Late Pop III Star Formation During the Epoch of Reionization: Results from the Renaissance Simulations

Xu, H., Norman, M.L., O’Shea, B.W., & Wise, J.H., 2016, ApJ, 823, 140.

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Summary

Abstract

Xu and collaborators use the Renaissance Simulations to study Population III star formation as late as redshift z = 7.6. They find 14 galaxies with recent Population III star formation in a low-density region. These stars can form late because metal enrichment spreads slowly and leaves pockets of pristine gas, while Lyman-Werner radiation delays their formation until their halos become more massive.

1. Introduction

  • Population III stars form from metal-free gas, but their supernovae quickly enrich nearby material with heavier elements.
  • Lyman-Werner radiation destroys molecular hydrogen and delays Population III star formation in small halos.
  • The paper investigates whether Population III stars can continue forming during the later stages of reionization.

2. The Renaissance Simulations

  • The authors analyze the Void region of the Renaissance Simulations, covering about 220 comoving cubic megaparsecs.
  • The simulation includes Population III and metal-enriched star formation, chemistry, cooling, radiation, and supernova feedback.
  • The Void region has relatively slow star formation and was evolved to z = 7.6 to study late Population III formation.

3. Results

  • At z = 7.6, the region contains about 500 metal-enriched galaxies but only 14 galaxies with recent Population III star formation.
  • The late-forming Population III stars are mostly found in halos between roughly 3 × 107 and 108 solar masses.
  • One Population III galaxy forms close to a larger metal-enriched galaxy, but it is much less massive and luminous than the proposed Population III interpretation of CR7.

4. Summary and Discussion

  • Slow, local metal enrichment leaves pristine pockets of gas available even inside regions containing metal-enriched galaxies.
  • Strong Lyman-Werner radiation delays star formation until pristine halos grow larger, extending the Population III era to lower redshifts.
  • Late Population III star clusters may be easier to observe than isolated Population III stars, although the paper finds that they are unlikely to explain CR7.

Best Figures

Figure 1
Distribution of late-forming Population III galaxies in the simulated Void region
The locations of the 14 Population III galaxies are shown across the simulated Void region alongside the gas density, Lyman-Werner radiation, and metallicity.
Figure 2
Population III star-formation history and host halo mass distribution
Population III stars continue forming through z = 7.6. Late-forming stars are concentrated in relatively massive halos because Lyman-Werner radiation suppresses their formation in smaller systems.