Hα photometry of Abell 2390

We present the results of a search for strong Hα emission line galaxies (rest frame equivalent widths greater than 50 A) in the z≈0.23 cluster Abell 2390. The survey contains 1189 galaxies over 270 arcmin2, and is 50 per cent complete at Mr≈−17.5+5 log h. The fraction of galaxies in which Hα is detected at the 2σ level rises from 0.0 in the central regions (excluding the cD galaxy) to 12.5±8 per cent at R200. For 165 of the galaxies in our catalogue, we compare the Hα equivalent widths with their [O ii] λ3727 equivalent widths, from the Canadian Network for Observational Cosmology (CNOC1) spectra. The fraction of strong Hα 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 (>2σ) Hα 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 [W0(O ii)<5 A and W0(Hδ)≳5 A]; at least two of these show strong signs in Hα 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 A 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.