An Hα survey of the rich cluster A 1689

We present results of an Halpha survey in the rich cluster A 1689 at z = 0.18, using the LDSS++ spectrograph on the Anglo-Australian Telescope (AAT). We obtained spectra covering redshifted Halpha at z = 0.16-0.22, for 522 galaxies brighter than I = 22.5, covering a field of 8.7 x 8.7 arcmin(2) (similar to1.1 x 1.1h(-1) Mpc at z = 0.18). We securely detect Halpha emission in 46 of these galaxies; accounting for selection effects arising from sampling and cluster membership, we determine that 24 +/- 4 per cent of cluster members brighter than M-R = 16.5 +/- 5 log h are detected with Halpha flux greater than 4h(-2) x 10(38) erg s(-1). This corresponds to a limiting star formation rate of 0.008h(-2) M. yr(-1), assuming 1 mag of dust extinction. Our observations are sufficiently sensitive to detect galaxies with star formation rates comparable to that of the Milky Way (greater than or similar to3 M. yr(-1)), unless they are obscured by more than 7 mag of extinction. From a Hubble Space Telescope mosaic covering 7.5 x 10.0 arcmin, we determine morphologies for 199 galaxies brighter than I = 21, and find that similar to20 per cent of the cluster members are of type Sa or later. More than 90 per cent of cluster spirals show Halpha emission, compared with less than 10 per cent of E and S0 galaxies. The cluster Halpha luminosity function has a low normalization relative to the zsimilar to 0.2 field, by similar to50 per cent, 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 6.7 and 15 mum. We detect all 15-mum sources at Halpha, so there is no evidence for any star formation completely hidden at Halpha. Comparing the 15-mum and Halpha fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 mag of extinction at Halpha, although this depends on the largely unknown contribution from any active galactic nucleus heated dust to the mid-infrared flux.