Simulating the Cosmic Dawn With Enzo

Summary

Abstract

Norman, Smith, and Bordner review about two decades of simulations made with the Enzo cosmology code to study Cosmic Dawn. The paper follows the development of the early universe from the formation of the first Population III stars through chemical enrichment, galaxy formation, and reionization. It also discusses the physical questions that remain difficult to model and the computing advances needed to produce larger and more detailed simulations.

1. Introduction

• Cosmic Dawn begins with the first stars and ends when ultraviolet radiation from early galaxies reionizes the intergalactic medium.
• Simulations help connect the small and faint galaxies observed by telescopes with the physical processes occurring inside them.
• The paper builds a connected picture of this period using results from several generations of Enzo simulations.

2. Numerical Simulations

• Enzo uses adaptive mesh refinement to place high resolution around collapsing gas, stars, and galaxies without resolving the entire simulation volume equally.
• The simulations include dark matter, gas dynamics, primordial chemistry, radiative cooling, stellar radiation, and supernova feedback.
• Increases in computing power allowed the simulations to grow from small models of individual stars to large volumes containing many galaxies.

3. Two Decades of Enzo Results

• Molecular hydrogen cooling allows primordial gas inside small dark-matter halos to collapse and form the first Population III stars.
• Supernovae from these stars spread the first heavy elements, allowing metal-enriched Population II stars and the first galaxies to form.
• The Renaissance Simulations show Population II star formation quickly overtaking Population III formation, while faint galaxies make an important contribution to reionization.

4. Simulations and Analytic Models

• Analytic models help simulations account for processes occurring on scales larger or smaller than the simulated region.
• Simulations can then test these models and supply more realistic descriptions of star formation, feedback, and galaxy populations.
• Together, these approaches suggest that Population III star formation becomes regulated by radiation but continues at a low level for a long period.

5. Physical Complications and Future Simulations

• Important uncertainties remain in Population III fragmentation, metal mixing, dust physics, magnetic fields, and the nature of dark matter.
• Radiation backgrounds from ultraviolet and X-ray sources can change the temperature and chemistry of gas across very large distances.
• Future simulations must follow more detailed physics while also covering larger volumes and statistically useful galaxy populations.

6. Technical Advances

• Open-source software lets different research groups contribute new physics models and reuse tools such as Enzo, Grackle, and yt.
• Public simulation archives allow more researchers to analyze expensive data sets without having to rerun the original simulations.
• Enzo-P/Cello is being developed to run these calculations efficiently on highly parallel and exascale computers.

Best Figures

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