Abstract
We present an analysis of the colour-magnitude relation for a sample of 54 Abell X-ray underluminous (AXU) clusters aiming at unveiling properties that may elucidate the evolutionary stages of the galaxy populations which compose such systems. We compared the parameters of their colour-magnitude relations with the ones found for another sample of 50 Abell X-ray ``normal'' (AXN) emitting clusters. The g and r magnitudes from the SDSS-DR7 were used for constructing the colour-magnitude relations. We found that both samples show the same trend: the red sequence slopes change with redshift, but the slopes for AXU clusters are always flatter than AXN clusters, by a difference of about 42% 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, but for the AXU clusters, this is systematically larger by about 31%. When we subdivide the AXU clusters in two subsamples, one with significant and the other with little or no substructure, we find that the former shows red-sequence slopes that are significantly flatter than those for the latter. This points to AXU clusters being younger systems than normal clusters, possibly accreting groups of galaxies, individual galaxies and gas.
