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

Abstract We present the first results from AMBRA simulation ( A STRID with M BH seeding from BRA HMA) evolved to z = 8. AMBRA combines the large cosmological volume and statistical power of ASTRID with the physically motivated gas-based black hole (BH) seeding models from BRAHMA . Motivated by James Webb Space Telescope’s (JWST) discoveries of massive BHs at z  ≳ 9, AMBRA adopts a lenient heavy-seed prescription from the BRAHMA suite, allowing for the formation of 4 × 10 4−5   M ⊙ 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 ⊙ . 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, ∼50% 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 ≈4 Laser Interferometer Space Antenna (LISA) detectable BH merger events per year at z ≥ 8, which is three orders of magnitude higher than that in ASTRID .