# The close pair fraction of BCGs since $z=0.5$: major mergers dominate recent BCG stellar mass growth

## TL;DR

This study uses SDSS data to show that major mergers significantly contribute to the stellar mass growth of brightest cluster galaxies since z=0.5, with an increasing importance towards the present day.

## Contribution

It provides new observational evidence quantifying the role of major mergers in BCG stellar mass growth since z=0.5, combining spectroscopic data with semi-analytical models.

## Key findings

- Major mergers contribute up to 29% of BCG stellar mass since z=0.45.
- The BCG pair fraction increases slightly towards lower redshifts.
- Major mergers can account for the stellar mass growth of intracluster light between z=0.3 and z=0.

## Abstract

Using the redMaPPer cluster catalogue based on the Sloan Digital Sky Survey (SDSS) photometry, we investigate the importance of major mergers in the stellar mass build-up of brightest cluster galaxies (BCGs) between $0.08 \leq z \leq 0.50$. We use the SDSS spectroscopy, supplemented with spectroscopic observations from the Southern African Large Telescope at higher redshifts, to identify which BCGs and nearby companions are potential major merger candidates. We use the pair fraction as a proxy for the merger fraction in order to determine how much stellar mass growth the BCGs have experienced due to major mergers. We observe a weak trend of the BCG pair fraction increasing with decreasing redshift, suggesting that major mergers may become more important towards the present day. Major mergers are found to contribute, on average, $24 \pm 14 $ $(29 \pm 17)$ per cent towards the stellar mass of a present day BCG since $z=0.32$ (0.45), assuming that half of the companion's stellar mass is accreted onto the BCG. Furthermore, using our merger results in conjunction with predictions from two recent semi-analytical models along with observational measurements from the literature, we find that major mergers have sufficient stellar material to account for the stellar mass growth of the intracluster light between $z=0.3$ and $z=0$.