Galaxy colours in high-redshift, X-ray-selected clusters – I. Blue galaxy fractions in eight clusters

ABSTRA C T We present initial results from a wide-field, multicolour imaging project, designed to study galaxy evolution in X-ray-selected clusters at intermediate Oz , 0:25U and high redshifts Oz , 0:5U. We give blue galaxy fractions from eight X-ray-selected clusters, drawn from a combined sample of three X-ray surveys. We find that all the clusters exhibit excess blue galaxy populations over the numbers observed in local systems, although a large scatter is present in the results. We find no significant correlation of blue fraction with redshift at z . 0:2, although the large scatter could mask a positive trend. We also find no systematic trend of blue fraction with X-ray luminosity. We show that the blue fraction is a function of (a) radius within a cluster, (b) absolute magnitude and (c) the passbands used to measure the colour. We find that our blue fractions (fb) from galaxy colours close to rest frameOU 2 BU0, f b , 0:4, are systematically higher than those from rest frameOB 2 VU0 colours, f b , 0:2. We conclude this effect is real, may offer a partial explanation of the widely differing levels of blue fraction found in previous studies, and may have implications for biases in optical samples selected in different bands. While the increasing blue fraction with radius can be interpreted as evidence of cluster infall of field galaxies, the exact physical processes which these galaxies undergo is unclear. We estimate that, in the cores of the more massive clusters, galaxies should be experiencing ram-pressure stripping of galactic gas by the intracluster medium. The fact that our low X-ray luminosity systems show a similar blue fraction as the high-luminosity systems, as well as a significant blue fraction gradient with radius, implies other physical effects are also important.