Oral Presentation
The brightest UV-selected galaxies in protoclusters at z~4: Ancestors of Brightest Cluster Galaxies?
Presenter: Kei Ito (SOKENDAI/NAOJ)
The Brightest Cluster Galaxy (BCG) is the brightest galaxy in a galaxy cluster. In the nearby universe, BCGs are known to have distinct characteristics compared to other cluster galaxies and field galaxies and essential for understanding the environmental effect. Theoretical researches predict that most of the stellar mass is already formed at z~3 (De Lucia and Blaizot 2007); therefore, it is a key to find the progenitor of BCGs (proto-BCGs) at high-redshift in order to understand the formation of BCGs. However, it has been difficult to systematically study the high-z BCGs due to the small size of previous protocluster sample. Here, we report our survey of proto-BCGs candidates at z~4 by using Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We are ongoing the largest survey of high-redshift protoclusters to date by using imaging data produced by HSC-SSP. We constructed a g-dropout galaxies catalog and found 179 protocluster candidates at z~4 (Toshikawa et al. 2018) over 121 square degrees. This number is roughly twenty times larger than any previous survey, enabling us, for the first time, to also construct a large sample of proto-BCGs. After removing contaminants (e.g., foreground galaxies or stars) which dominate bright-end, we identified 63 proto-BCGs candidates by 1) selecting the brightest member galaxy in each protocluster and 2) requiring the magnitude difference between the fifth and the first in a region is greater than 1.1 mag in i-band. Interestingly, proto-BCGs and members of protoclusters hosting proto-BCGs are found to be redder in (i-z), indicating that the dust extinction is more severe than that in field galaxies and members of protoclusters without proto-BCGs. We also compared the size of the stacked image of proto-BCGs and that of field galaxies, and the effective radius of proto-BCG is ~16% larger than that of field galaxies. In this presentation, we introduce these results and discuss the cause of these results and the BCG evolution.
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