# Astro2020 Science White Paper: Assembly of the Most Massive Clusters at Cosmic Noon

## TL;DR

This white paper discusses the importance of studying protoclusters at z>2 to understand galaxy evolution and cluster assembly during the critical period before quenching, emphasizing upcoming observational advancements.

## Contribution

It highlights the need for comprehensive spectroscopic and imaging surveys of protoclusters at high redshift to uncover the physical processes driving galaxy and cluster evolution.

## Key findings

- Protoclusters at z>2 are key to understanding galaxy quenching.
- Upcoming deep spectroscopy will enable detailed studies of faint galaxies.
- Understanding protoclusters informs models of massive cluster formation.

## Abstract

Galaxy evolution is driven by many complex interrelated processes as galaxies accrete gas, form new stars, grow their stellar masses and central black holes, and subsequently quench. The processes that drive these transformations is poorly understood, but it is clear that the local environment on multiple scales plays a significant role. Today's massive clusters are dominated by spheroidal galaxies with low levels of star formation while those in the field are mostly still actively forming their stars. In order to understand the physical processes that drive both the mass build up in galaxies and the quenching of star formation, we need to investigate galaxies and their surrounding gas within and around the precursors of today's massive galaxy clusters -- protoclusters at z>2. The transition period before protoclusters began to quench and become the massive clusters we observe today is a crucial time to investigate their properties and the mechanisms driving their evolution. However, until now, progress characterizing the galaxies within protoclusters has been slow, due the difficulty of obtaining highly complete spectroscopic observations of faint galaxies at z>2 over large areas of the sky. The next decade will see a transformational shift in our understanding of protoclusters as deep spectroscopy over wide fields of view will be possible in conjunction with high resolution deep imaging in the optical and near-infrared.