The rest-frame UV emission from massive stars contains a wealth of
information about the physical nature and conditions of star formation in
galaxies. Using studies of the rest-frame UV, the past decade has witnessed the
beginning of knowledge about the existence and properties of galaxies during
the first few billion years after the Big Bang. This period of history
corresponds to the formation of the first stars, the rapid formation of galaxy
stellar populations, the reionization of the IGM, the production and
dissemination of heavy elements, and the formation of the first black holes.
Massive stars in these galaxies drive all of these events, and their light
dominates the spectral energy distributions of galaxies. As we look to the
2020s, fundamental questions remain about the nature of these stellar
populations and their evolution, from just before the peak of the cosmic star
formation density (z~3), up to the epoch of reionization (z > 6). This next
decade will provide transformative gains both in our ability to identify
star-forming galaxies and accreting supermassive black holes at these early
epochs with imaging surveys in the rest-frame UV (e.g., LSST, WFIRST).
Ground-based, rest-frame UV spectroscopy on >20 m-class telescopes (e.g.,
GMT/TMT) offers the ability to investigate the astrophysical conditions in
galaxies at the earliest cosmic times. This includes studies of the evolution
in galaxy stellar populations, gas ionization (temperature, pressure),
metallicity, and interstellar (and circumgalactic) gas kinematics and covering
fractions. In this white paper, we describe the scientific prospects and the
requirements for research in this area.