How Overmassive Black Holes Formed at Cosmic Dawn

Muhammad A. Latif, Daniel J. Whalen, Sadegh Khochfar, Fergus Cullen

First listed 2026-03-30 | Last updated 2026-05-04

Abstract

Overmassive black hole galaxies (OBGs) at redshifts $z \sim$ 10, or 450 Myr after the Big Bang, are one of the most puzzling discoveries by the James Webb Space Telescope to date because they formed by such early epochs and their black-hole to stellar mass ratios are a hundred times higher than those in galaxies today. Here we show that OBGs are simply the result of DCBH birth in primordial halos at early times. A 70,000 M$_{\odot}$ DCBH forming at $z =$ 25.7 in our cosmological simulation grows at about half the Eddington rate to $6.0 \times 10^6$ M$_{\odot}$ by $z =$ 10.1. Its host galaxy reaches a stellar mass of $4 \times 10^8$ M$_{\odot}$, a metallicity $Z =$ 0.1 Z$_{\odot}$, a star formation rate of 2 M$_{\odot}$ yr$^{-1}$, and $M_{\rm BH}/M_{\ast}$ $\sim$ 0.01, on par with OBGs like GN-z11, UHZ1, and GHZ9 at $z =$ 10.6, 10.1, and 10.2, respectively. Our simulation, the first to follow the coevolution of a DCBH and its host galaxy for several hundred Myr, shows that this ratio is a natural result of initial suppression of star formation by the DCBH and the later, violent blowout of metals by Pop III supernovae. Our models provide an excellent match to the spectra of UHZ1 and GHZ9 at $z =$ 10.1 and 10.4, respectively.

Short digest

This paper asks whether the newly discussed overmassive black hole galaxies at cosmic dawn can arise naturally from early direct-collapse black-hole formation rather than from a more exotic late-time assembly path. The main result is that a 70,000-solar-mass DCBH formed at very high redshift can grow into a system with the black-hole-to-stellar-mass ratio, metallicity, star-formation rate, and broad spectral properties seen in objects like UHZ1 and GHZ9 by z around 10. The paper matters because it offers a concrete physical channel for the broader JWST overmassive-black-hole population that sits adjacent to the little-red-dot and early-AGN discussion.

Key figures to inspect

• Figure 4 is the must-see comparison plot: it places the simulated Cloudy spectra against GHZ9, UHZ1, and GN-z11, so this is the figure to inspect first if you want to judge whether the model actually resembles the observed overmassive-BH systems.
• Figure 3 is the key growth-history figure: it tracks the accretion rate, black-hole mass, and BH-to-stellar-mass ratio over time, showing how the system reaches the extreme host ratio by cosmic dawn.
• Figure 2 is worth checking next: it shows how the host stellar mass, metallicity, and star-formation rate evolve, which is what makes the simulated object comparable to the observed JWST galaxies.
• Figure 1 is the orientation figure: it shows the DCBH at birth and the later overmassive-black-hole galaxy near the end of the simulation, giving you the morphological context for the full evolutionary argument.