The Emergence of the First Star-Free Atomic Cooling Haloes in the Universe

Summary

Abstract

Regan and collaborators use the Renaissance Simulations to search for metal-free, star-free atomic-cooling halos that could form direct-collapse black holes. They identify 79 candidates, most of which form in the dense Rarepeak region. The results show that rapid halo growth, moderate Lyman-Werner radiation, and avoiding metal pollution must work together to keep these halos star-free until they reach the atomic-cooling threshold.

1. Introduction

• Direct-collapse black holes are heavy seeds that could help explain how supermassive black holes formed so early.
• Their host halos must avoid ordinary Population III star formation and remain free of metals while growing.
• The paper tests whether rapid halo growth can delay star formation without requiring an extremely strong radiation field.

2. Renaissance Simulation Suite

• The simulations follow early star formation, radiation, chemistry, metal enrichment, and halo growth in dense, average-density, and void regions.
• Candidate halos are required to be metal-free, star-free, and massive enough for atomic hydrogen cooling.

3.1 Candidate Abundance

• The study finds 79 candidate halos: 76 in the Rarepeak region and three in the Normal region.
• No candidates appear in the Void region, showing that dense environments strongly favor their formation.

3.2 Required Physical Conditions

• Most candidates are located about 10–100 kiloparsecs from the nearest massive galaxy and receive moderate Lyman-Werner radiation.
• Their radiation levels are usually far below the intensity previously thought necessary to completely suppress molecular-hydrogen cooling.

3.3 Radiation, Metallicity, and Rapid Growth

• Most candidate halos grow rapidly before reaching the atomic-cooling limit, producing dynamical heating that delays gas collapse.
• Rapid growth alone is not enough: a halo that becomes metal-enriched can cool and form stars even while growing quickly.
• Successful candidates combine fast growth, low metallicity, and enough Lyman-Werner radiation to limit molecular cooling.

3.4 Synchronized Halos

• The authors identify five pairs of pristine atomic-cooling halos that form close together in both space and time.
• Radiation from the first halo in a pair could help keep the second halo star-free and support direct-collapse black-hole formation.

4. Discussion and Conclusions

• Dynamical heating from rapid growth is the main process keeping most candidate halos star-free until atomic cooling begins.
• Metal transport must be modeled carefully because external enrichment can remove an otherwise promising candidate.
• High-resolution follow-up simulations are needed to determine whether these halos form supermassive stars, dense star clusters, or black holes.

Best Figures

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