A New Test of the Statistical Nature of the Brightest Cluster Galaxies

TL;DR

This paper introduces a new statistical method to test whether brightest cluster galaxies (BCGs) are simply the most luminous members drawn from the same galaxy luminosity distribution, revealing they are often statistically distinct in high-luminosity clusters.

Contribution

The paper proposes a novel statistical test for the nature of BCGs and demonstrates its effectiveness using a large galaxy cluster sample, challenging previous assumptions about their origins.

Findings

BCGs in high-luminosity clusters are unlikely to be drawn from the same luminosity distribution as other cluster galaxies.

BCGs in less luminous clusters are consistent with being the statistical extreme of the galaxy population.

Second brightest galaxies are consistent with being the statistical extreme, indicating BCGs are often distinct from other luminous cluster galaxies.

Abstract

A novel statistic is proposed to examine the hypothesis that all cluster galaxies are drawn from the same luminosity distribution (LD). In such a "statistical model" of galaxy LD, the brightest cluster galaxies (BCGs) are simply the statistical extreme of the galaxy population. Using a large sample of nearby clusters, we show that BCGs in high luminosity clusters (e.g., L_tot > 4x10^11 L_sun) are unlikely (probability <3x10^-4) to be drawn from the LD defined by all red cluster galaxies more luminous than M_r=-20. On the other hand, BCGs in less luminous clusters are consistent with being the statistical extreme. Applying our method to the second brightest galaxies, we show that they are consistent with being the statistical extreme, which implies that the BCGs are also distinct from non-BCG luminous, red, cluster galaxies. We point out some issues with the interpretation of the classical tests proposed by Tremaine & Richstone (1977) that are designed to examine the statistical nature of BCGs, investigate the robustness of both our statistical test and those of TR against difficulties in photometry of galaxies of large angular size, and discuss the implication of our findings on surveys that use the luminous red galaxies to measure the baryon acoustic oscillation features in the galaxy power spectrum.