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

In hierarchical models, where spheroidals are primarily produced via the merger of disk galaxies, the number of intrinsically red systems at faint limits will be substantially lower than that expected in “traditional ” models where the bulk of star formation was completed at high redshifts. In this paper we analyse the optical–nearinfrared colour distribution of a large flux-limited sample of field spheroidal galaxies identified morphologically from archival Hubble Space Telescope data. The I814 −HK ′ colour distribution for a sample jointly limited at I814 23 mag and HK ′ 19.5 mag is used to constrain the star formation history of elliptical and S0 galaxies. We compare visual and automated methods for selecting spheroidals from our deep HST images and, in both cases, detect a significant deficit of intrinsically red spheroids. From limited spectroscopic and deeper photometric redshift data, we demonstrate that this deficit results from a strong evolutionary effect. The space density of spheroidals of all colours at faint limits is lower than expected for a population of constant comoving density. We conclude that field ellipticals cannot have formed all of their stars at high redshift, as has been suggested for the bulk of their clustered counterparts. We compare alternative evolutionary histories with our data, including an analytical representation of recent models based on the hierarchical assembly of ellipticals from the continuous merger of disk galaxies embedded in dark matter halos which provides a better, though not ideal fit. The accumulation of spectroscopic redshifts for our sample will allow a quantitative discrimination between models with extended star formation and provide an important test of the physical basis of the cold dark matter model. Key words: 1