Mass growth and mergers: direct observations of the luminosity function of LRG satellite galaxies out to z=0.7 from SDSS and BOSS images

TL;DR

This study analyzes the luminosity functions of galaxies around luminous red galaxies (LRGs) at redshifts 0.34 and 0.65, revealing a luminosity gap that suggests minor mergers dominate their growth and that LRGs are a small fraction of central galaxies in their halos.

Contribution

It introduces a two-component fit model for luminosity functions around LRGs, highlighting the luminosity gap and its implications for galaxy growth mechanisms.

Findings

LRGs are brighter than their satellites by about 1.3 magnitudes.

Approximately 35% of environmental mass is in the LRG itself.

The luminosity gap can be explained by sampling a Schechter function, indicating LRGs are a small subset of central galaxies.

Abstract

We present a statistical study of the luminosity functions of galaxies surrounding luminous red galaxies (LRGs) at average redshifts <z>=0.34 and <z>=0.65. The luminosity functions are derived by extracting source photometry around more than 40,000 LRGs and subtracting foreground and background contamination using randomly selected control fields. We show that at both studied redshifts the average luminosity functions of the LRGs and their satellite galaxies are poorly fitted by a Schechter function due to a luminosity gap between the centrals and their most luminous satellites. We utilize a two-component fit of a Schechter function plus a log-normal distribution to demonstrate that LRGs are typically brighter than their most luminous satellite by roughly 1.3 magnitudes. This luminosity gap implies that interactions within LRG environments are typically restricted to minor mergers with mass ratios of 1:4 or lower. The luminosity functions further imply that roughly 35% of the mass in the environment is locked in the LRG itself, supporting the idea that mass growth through major mergers within the environment is unlikely. Lastly, we show that the luminosity gap may be at least partially explained by the selection of LRGs as the gap can be reproduced by sparsely sampling a Schechter function. In that case LRGs may represent only a small fraction of central galaxies in similar mass halos.