Galaxy Properties and UV Escape Fractions during the Epoch of Reionization: Results from the Renaissance Simulations

Xu, H., Wise, J.H., Norman, M.L., Ahn, K., & O’Shea, B.W., 2016, ApJ, 833, 84.

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Summary

Abstract

Xu and collaborators use the Renaissance Simulations to study the properties of early galaxies and the fraction of their ionizing ultraviolet radiation that escapes into the IGM. Small galaxies form stars irregularly, but their ionizing photons escape relatively easily. The average escape fraction reaches 40–60 percent near a halo mass of 107 solar masses, compared with about 5 percent between 108 and 109 solar masses.

1. Introduction

  • Early galaxies are thought to provide most of the ultraviolet photons responsible for cosmic reionization.
  • The escape fraction measures how much ionizing radiation leaves a galaxy instead of being absorbed by its gas.
  • The paper examines how star formation and escape fraction depend on halo mass, redshift, and cosmic environment.

2. Simulations

  • The authors use the Rarepeak, Normal, and Void regions of the Renaissance Simulations to represent different cosmic environments.
  • Each region contains more than 3,000 halos with masses between about 107 and 109.5 solar masses.
  • The simulations include Population III and metal-enriched stars, radiation transport, chemistry, cooling, and supernova feedback.

3. Simulated Galaxy Properties

  • Halos as small as 107 solar masses can form stars after earlier Population III supernovae enrich their gas with metals.
  • Small galaxies alternate between active star formation and quiet periods because radiation and supernovae repeatedly remove or disturb their gas.
  • Galaxy properties depend mainly on halo mass and remain fairly similar across different redshifts and environments.

4. Ultraviolet Escape Fractions

  • The average escape fraction is about 40–60 percent in halos near 107 solar masses but falls to about 5 percent between 108 and 109 solar masses.
  • In the largest simulated halos, the escape fraction increases again to about 10–20 percent during strong, continuous star formation.
  • Ionizing radiation escapes unevenly through low-density channels created by radiation and supernova feedback.

5. Conclusions

  • The smallest galaxies provide an important part of the ionizing photon budget during the early stages of reionization.
  • As star formation in small halos becomes suppressed, larger galaxies provide more of the radiation needed to complete reionization.
  • A galaxy’s contribution depends on both its escape fraction and how often it is actively forming massive stars.

Best Figures

Figure 18
Ultraviolet escape fraction and escaped ionizing photons as functions of halo mass
Smaller halos have higher average escape fractions, while more massive galaxies produce a larger total number of ionizing photons. The figure shows how both factors determine which galaxies contribute most strongly to reionization.
Figure 22
Fractions of halos with active star formation and escaping ionizing photons
Only about 10 percent of halos near 107 solar masses are actively forming stars at a given time. This fraction rises with halo mass until nearly every massive halo contains young stars and releases ionizing radiation.