An H α survey of the rich cluster A 1689

We present results of an Hα survey in the rich cluster A 1689 at z = 0.18, using the LDSS++ spectrograph on the AAT. We obtained spectra covering redshifted Hα at z = 0.16–0.22, for 522 galaxies brighter than I = 22.5, covering a field of 8.7×8.7(∼ 1.1×1.1h Mpc at z = 0.18). We securely detect Hα emission in 46 of these galaxies; accounting for selection effects due to sampling and cluster membership, we determine that 24±4% of cluster members brighter than MR = −16.5+5 logh are detected with Hα flux greater than 4h×10 ergs s. This corresponds to a limiting star formation rate of 0.008 hM⊙ yr , assuming 1 magnitude of dust extinction. Our observations are sufficiently sensitive to detect galaxies with star formation rates comparable to that of the Milky Way ( ∼ > 3M⊙ yr ), unless they are obscured by more than 7 magnitudes of extinction. From a Hubble Space Telescope mosaic covering 7.5× 10.0, we determine morphologies for 199 galaxies brighter than I = 21, and find that ∼20% of the cluster members are of type Sa or later. More than 90% of cluster spirals show Hα emission, compared with less than 10% of E and S0 galaxies. The cluster Hα luminosity function has a low normalisation relative to the z ∼ 0.2 field, by ∼ 50%, after accounting for the different fraction of spiral galaxies in the two environments. When compared with local field galaxies, this suggests that star formation activity is suppressed in early-type cluster galaxies, relative to their field counterparts. Our sample includes 29 galaxies previously observed with ISOCAM at 6.7μm and 15μm. We detect all 15μm sources at Hα, so there is no evidence for any star formation completely hidden at Hα. Comparing the 15μm and Hα fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 magnitudes of extinction at Hα, although this depends on the largely unknown contribution from any AGN-heated dust to the mid-infrared flux.