A Low Global Star Formation Rate in the Rich Galaxy

We present the results of a wide-field survey for H -emitting galaxies in the cluster AC 114 at z = 0:32. Spectra centred on H at the cluster redshift have been obtained for 586 galaxies to Itot 22 out to a radius of 2 h−1 50 Mpc. At most, only 10% of these were found to be H –emitting cluster members. These objects are predominantly blue and of late-type spiral morphology, consistent with them hosting star formation. However, 65% of the cluster members classified morphologically as spirals (with HST), have no detectable H emission; star-formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the H detection rate and the strength of H emission with environment (as traced by local galaxy density) are found to be weak within the region studied. Star formation within the cluster members is also found to be strongly and uniformly suppressed with the rates inferred from the H emission not exceeding 4 M yr−1, and AC 114’s H luminosity function being an order of magnitude below that observed for field galaxies at the same redshift. None of the galaxies detected have the high star formation rates associated with ‘starburst’ galaxies; however, this may still be reconciliable with the known (8 3%) fraction of ‘post-starburst’ galaxies within AC 114, given the poorly determined but short lifetimes of starbursts and the possibility that much of the associated star formation is obscured by dust. Subject headings: galaxies: clusters — galaxies: evolution — galaxies: formation