Massive Star Formation in Metal-Enriched Haloes at High Redshift

Summary

Abstract

Regan and collaborators use the Renaissance Simulations to investigate whether supermassive stars can form inside halos that already contain metals. They search for atomic-cooling halos with central gas-infall rates above 0.1 solar masses per year and find that about two-thirds are metal-enriched. If poorly mixed halos can retain pockets of metal-poor gas, including these systems could increase the number of possible supermassive-star hosts by at least a factor of four.

1. Introduction

• Supermassive stars could produce the heavy black-hole seeds needed to explain massive quasars in the early universe.
• Although this process is usually studied in metal-free gas, recent work suggests that some metal-enriched halos may also support it.
• The paper tests how many additional candidates appear when halos are selected by gas-infall rate instead of metallicity alone.

2. Renaissance Datasets

• The authors analyze atomic-cooling halos from the dense Rarepeak region and the average-density Normal region of the Renaissance Simulations.
• Gas-infall rates are measured within 20 parsecs of each halo’s center, where material is being delivered toward a possible forming star.

3.1 Atomic-Cooling Halos

• The sample contains 3,244 halos from the Rarepeak region and 2,335 from the Normal region across multiple simulation outputs.
• The halos are divided into metal-free, extremely metal-poor, and metal-enriched groups.

3.2 Central Mass-Infall Rates

• Many atomic-cooling halos have central infall rates above 0.1 solar masses per year, including halos that already contain metals.
• The densest Rarepeak environment generally produces more high-infall halos than the Normal region.

3.3 Abundance and Metallicity

• Among halos with infall rates above 0.1 solar masses per year, about two-thirds have metallicities above 10⁻³ times solar.
• Including extremely metal-poor halos increases the candidate population, while including metal-enriched halos could raise it by a factor of four or more.

3.4 The Most Rapid Accretors

• The fastest-accreting halos span a wide range of metallicities but maintain high gas-infall rates over much of their inner regions.
• Some enriched halos have lower metallicity near their centers, suggesting that metal mixing can be incomplete and uneven.
• These low-metallicity pockets may still allow a supermassive star or an existing black hole to grow rapidly.

4. Discussion and Conclusions

• High gas-infall rates may allow supermassive-star formation in a wider variety of environments than metal-free models suggest.
• Metal-enriched candidates require poorly mixed, metal-poor central gas; otherwise cooling and fragmentation may produce ordinary stars instead.
• Higher-resolution simulations are needed to determine how many of these halos actually form supermassive stars or rapidly growing black holes.

Best Figures

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