Fully Coupled Simulation of Cosmic Reionization. III. Stochastic Early Reionization by the Smallest Galaxies

Summary

Abstract

Norman and collaborators study how the smallest early galaxies contribute to cosmic reionization. Using ionizing radiation properties measured in the Renaissance Simulations, they show that low-mass metal-cooling halos help begin reionization earlier, while larger galaxies eventually dominate and drive the universe toward complete ionization.

1. Introduction and Background

• Small metal-cooling halos can form stars after being enriched by metals from earlier Population III supernovae.
• Although each of these galaxies forms relatively few stars, their large numbers and high ionizing escape fractions allow them to contribute significantly to early reionization.
• The paper asks how including these small galaxies changes the timing and progression of cosmic reionization.

2. Ionizing Photons from the Smallest Galaxies

• The authors use star formation and ionizing escape properties measured in the Rarepeak, Normal, and Void Renaissance Simulations.
• Low-mass galaxies form stars intermittently, so their ionizing output can turn on and off over time.
• The probability of active star formation increases with halo mass, approaching unity near 10^8.5 solar masses.

3. Research Methods

• The authors run fully coupled cosmological radiation-hydrodynamic simulations using the Enzo code.
• Halos are identified during the simulation and assigned ionizing luminosities using probability distributions derived from the Renaissance Simulations.
• The ionizing source population is updated every 20 million years to reproduce the bursty star formation of low-mass galaxies.

4. Results

• Small metal-cooling halos provide an important fraction of the ionizing photons during the earliest stages of reionization.
• Their intermittent star formation causes small ionized regions to form and sometimes recombine, producing a stochastic early phase.
• By about z ≈ 10, galaxies above roughly 10⁹ solar masses dominate the ionizing photon budget and reionization becomes smoother.

5. Discussion

• The smallest galaxies are most important for starting reionization rather than finishing it.
• Their importance comes from their high abundance and ionizing escape fractions rather than from large star formation rates in individual galaxies.
• Low-mass atomic-cooling halos contribute less than expected because their ionizing escape fractions are generally small.

6. Conclusions

• Including small metal-cooling galaxies causes reionization to begin earlier, but changes the completion time only slightly.
• The simulation begins ionizing before z = 20, reaches about 10% ionization near z = 10, and becomes fully ionized around z = 7.1.
• The early universe therefore experiences a stochastic period of reionization before larger galaxies take over and drive smooth, large-scale ionization.

Best Figures

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