How Long do High-Redshift Massive Black Hole Seeds Remain Outliers in Black Hole vs. Host Galaxy Relations?

Summary

Abstract

Scoggins, Haiman, and Wise study how long galaxies containing newly formed heavy black-hole seeds remain unusual compared with ordinary galaxies. Using two halos from the Renaissance Simulations, they find that the black holes remain more massive than their host galaxies’ stellar populations for about 500 million years. The galaxies remain separate from their larger neighbors until approximately redshift 8, creating a possible window for detection with JWST and X-ray telescopes.

1. Introduction

• Heavy-seed models create black holes with initial masses of approximately 10⁴–10⁶ solar masses inside galaxies with very few stars.
• These systems are called overly massive black-hole galaxies because their black holes can outweigh their entire stellar populations.
• This extreme black-hole-to-stellar-mass ratio could help distinguish heavy seeds from smaller Population III remnants.

2.1 Target Halos

• The study follows the most massive and most irradiated direct-collapse candidates previously identified in the Renaissance Simulations.
• Both form near redshift 15 and later merge with separate, much larger “Superhost” halos near redshift 8.

2.2 Black-Hole and Stellar Masses

• Stellar masses are estimated from each halo’s growth and merger history, including stellar loss during tidal stripping.
• The models test initial black-hole masses from 10⁴–10⁶ solar masses and several possible accretion rates.

3.1 Black-Hole-to-Stellar-Mass Relation

• In nearly every model, the black holes remain more massive than their galaxies’ stars from formation until the final mergers near redshift 8.
• Even the most conservative models remain well above the mass ratios expected for Population III black-hole seeds.

3.2 Alternative Black-Hole Growth

• The authors also test a model that permits periods of super-Eddington black-hole accretion.
• Faster growth makes the systems even stronger outliers and does not change the main conclusion.

3.3 Alternative Stellar-Mass Calculation

• A second method gives somewhat larger stellar masses, especially during periods of tidal stripping.
• The black-hole-to-stellar-mass ratio still remains much larger than the values expected for light seeds.

3.4 Detection

• X-ray observations could estimate the black-hole mass while JWST could measure or limit the host galaxy’s stellar mass.
• The two galaxies remain far enough from their Superhosts for JWST to resolve them separately until their mergers finish near redshift 8.

3.5 Other Detection Methods

• Heavy seeds may also produce unusual emission-line ratios and infrared and X-ray spectral features.
• The mass ratio may remain identifiable longer than these short-lived spectral signatures.

4. Conclusions

• Both simulated galaxies maintain black-hole-to-stellar-mass ratios above one for roughly 500 million years.
• The unusual ratio disappears once the galaxies completely merge with much larger hosts near redshift 8.
• Combined JWST and X-ray observations could detect similar systems and provide evidence for heavy black-hole seeds.

Best Figures

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