The early-type galaxy population in Abell 2218

We present high signal-to-noise, moderate-resolution spectroscopy of 48 early-type members of the rich cluster Abell 2218 at z = 0.18, taken with the LDSS2 spectrograph on the 4.2-m William Herschel Telescope. This sample both is larger and spans a wider galaxy luminosity range, down to MBB* + 1, than previous studies. In addition to the relatively large size of the sample, we have detailed morphological information from archival Hubble Space Telescope imaging for 20 of the galaxies. We combine the morphological, photometric, kinematic and line-strength information to compare A2218 with similar samples drawn from local clusters and to identify evolutionary changes between the samples which have occurred over the last approximate to3 Gyr. The overall picture is one of little or no evolution in nearly all of the galaxy parameters. Zero-point offsets in the Faber-Jackson, MBb-sigma and Fundamental Plane relations are all consistent with passively evolving stellar populations. The slopes of these relations have not changed significantly in the 3Gyr between A2218 and today. We do however find a significant spread in the estimated luminosity-weighted ages of the stellar populations in the galaxies, based on line diagnostic diagrams. This age spread is seen in both the discy early-type galaxies (SO) and also the ellipticals. We observe both ellipticals with a strong contribution from a young stellar population and lenticulars dominated by old stellar populations. On average, we find no evidence for systematic differences between the populations of ellipticals and lenticulars. In both cases there appears to be little evidence for differences between the stellar populations of the two samples. This points to a common formation epoch for the bulk of the stars in most of the early-type galaxies in A2218. This result can be reconciled with the claims of rapid morphological evolution in distant clusters if the suggested transformation from spirals to lenticulars does not involve significant new star formation.