H-alpha Photometry of Abell 2390

Abstract

We present the results of a search for strong H-alpha 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 square arcminutes, and is 50% complete at M_r=-17.5+5 log h. The fraction of galaxies in which H-alpha is detected at the 2sigma level rises from 0.0 in the central regions (excluding the cD galaxy) to 12.5+/-8% at R200. For 165 of the galaxies in our catalogue, we compare the H-alpha equivalent widths with their [OII] equivalent widths, from the CNOC1 spectra. The fraction of strong H-alpha emission line galaxies is consistent with the fraction of strong [OII] emission galaxies in the CNOC1 sample: only 2+/-1% have no detectable [OII] emission and yet significant (>2sigma) 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(OII)<5 A and W(Hdelta)>5 A); 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 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.