A photometric study of the ages and metallicities of early-type galaxies in A 2218

We present deep optical and near-infrared imaging of the rich cluster A2218 at z = 0.17. Our optical imaging comes from new multicolour Hubble Space T elescope WFPC2 observations in the F450W (B-450), F606W (V-606) and F814W ( I-814) passbands. These observations are complemented by deep near-infrared , K-s-band imaging from the new INGRID imager on the 4.2-m William Herschel Telescope. This combination provides unique high-precision multicolour opt ical-infrared photometry and morphological information for a large sample o f galaxies in the core of this rich cluster, at a look-back time of similar to3 Gyr. We analyse the (B-450 - I-814), (V-606 - I-814) and (I-814 - K-s) colours of galaxies spanning a range of a factor of 100 in K-band luminosi ty in this region, and compare them with grids of stellar population models , We find that the locus of the colours of the stellar populations in the l uminous (greater than or similar to L*(K)) early-type galaxies, both ellipt icals and SOs, traces a sequence of varying metallicity at a single age. At fainter luminosities (less than or similar to 0.1L*(K)), this sequence is extended to lower metallicities by the morphologically classified elliptica ls. However, the faintest SOs exhibit very different behaviour, showing a w ide range of colours, including a large fraction (30 per cent) with relativ ely blue colours that appear to have younger luminosity-weighted ages for t heir stellar populations; 2-5 Gyr. We show that the proportion of these you ng SOs in the cluster population is consistent with the observed decrease i n the SO population seen in distant clusters, when interpreted within the f ramework of a two-step spectroscopic and morphological transformation of ac creted spiral field galaxies into cluster SOs.