ALBERT

Description: We present an analysis of the colour–magnitude relation for a sample of 56 Abell X-ray underluminous clusters. Our aim is to unveil properties that might elucidate the evolutionary stages of the galaxy populations that compose such systems. To do so, we have compared the parameters of their colour–magnitude relations with the ones found for another sample of 50 normal Abell X-ray emitting clusters, both samples selected in an objective way. The g and r magnitudes from Data Release 7 of the Sloan Digital Sky Survey were used for constructing the colour–magnitude relations. We have found that both samples show the same trend: the red-sequence slopes change with redshift, but the slopes for X-ray underluminous clusters are always flatter than those for normal clusters, with a difference of about 69 per cent along the surveyed redshift range of 0.05 ≤ z  〈 0.20. Also, the intrinsic scatter of the colour–magnitude relation was found to grow with redshift for both samples; however, for the X-ray underluminous clusters, this is systematically larger by about 28 per cent. By applying the Cramér test to the result of this comparison between X-ray normal and underluminous cluster samples, we obtain probabilities of 92 and 99 per cent that the red-sequence slope and intrinsic scatter distributions, respectively, differ, in the sense that X-ray underluminous clusters red sequences show flatter slopes and higher scatters in their relations. No significant differences in the distributions of red-sequence median colours are found between the two cluster samples. This points to X-ray underluminous clusters being younger systems than normal clusters, possibly in the process of accreting groups of galaxies, individual galaxies and gas.