The ratio of luminous to faint red-sequence galaxies in X-ray and optically selected low-redshift clusters

We study the ratio of luminous to faint red-sequence galaxies in both optically and X-ray selected galaxy clusters in the poorly studied redshift range 0.05 < z < 0.19. The X-ray-selected sample consists of 112 clusters based on the ROSAT All-Sky Survey, while the optical sample consists of 266 clusters from the Sloan Digital Sky Survey. Our results are consistent with the presence of a trend in luminous-to-faint ratio with redshift, confirming that downsizing is continuous from high to low redshift. After correcting for the variations with redshift using a partial Spearman analysis, we find no significant relationship between luminous-to-faint ratio and X-ray luminosity of the host cluster sample, in contrast to recent suggestions. Finally, we investigate the stacked colour-magnitude relations of these samples, finding no significant differences between the slopes for optically and X-ray selected clusters. The colour-magnitude slopes are consistent with the values obtained in similar studies, but not with predictions of theoretical models.