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