Over-Massive Black Hole Galaxy Phase

• The over-massive black hole galaxy phase is a transient epoch in early galaxy evolution where SMBHs exceed local scaling relations by factors of 10–10³.
• Rapid, merger-driven super-Eddington accretion and heavy seed formation trigger episodic SMBH growth while the host galaxy lags in stellar mass buildup.
• Observations and simulations indicate this phase influences feedback processes and links early quasar activity to the emergence of compact quiescent galaxies.

An "over-massive black hole galaxy phase" denotes a transient evolutionary state in which a galaxy's central supermassive black hole (SMBH) has acquired a mass far exceeding the present-day scaling relations between the SMBH and its host galaxy stellar mass, often by orders of magnitude. This phase is observed predominantly at high redshift ($z \gtrsim 4$–10), and is characterized by rapid and episodic black hole growth, typically outpacing the buildup of the galaxy's stellar population. Empirical examples and theoretical models highlight this phenomenon as generic and ubiquitous during the earliest phases of galaxy assembly, challenging the notion of tightly coupled or "lock-step" coevolution.

1. Defining Characteristics of the Over-Massive Black Hole Galaxy Phase

The over-massive phase is operationally defined by a black-hole-to-stellar mass ratio

$\mu \equiv \frac{M_{\rm BH}}{M_*}$

exceeding the local value (typically $\mu_{\rm local} \sim 0.2-0.5\%$) by factors of $10$–$10^3$. Canonical observed and simulated systems include:

Example	$z$	$M_{\rm BH}$	$M_*$	$\mu$	Reference
GN-1001830	6.68	$4 \times 10^8~M_\odot$	$\mu \equiv \frac{M_{\rm BH}}{M_*}$0	0.4	(Juodžbalis et al., 2024)
UHZ1	10.1	$\mu \equiv \frac{M_{\rm BH}}{M_*}$1	$\mu \equiv \frac{M_{\rm BH}}{M_*}$2	$\mu \equiv \frac{M_{\rm BH}}{M_*}$3	(Natarajan et al., 2023)
CID-947	3.328	$\mu \equiv \frac{M_{\rm BH}}{M_*}$4	$\mu \equiv \frac{M_{\rm BH}}{M_*}$5	0.12	(Trakhtenbrot et al., 2015)

These ratios are $\mu \equiv \frac{M_{\rm BH}}{M_*}$6–$\mu \equiv \frac{M_{\rm BH}}{M_*}$7 above the local relation. Over-massive systems exhibit broad emission lines (H$\mu \equiv \frac{M_{\rm BH}}{M_*}$8, H$\mu \equiv \frac{M_{\rm BH}}{M_*}$9), low or quiescent Eddington ratios ($\mu_{\rm local} \sim 0.2-0.5\%$0–$\mu_{\rm local} \sim 0.2-0.5\%$1 in dormant states), compact host morphologies, suppressed or low star formation rates, and are often detected in deep JWST and Chandra/X-ray surveys.

2. Physical Mechanisms Driving the Over-Massive Phase

Two primary pathways to the over-massive state have been established:

• Episodic, merger-driven super-Eddington accretion: Semi-analytic models and cosmological simulations show that short (0.5–4 Myr) bursts of super-Eddington accretion, frequently triggered by major mergers, drive rapid BH mass growth before stellar mass can catch up. The net duty cycle of such bursts is low ($\mu_{\rm local} \sim 0.2-0.5\%$2 = 1–4%), such that most of the over-massive population is dormant at any given time (Trinca et al., 2024, Juodžbalis et al., 2024).
• Heavy seed formation scenarios: Direct collapse black hole (DCBH) pathways—seed masses $\mu_{\rm local} \sim 0.2-0.5\%$3–$\mu_{\rm local} \sim 0.2-0.5\%$4—arise in atomic cooling halos and rapidly grow under conditions that inhibit star formation (e.g., strong Lyman–Werner flux, baryonic streaming motion, violent mergers) (Chon et al., 8 Jan 2026, Inayoshi et al., 2018, Natarajan et al., 2023). This initiates the OBG phase, often with $\mu_{\rm local} \sim 0.2-0.5\%$5.

Both channels are augmented by environmental factors like high gas and dark matter fractions, which suppress stellar-to-dynamical mass ratios at early epochs (McClymont et al., 16 Jun 2025).

3. Scaling Relations and Deviation from Local Coevolution

The over-massive phase is best diagnosed by severe positive deviation from the local $\mu_{\rm local} \sim 0.2-0.5\%$6–$\mu_{\rm local} \sim 0.2-0.5\%$7 relation. For example, in the JADES system GN-1001830, $\mu_{\rm local} \sim 0.2-0.5\%$8—$\mu_{\rm local} \sim 0.2-0.5\%$9 above the canonical $10$0. In contrast, such systems tend to remain closer to the local $10$1–$10$2 and $10$3–$10$4 relations, implying rapid SMBH assembly in the context of a growing gravitational potential well, with delayed stellar mass growth (Juodžbalis et al., 2024, McClymont et al., 16 Jun 2025).

This decoupling is inconsistent with synchronized coevolution models stipulating

$10$5

at all epochs. Instead, the over-massive phase is a manifestation of asynchronous SMBH–galaxy growth, with $10$6 potentially evolving as $10$7, but lying well above such an extrapolation at $10$8 (Trakhtenbrot et al., 2015).

