# Bimodal morphologies of massive galaxies at the core of a protocluster at z=3.09 and the strong size growth of a brightest cluster galaxy

## TL;DR

This study uses high-resolution near-infrared imaging to explore galaxy morphologies at a protocluster's core at z=3.09, revealing diverse structures, evidence for galaxy evolution, and insights into the size and mass growth of a potential brightest cluster galaxy.

## Contribution

It provides detailed morphological analysis of dense galaxy groups at high redshift, supporting the two-phase formation scenario of ellipticals and examining size growth of BCG progenitors.

## Key findings

- A compact elliptical galaxy supports the two-phase formation scenario.
- Massive star-forming galaxies are larger than field counterparts.
- No new low-mass galaxy candidates were detected in the group.

## Abstract

We present the near-infrared high resolution imaging of an extremely dense group of galaxies at the core of the protocluster at $z=3.09$ in the SSA22 field by using the adaptive optics AO188 and the Infrared Camera and Spectrograph (IRCS) on Subaru Telescope. Wide morphological variety of them suggests their on-going dramatic evolutions. One of the two quiescent galaxies (QGs), the most massive one in the group, is a compact elliptical with an effective radius $r_{e} = 1.37\pm0.75$ kpc. It supports the two-phase formation scenario of giant ellipticals today that a massive compact elliptical is formed at once and evolves in the size and stellar mass by series of mergers. Since this object is a plausible progenitor of a brightest cluster galaxy (BCG) of one of the most massive clusters today, it requires strong size ($\ga10$) and stellar mass ($\sim$ four times by $z=0$) growths. Another QG hosts an AGN(s) and is fitted with a model composed from an nuclear component and S\'ersic model. It shows spatially extended [O{\footnotesize III}]$\lambda$5007 emission line compared to the continuum emission, a plausible evidence of outflows. Massive star forming galaxies (SFGs) in the group are two to three times larger than the field SFGs at similar redshift. Although we obtained the $K$-band image deeper than the previous one, we found no candidate new members. This implies a physical deficiency of low mass galaxies with stellar mass $M_{\star}\la4\times10^{10}~M_{\odot}$ and/or poor detection completeness of them owing to their diffuse morphologies.