Evidence for evolving spheroidals in the Hubble Deep Fields North and South

Abstract

We investigate the dispersion in the internal colours of faint spheroidals in the Hubble Deep Fields North and South. In high-redshift rapid-collapse scenarios, the dispersion in internal colours should be small at moderate redshift apart from a small metallicity induced reddening in the enriched cores. However, recently assembled spheroidals are likely to show non-homologous internal colours, at least until younger stellar populations become fully mixed. Here we find that a remarkably large fraction (≳30 per cent) of the morphologically classified spheroidals with I₈₁₄ᵥᵥ<24 mag show strong variations in internal colour, which we take as evidence for recent episodes of star formation. In most cases these colour variations manifest themselves via the presence of blue cores, an effect of opposite sign to that expected from metallicity gradients. Examining similarly selected ellipticals in five rich clusters with 0.37

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