A Low Global Star Formation Rate in the Rich Galaxy Cluster AC 114 atz = 0.32

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Abstract

We present the results of a wide-field survey for Hα-emitting galaxies in the cluster AC 114 at z = 0.32. Spectra centered on Hα at the cluster redshift have been obtained for 586 galaxies to Itot ∼ 22 out to a radius of ∼2 h-150 Mpc. At most, only ∼10% of these were found to be Hα-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 Hα emission; star formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the Hα detection rate and the strength of Hα 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 Hα emission not exceeding 4 M⊙ yr-1, and AC 114's Hα 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 reconcilable 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.

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