The optical-infrared colour distribution of a statistically complete sample of faint field spheroidal galaxies

In hierarchical models, where spheroidal galaxies are primarily produced vi a a continuous merging of disc galaxies, the number of intrinsically red sy stems at faint limits will be substantially lower than in 'traditional' mod els where the bulk of star fomation was completed at high redshifts. In thi s paper we analyse the optical-near-infrared colour distribution of a large flux-limited sample of field spheroidal galaxies identified morphologicall y from archival Hubble Space Telescope data. The I-814 - HK ' colour distri bution for a sample jointly limited at I-814 < 23 mag and HK ' < 19.5 mag i s used to constrain their star formation history. We compare visual and aut omated methods for selecting spheroidals from our deep HST images and, in b oth cases, detect a significant deficit of intrinsically red spheroidals re lative to the predictions of high-redshift monolithic-collapse models. Howe ver, the overall space density of spheroidals (irrespective of colour) is n ot substantially different from that seen locally. Spectral synthesis model ling of our results suggests that high-redshift spheroidals are dominated b y evolved stellar populations polluted by some amount of subsidiary star fo rmation. Despite its effect on the optical-infrared colour, this star forma tion probably makes only a modest contribution to the overall stellar mass. We briefly discuss the implications of our results in the context of earli er predictions based on models where spheroidals assemble hierarchically.