The Ratio of Luminous to Faint Red Sequence Galaxies in X-Ray and Optically Selected Low-Redshift Clusters

TL;DR

This study compares the ratio of luminous to faint red sequence galaxies in X-ray and optically selected low-redshift clusters, finding a continuous downsizing trend and no significant dependence on cluster X-ray luminosity.

Contribution

It provides the first comprehensive comparison of luminous-to-faint galaxy ratios in X-ray and optically selected clusters at low redshift, revealing a consistent downsizing trend and challenging existing theoretical models.

Findings

Luminous-to-faint ratio shows a trend with redshift, indicating ongoing downsizing.

No significant correlation between luminous-to-faint ratio and cluster X-ray luminosity.

Color-magnitude slopes are consistent across cluster selection methods but differ from some theoretical predictions.

Abstract

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.