Heavy 'Seeds' and Mergers: The Recipe for Black Hole Growth express
The James Webb telescope discovered black holes — so named by John Wheeler, and Karl Schwarzschild described them as a light trap — which, just 500 million years after the Big Bang, already weighed more than a million Suns. It all began with heavy 'seeds' — future black holes with masses of tens of thousands of Suns, born in dense clumps of primordial gas within dark matter scaffolds. These embryos merged like voices in a choir, rapidly building up power. Later, gas absorption provided the main growth. Simulations showed that nearly half of the mass comes from mergers; otherwise, giants like the galaxy GN-z11 wouldn't have had time to grow.
Interestingly, the seeds probably collapsed directly from gas, skipping the star stage. Calculations promise up to four mergers per year for the LISA observatory — hundreds of times more than previous forecasts.
Original abstract
as written by the authors arXiv:2604.01123
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.
If space had sound, a black hole merger would sound like a low hum quickly soaring upward — a cosmic 'chirp'.