# The brightest UV-selected galaxies in protoclusters at $z\sim4$: Ancestors of Brightest Cluster Galaxies?

## TL;DR

This study identifies and characterizes the brightest UV-selected galaxies in protoclusters at z~4, revealing their distinct properties and suggesting early environmental effects influence galaxy evolution.

## Contribution

First systematic analysis of proto-BCG candidates at z~4, highlighting their unique properties and early environmental influences compared to other protocluster galaxies.

## Key findings

- Proto-BCG candidates are brighter and dustier than other galaxies in protoclusters.
- Proto-BCGs have larger effective radii than field galaxies.
- Environmental effects are evident in galaxy properties at z~4.

## Abstract

We present the results of a survey of the brightest UV-selected galaxies in protoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates are drawn from 179 overdense regions of $g$-dropout galaxies at $z\sim4$ from the Hyper Suprime-Cam Subaru Strategic Program identified previously as good protocluster candidates. This study is the first to extend the systematic study of the progenitors of BCGs from $z\sim2$ to $z\sim4$. We carefully remove possible contaminants from foreground galaxies and, for each structure, we select the brightest galaxy that is at least 1 mag brighter than the fifth brightest galaxy. We select 63 proto-BCG candidates and compare their properties with those of galaxies in the field and those of other galaxies in overdense structures. The proto-BCG candidates and their surrounding galaxies have different rest-UV color $(i - z)$ distributions to field galaxies and other galaxies in protoclusters that do not host proto-BCGs. In addition, galaxies surrounding proto-BCGs are brighter than those in protoclusters without proto-BCGs. The image stacking analysis reveals that the average effective radius of proto-BCGs is $\sim28\%$ larger than that of field galaxies. The $i-z$ color differences suggest that proto-BCGs and their surrounding galaxies are dustier than other galaxies at $z\sim4$. These results suggest that specific environmental effects or assembly biasses have already emerged in some protoclusters as early as $z \sim 4$, and we suggest that proto-BCGs have different star formation histories than other galaxies in the same epoch.