[BCS2002] 3810 , the SIMBAD biblio

We present results of an Hα 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 Hα at z = 0.16-0.22, for 522 galaxies brighter than I=22.5, covering a field of 8.7x8.7arcmin² (∼1.1x1.1 h^–1 Mpc at z=0.18). We securely detect Hα 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 + 5log h are detected with Hα flux greater than 4h^–2x10³⁸erg.s^–1. This corresponds to a limiting star formation rate of 0.008h^–2M_☉.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 (≳3M_☉.yr^–1), unless they are obscured by more than 7 mag of extinction. From a Hubble Space Telescope mosaic covering 7.5x10.0 arcmin, we determine morphologies for 199 galaxies brighter than I = 21, and find that ∼20 per cent of the cluster members are of type Sa or later. More than 90 per cent of cluster spirals show Hα emission, compared with less than 10 per cent of E and S0 galaxies. The cluster Hα luminosity function has a low normalization relative to the z ∼ 0.2 field, by ∼50 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 µm. We detect all 15-µm sources at Hα, so there is no evidence for any star formation completely hidden at Hα. Comparing the 15-µm and Hα fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 mag of extinction at Hα, although this depends on the largely unknown contribution from any active galactic nucleus heated dust to the mid-infrared flux.