4. Observational Probes and Diagnostics

Robust identification relies on:

• Broad-line diagnostics: Virial mass estimators utilizing broad H$10$9 or H$10^3$0 widths ($10^3$1 km/s), and nuclear SED/broad-line fluxes to infer $10^3$2 (Juodžbalis et al., 2024).
• Stellar and dynamical mass estimates: SED fitting (e.g., Prospector + BAGPIPES) constrains $10^3$3, while velocity dispersion proxies and resolved kinematics allow calculation of $10^3$4.
• Cold gas and morphology: ALMA [C II] 158 $10^3$5m sizes indicate compact hosts ($10^3$6 kpc at $10^3$7) with low gas fractions tightly associated with enhanced $10^3$8, and overlap with quiescent post-starburst systems at $10^3$9–5 (Wu et al., 17 Jun 2025).
• AGN activity: Most over-massive systems are quiescent ($z$0), but the luminosity function is enhanced during brief super-Eddington phases, reconciling with the observed population of "little red dots" and faint AGNs at $z$1–8 (Trinca et al., 2024).

5. Theoretical Scenarios and Simulation Constraints

Hydrodynamical and semi-analytic models, including the COLIBRE, CAT, FABLE, and THESAN-ZOOM simulations, have demonstrated key elements of this phase:

• Duty cycles of super-Eddington bursts are $z$21–4%, with burst durations $z$3–$z$4 Myr (Trinca et al., 2024, Chaikin et al., 21 Jan 2026).
• At high $z$5, a majority of BH mass growth occurs in the super-Eddington regime, although these events occupy a minority of the time ($z$62–5%) (Chaikin et al., 21 Jan 2026).
• The BH mass to stellar mass ratio evolves with host properties: $z$7 at $z$8, decreasing to $z$9 at $M_{\rm BH}$0 (McClymont et al., 16 Jun 2025).
• Delayed stellar-mass buildup (suppressed $M_{\rm BH}$1) is the result of high gas and dark-matter fractions at early cosmic times, rather than anomalously rapid SMBH growth per se (McClymont et al., 16 Jun 2025).
• Simulations populate the observed region of $M_{\rm BH}$2 at high-$M_{\rm BH}$3 without invoking exotic inputs, indicating the phase is a natural consequence of fundamental BH–host potential coupling (McClymont et al., 16 Jun 2025, Chaikin et al., 21 Jan 2026).

6. Evolutionary Outcomes and Cosmological Implications

The over-massive phase represents a transient, but common, evolutionary epoch in which:

• Rapid SMBH growth outputs feedback capable of regulating early star formation, evidenced by suppressed or quenched SFRs in over-massive hosts (e.g., SFR %%%%62$10^3$63%%%% below main sequence in GN-1001830), and leading to the emergence of early quiescent galaxies (Juodžbalis et al., 2024, Wu et al., 17 Jun 2025, Chaikin et al., 21 Jan 2026).
• As gas depletion proceeds via feedback, host morphologies evolve toward high stellar surface density, and the $M_{\rm BH}$6 ratio returns to the local value as stellar mass builds over the next few gigayears (Wu et al., 17 Jun 2025, Trakhtenbrot et al., 2015).
• This phase provides a physical and evolutionary link between the first SMBHs, their quasar descendants, and compact quiescent systems observed at $M_{\rm BH}$7–5, and ultimately massive local ellipticals (Wu et al., 17 Jun 2025).
• The existence of a large population of dormant, over-massive SMBHs is predicted at $M_{\rm BH}$8, which may substantially exceed the numbers detectable via AGN signatures, implying the observed phase is but the "tip of the iceberg" (Juodžbalis et al., 2024, Trinca et al., 2024).

7. Outstanding Issues and Local Universe Analogs

At $M_{\rm BH}$9, rare compact galaxies such as NGC 1277 ($M_*$0) and similar compact lenticulars have been identified as possible remnants of the high-redshift over-massive phase (Bosch et al., 2012). However, systems with substantial spheroids and correctly assessed bulge masses may not be as extreme outliers as once thought, typically lying within $M_*$1 of local M$M_*$2–M$M_*$3 scaling relations (Graham et al., 2016).

Open questions persist as to whether extreme outliers represent stochastic scatter ("tail") or brief, consequential evolutionary stages ("phase"). High-redshift and local studies advocate for the latter, at least for a non-negligible fraction of massive galaxy formation pathways (Bosch et al., 2012, Trakhtenbrot et al., 2015).

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In summary, the over-massive black hole galaxy phase is a generic, physically-motivated, and directly observed stage of early SMBH–galaxy evolution, in which SMBHs briefly attain a mass ratio $M_*$4–$M_*$5 times above local relations due to rapid, burst-driven accretion in a gas-rich, merger-prone, and dynamically evolving environment. This phase is crucial for understanding SMBH assembly, feedback-regulated quenching, the origin of early quiescent galaxies, and the physical diversity of host-BH scaling relations across cosmic time (Juodžbalis et al., 2024, Trinca et al., 2024, Chaikin et al., 21 Jan 2026, Wu et al., 17 Jun 2025, McClymont et al., 16 Jun 2025, Natarajan et al., 2023).