The statistical nature of the brightest group galaxies

TL;DR

This study investigates the statistical properties of brightest group galaxies (BGGs) in galaxy clusters, revealing that BGGs are systematically brighter than predicted by order statistics due to growth, which correlates with the magnitude gap.

Contribution

It demonstrates that BGGs have grown about 20% in mass beyond expectations, and this growth is linked to the magnitude gap, providing new insights into galaxy evolution within groups.

Findings

BGGs are brighter than order statistic predictions.

BGG growth correlates with the magnitude gap.

BGGs have grown about 20% in mass relative to other members.

Abstract

We examine the statistical properties of the brightest group galaxies (BGGs) using a complete spectroscopic sample of groups/clusters of galaxies selected from the Data Release 7 of the Sloan Digital Sky Survey. We test whether BGGs and other bright members of groups are consistent with an ordered population among the total population of group galaxies. We find that the luminosity distributions of BGGs do not follow the predictions from the order statistics (OS). The average luminosities of BGGs are systematically brighter than OS predictions. On the other hand, by properly taking into account the brightening effect of the BGGs, the luminosity distributions of the second brightest galaxies are in excellent agreement with the expectations of OS. The brightening of BGGs relative to the OS expectation is consistent with a scenario that the BGGs on average have over-grown about 20 percent masses relative to the other member galaxies. The growth ($\Delta M$) is not stochastic but correlated with the magnitude gap ($G_{1,2}$) between the brightest and the second brightest galaxy. The growth ($\Delta M$) is larger for the groups having more prominent BGGs (larger $G_{1,2}$) and averagely contributes about 30 percent of the final $G_{1,2}$ of the groups of galaxies.