Simulating the Cosmic Dawn with Enzo
  We review two decades of progress using the Enzo hydrodynamic cosmology code
to simulate the Cosmic Dawn, a period of roughly 1 billion years beginning with
the formation of the first stars in the universe, and ending with cosmic
reionization. Using simulations of increasing size and complexity, working up
in length and mass scale and to lower redshifts, a connected narrative is built
up covering the entire epoch. In the first part of the paper, we draw on
results we and our collaborators have achieved using the Enzo cosmological
adaptive mesh refinement code. Topics include the formation of Population III
stars, the transition to Population II star formation, chemical enrichment, the
assembly of the first galaxies, their high redshift galaxy statistics, and
their role in reionization. In the second part of the paper we highlight
physical difficulties that will require new, more physically complex
simulations to address, drawing from a broader literature survey. We discuss
the healthy interplay between self-consistent numerical simulations and
analytic and semi-analytic approaches. Finally, we discuss technical advances
in hardware and software that will enable a new class of more realistic
simulations to be carried out on exascale supercomputers in the future.
