Rapid emergence of overmassive black holes in the early Universe

Sunmyon Chon, Shingo Hirano, Tomoaki Ishiyama, Seok-Jun Chang, Volker Springel

First listed 2026-01-08 | Last updated 2026-01-08

Abstract

The origin of supermassive black holes (SMBHs) remains a long-standing problem in astrophysics. Recent JWST observations reveal an unexpectedly abundant population of overmassive black holes at z>4-6, where the BH masses lie far above local scaling relations and not reproduced by current cosmological models. How such overmassive black holes form and rapidly grow within young galaxies has remained unclear. Here we present fully cosmological radiation-hydrodynamic simulations that, for the first time, self-consistently follow the birth, early growth, and emergent observable signatures of SMBHs in proto-cluster environments. We find that heavy seeds of order $10^6 M_\text{sun}$ naturally form, exceeding typical theoretical expectations by an order of magnitude. These seeds rapidly develop dense, optically thick disks whose strong electron scattering produces broad H$α$ emission comparable to that seen in little red dots (LRDs). Sustained super-Eddington accretion then drives fast growth to $\sim 3 \times 10^7 ~M_\text{sun}$ by $z \sim 8$. These results provide a unified physical scenario in which LRDs correspond to a short-lived, enshrouded phase of heavy-seed formation, naturally evolving into the overmassive quasars detected by JWST and ultimately the progenitors of today's SMBHs.

Short digest

Cosmological radiation–hydrodynamic simulations (AREPO) track SMBH birth and early growth in overdense proto-cluster regions illuminated by strong FUV fields. The runs naturally form ~10^6 Msun heavy seeds from supermassive-star collapse that quickly assemble dense, optically thick disks whose electron scattering yields broad Halpha and Balmer absorption akin to little red dots. Brief (<Myr), super-Eddington episodes propel growth to ~3×10^7 Msun by z~8, keeping BH-to-stellar mass ratios above local relations and overlapping the observed LRD locus. Light (Pop III–remnant) seeds in the same environment fail to grow comparably, linking LRDs to a short-lived, enshrouded heavy-seed phase that precedes overmassive quasars.

Key figures to inspect

• Figure 1: Use panels (e)–(f) to read off the heavy-seed (MBH1/MBH2) mass build-up and the duration/peak of the super-Eddington bursts, and contrast with the stalled Pop III light-seed track; maps (a–d) show the FUV-illuminated environment and subsequent migration/merger into the neighbor halo.
• Figure 2: Inspect the simulated M_BH–M_star track versus LRD and quasar points to see BH-to-stellar mass ratios exceeding the local relation, and note when the BH enters the host’s virial radius along the path that matches LRDs.
• Figure 3: Check how the progenitor cloud fragments into a compact multiple system, with bursty accretion and mergers keeping stars in an inflated supergiant phase that mutes UV feedback—setting up collapse to ~10^6 Msun heavy seeds.
• Figure 4: Follow the circum-BH disk from thick to thin; the radial density and emissivity profiles demonstrate the conditions for Balmer absorption and electron-scattering–broadened Halpha, connecting the simulated disk directly to LRD spectra.