Back to Normal Again: Possible Destinies of JWST overmassive SMBHs and "Little Red Dots" in the View of Shin-Uchuu Simulation

Haojie Hu, Hiroto Yanagisawa, Moka Nishigaki, Tomokazu Kiyota, Tomoaki Ishiyama, Ken Ohsuga

First listed 2026-02-16 | Last updated 2026-02-16

Abstract

The James Webb Space Telescope (JWST) has enabled the discovery of hundreds of supermassive black holes (SMBHs) at redshifts $z\gtrsim 4-7$. A non-negligible fraction of these SMBHs are hosted in galaxies with BH-to-galaxy mass ratios ($M_{\rm BH}/M_\star$) being excessively larger than that for local SMBHs by $\sim 1-2$ dex. The origin of these ``overmassive'' BHs remains elusive, demanding either a heavy seed formation scenario or rapid growth of seed BHs. Their deviation from local scaling relations challenges our understanding of how SMBHs and their host galaxies coevolve across cosmic time. In this paper, we apply phenomenological modelings for BHs and galaxies to dark matter halo merger histories from N-body simulations to investigate the subsequent evolution of JWST-discovered ``overmassive'' SMBHs. We find that early evolution of ``overmassive'' SMBHs is dominated by stunted accretion leading to gradual decreases in $M_{\rm BH}/M_\star$ ratios. In contrast, less massive SMBHs experience super-Eddington accretion during their early evolution, resulting in a slow increase of mass ratios toward $M_{\rm BH}/M_\star \sim 0.01$. Convergence occurs at $M_{\rm BH}\sim 10^8~M_\odot$ with $M_{\rm BH}/M_\star \sim 0.01$. At lower redshift, nearly all SMBHs evolve onto local relations, as expected given that our models adopt empirical relations derived from low-redshift observations. This suggests that the global feedback mechanisms regulating the coevolution of $M_{\rm BH}/M_\star$ ratios are implicitly encoded in local relations in terms of star-formation rate distribution, black hole accretion rate distribution and their active (quiescent) fractions.

Short digest

Using Shin-Uchuu halo merger trees with phenomenological prescriptions for BH growth and galaxy evolution, the authors evolve JWST “overmassive” SMBHs and LRD analogs (e.g., GN-z11, CEERS-20496, UNZ1, Abell-2744-QSO1). They find initially overmassive SMBHs undergo stunted accretion that reduces M_BH/M_*, while lower-mass SMBHs pass through super-Eddington phases that lift the ratio toward ~0.01; tracks converge near M_BH ≈ 10^8 Msun with M_BH/M_* ≈ 0.01 before moving onto local relations at later times. This points to global regulation encoded by observed SFR/BHAR distributions and active–quiescent fractions, implying many LRD-like systems can return to normal without fine-tuned feedback. A noted caveat is that the late-time approach to local relations is influenced by the adoption of low‑z empirical relations in the modeling.

Key figures to inspect

• Figure 3: Follow GN-z11, CEERS-20496, UNZ1, and Abell-2744-QSO1 tracks to see when growth is super-Eddington (red stars) versus quasar/radio modes and how M_BH/M_* declines or rises before converging near 10^8 Msun and ~1%.
• Figure 2: Inspect halo and stellar-mass assembly histories selected for each object; check how the merger-tree scatter brackets the observed M_* of the SMBH/LRD sample and the implied gas supply history.
• Figure 1: Examine the adopted redshift- and mass-dependent quiescent fractions and BH active fractions; assess how the small high-mass active fraction constrains early fueling and helps drive the stunted-growth phase.
• Figure 4: Compare the model SFR–M_* distribution and SMHM ratios to UniverseMachine and SDSS fits to verify that the phenomenological galaxy model reproduces main-sequence trends that underpin the ~1% M_BH/M_* convergence.