Hα photometry of Abell 2390

We present the results of a search for strong H alpha emission line galaxies (rest frame equivalent widths greater than 50 Angstrom) in the z approximate to0.23 cluster Abell 2390. The survey contains 1189 galaxies over 270 arcmin(2), and is 50 per cent complete at M(r)approximate to -17.5+5 log h. The fraction of galaxies in which H alpha is detected at the 2 sigma level rises from 0.0 in the central regions (excluding the cD galaxy) to 12.5 +/-8 per cent at R-200. For 165 of the galaxies in our catalogue, we compare the H alpha equivalent widths with their [O II] lambda 3727 equivalent widths, from the Canadian Network for Observational Cosmology (CNOC1) spectra. The fraction of strong H alpha emission line galaxies is consistent with the fraction of strong [O II] emission galaxies in the CNOC1 sample: only 2 +/-1 per cent have no detectable [O II] emission and yet significant (>2 sigma) H alpha equivalent widths. Dust obscuration, non-thermal ionization, and aperture effects are all likely to contribute to this non-correspondence of emission lines. We identify six spectroscopically 'secure' k+a galaxies [W-0(O II)<5 Angstrom and W-0(H delta)greater than or similar to5 Angstrom]; at least two of these show strong signs in H alpha of star formation in regions that are covered by the slit from which the spectra were obtained. Thus, some fraction of galaxies classified as k+a based on spectra shortward of 6000 Angstrom are likely to be undergoing significant star formation. These results are consistent with a 'strangulation' model for cluster galaxy evolution, in which star formation in cluster galaxies is gradually decreased, and is neither enhanced nor abruptly terminated by the cluster environment.