Environmental effects on the bright end of the galaxy luminosity function in galaxy clusters

TL;DR

This study investigates how the luminosity function of galaxies, especially the brightest ones, varies with the dynamical state of galaxy clusters, revealing that brightest galaxies grow through hierarchical merging regardless of cluster relaxation.

Contribution

It compares the bright galaxy populations in unrelaxed and relaxed clusters, showing similar luminosity functions but differences in the number of brightest galaxies, highlighting their growth via merging.

Findings

Relaxed clusters have fewer bright galaxies than unrelaxed ones.

The total luminosity of bright galaxies is similar across cluster types.

Bright galaxies grow in luminosity and decrease in number as clusters evolve hierarchically.

Abstract

The dependence of the luminosity function of cluster galaxies on the evolutionary state of the parent cluster is still an open issue, in particular as concern the formation/evolution of the brightest cluster galaxies. We plan to study the bright part of the LFs of a sample of very unrelaxed clusters ("DARC" clusters showing evidence of major, recent mergers) and compare them to a reference sample of relaxed clusters spanning a comparable mass and redshift range. Our analysis is based on the SDSS DR7 photometric data of ten, massive, and X-ray luminous clusters (0.2<z<0.3), always considering physical radii (R_200 or its fractions). We consider r' band LFs and use the color-magnitude diagrams (r'-i',r') to clean our samples as well to consider separately red and blue galaxies. We find that DARC and relaxed clusters give similar LF parameters and blue fractions. The two samples differ for their content of bright galaxies BGs, M_r<-22.5, since relaxed clusters have fewer BGs, in particular when considering the outer cluster region 0.5R_200<R<R_200 (by a factor two). However, the cumulative light in BGs is similar for relaxed and DARC samples. We conclude that BGs grow in luminosity and decrease in number as the parent clusters grow hierarchically in agreement with the BG formation by merging with other luminous galaxies.