT R ASTRON SOC
Volume: 466
Issue: 4
First page : 4492
Last page: 4514
Number of pages: 23
ISSN: 0035-8711
DOI: https://doi.org/10.1093/mnras/stx003
Abstract
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 similar to 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 similar to 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 del(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 (less than or similar to 1 M-circle dot 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 Quasi-Stellar Object (QSO) cannot account for the observed UV excess on its own. We discuss these hypotheses on the basis of current observations and available models.
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 similar to 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 similar to 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 del(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 (less than or similar to 1 M-circle dot 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 Quasi-Stellar Object (QSO) cannot account for the observed UV excess on its own. We discuss these hypotheses on the basis of current observations and available models.
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