First results of AMBRA: Abundant Seeds and Early Mergers as a Pathway to the First Massive Black Holes

TL;DR

AMBRA models early massive black hole formation using a lenient seed prescription, showing that early seeds and mergers can explain JWST's high-redshift BH observations without super-Eddington accretion.

Contribution

It introduces an improved seeding model in cosmological simulations that enables early and efficient black hole seed formation, matching JWST observations.

Findings

AMBRA produces a higher number density of massive BHs at z=8 than ASTRID.

Approximately 50% of BH mass by z=11 results from mergers in AMBRA.

AMBRA predicts about 4 LISA-detectable BH mergers per year at z≥8.

Abstract

AMBRA combines the large cosmological volume and statistical power of ASTRID with the physically motivated gas-based black hole seeding models from BRAHMA. Motivated by JWST's discoveries of massive black holes (BHs) at $z\gtrsim 9$, AMBRA adopts a lenient heavy-seed prescription from the BRAHMA suite, allowing for the formation of $4\times 10^{4-5}\ M_{\odot}$ seeds in halos with star-forming, metal-poor gas. The seeding model is motivated by scenarios in which heavy seeds form through stellar collisions in star clusters or from the rapid growth of Population III remnants. The improved seeding model enables AMBRA to form BH seeds much earlier and more efficiently compared to ASTRID. This significantly enhances early BH growth, producing a $z=8$ BH number density more than an order of magnitude higher than that in ASTRID over the mass range $10^{5-7}\ M_{\odot}$. BHs reaching masses consistent with GN-z11 and CEERS-1019 typically originate in highly compact density peaks and undergo multiple early mergers. In these systems, $\sim50\%$ of BH masses by $z=11$ is from BH mergers, after which gas accretion becomes the dominant growth channel. Without this early merger-driven assembly, ASTRID cannot reproduce the high-mass BH detected by JWST. Our results indicate that abundant early seed formation combined with frequent mergers can explain several JWST massive BH candidates without requiring sustained super-Eddington accretion. As a testable prediction, AMBRA yields $\approx4$ LISA detectable BH merger events per year at $z\geq8$, which is three orders of magnitude higher than that in ASTRID.