Overmassive and Undermassive Massive Black Holes: The Role of Environment and Gravitational-Wave Recoils
Pith reviewed 2026-05-15 17:25 UTC · model grok-4.3
The pith
Outliers in the black hole-stellar mass relation arise through multiple channels whose importance shifts with galaxy mass and redshift.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Outliers of the M_BH-M_* relation do not arise from a single mechanism, but from the interplay between environmental effects, gravitational recoils, and diverse MBH fueling histories, whose relative importance varies with galaxy mass and redshift.
What carries the argument
Distinct evolutionary pathways identified in the L-Galaxies-BH semi-analytical model: enhanced merger and secular activity for overmassive cases, gravitational recoil ejections plus replacement black holes for undermassive massive galaxies, and quiescent limited-growth histories for undermassive low-mass galaxies.
If this is right
- Overmassive black holes at redshift greater than 4 commonly form via early rapid growth that includes super-Eddington accretion episodes.
- At low redshift, roughly 20 percent of overmassive cases result from environmental reduction of the host galaxy's stellar mass.
- In massive galaxies, gravitational recoil can temporarily leave the galaxy without its original nucleus, after which a lighter black hole from a prior merger becomes central but remains undermassive.
- In galaxies below 10^9 solar masses, undermassive black holes arise mainly from limited mergers and weak secular activity that suppress efficient growth.
Where Pith is reading between the lines
- The scatter around the scaling relation may encode recoverable information about a galaxy's merger rate and local density if black-hole masses can be measured across redshift bins.
- In dense environments such as galaxy clusters, environmental stripping could produce a higher fraction of overmassive outliers than in the field.
- Cosmological simulations that assume a single tight universal relation may under- or over-predict black-hole feedback in galaxy populations dominated by one of these outlier channels.
Load-bearing premise
The semi-analytical model and its calibration to the Millennium simulations accurately reproduce MBH accretion, mergers, recoils, and environmental interactions without systematic biases in the outlier populations.
What would settle it
A survey finding that undermassive black holes in massive galaxies are equally common in systems with no merger history or recoil signatures would undermine the recoil channel as a primary driver.
Figures
read the original abstract
Understanding the connection between galaxy properties and their central massive black holes (MBHs) is key to unveiling their co-evolution. We use the ${\tt L{-}Galaxies{-} \it BH}$ semi-analytical model and the ${\tt Millennium}$ suite of simulations to investigate the physical origin of galaxies hosting overmassive and undermassive MBHs with respect to the $M_{\rm BH}-M_*$ relation, across stellar mass and cosmic time. We find that distinct evolutionary pathways drive different offsets from the scaling relation. Overmassive MBHs are primarily associated with galaxies that experienced enhanced merger history and secular activity. At $z\,{>}\,4$, this activity often leads to early, rapid MBH growth, frequently involving super-Eddington accretion episodes. At low redshift, a minority of overmassive systems ($20\%$) instead arise from environmental effects that reduce the stellar mass of the host, shifting galaxies above the relation without requiring additional MBH growth. Undermassive MBHs originate from two main channels. In massive galaxies, gravitational recoil following MBH mergers can eject the central MBH, temporarily leaving the galaxy without a nucleus. During this phase, MBHs coming from previous galaxy mergers can become the new central MBHs, but their masses remain below the expected ones from the scaling relation, as they never co-evolved with their new host galaxy. In low-mass galaxies ($M_*<10^9 M_\odot$), undermassive MBHs are more commonly linked to a quiescent evolutionary history, with limited mergers and weak secular processes that suppress an efficient MBH growth. We therefore conclude that outliers of the $M_{\rm BH}-M_*$ do not arise from a single mechanism, but from the interplay between environmental effects, gravitational recoils, and diverse MBH fueling histories, whose relative importance varies with galaxy mass and redshift.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses the L-Galaxies-BH semi-analytical model on the Millennium simulations to trace the origins of overmassive and undermassive MBHs relative to the M_BH-M_* relation. Overmassive systems are linked to enhanced merger/secular activity (including super-Eddington episodes at z>4) or, in a minority (~20%) of low-z cases, to environmental reduction of stellar mass; undermassive systems arise either from post-recoil MBH replacement in massive galaxies or from quiescent histories with limited mergers in low-mass galaxies (M_*<10^9 M_⊙). The central claim is that outliers result from the interplay of environment, gravitational recoils, and fueling histories, with channel dominance varying by galaxy mass and redshift.
Significance. If the model's accretion, merger, and recoil prescriptions are reliable, the work provides a concrete multi-channel framework for interpreting M_BH-M_* outliers across cosmic time, with direct relevance to high-z JWST observations and future gravitational-wave constraints on recoil velocities.
major comments (1)
- [analysis of undermassive MBHs in low-mass galaxies] In the analysis of undermassive MBHs in low-mass galaxies (M_* < 10^9 M_⊙), the attribution to quiescent histories with limited mergers rests on accurate tracking of minor mergers and environmental interactions. The Millennium dark-matter particle mass (~8.6 × 10^8 M_⊙) implies that galaxies near this threshold are resolved by only a few particles, which can suppress detection of minor mergers and bias systems toward the quiescent channel. This resolution limitation is load-bearing for the mass-dependent channel claim and requires explicit validation or resolution tests.
minor comments (2)
- [Abstract] The abstract states that 20% of low-redshift overmassive systems arise from environmental effects; the precise redshift boundary used for 'low redshift' should be stated explicitly for reproducibility.
- [Figures] Notation for stellar mass (M_*) and black-hole mass (M_BH) is generally clear, but figure captions should explicitly note whether the plotted scaling relation is the model's own calibrated relation or an observational fit.
Simulated Author's Rebuttal
We thank the referee for their constructive and detailed review of our manuscript. We have carefully considered the major comment and provide a point-by-point response below. We agree that the resolution limitations warrant additional discussion and have revised the manuscript to address this.
read point-by-point responses
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Referee: In the analysis of undermassive MBHs in low-mass galaxies (M_* < 10^9 M_⊙), the attribution to quiescent histories with limited mergers rests on accurate tracking of minor mergers and environmental interactions. The Millennium dark-matter particle mass (~8.6 × 10^8 M_⊙) implies that galaxies near this threshold are resolved by only a few particles, which can suppress detection of minor mergers and bias systems toward the quiescent channel. This resolution limitation is load-bearing for the mass-dependent channel claim and requires explicit validation or resolution tests.
Authors: We thank the referee for raising this important numerical caveat. The Millennium simulation's dark-matter particle mass of ~8.6×10^8 M_⊙ indeed means galaxies with M_* ≲ 10^9 M_⊙ are only marginally resolved, which can affect the detection of minor mergers and environmental interactions in the merger trees. Our classification of the quiescent channel in low-mass systems is derived directly from the model's merger and accretion histories, but we acknowledge that this resolution floor may preferentially suppress minor-merger events and thereby bias some systems toward appearing quiescent. In the revised manuscript we will add an explicit paragraph in Section 2 (Methods) and the discussion of low-mass undermassive systems (Section 4.2) that (i) states the resolution limit, (ii) quantifies the typical number of particles per galaxy at the M_* = 10^9 M_⊙ threshold, and (iii) notes that the reported mass-dependent channel distinction should be interpreted with this caveat. We will also mention that higher-resolution runs (e.g., Millennium-II) would be required for a full convergence test. We do not believe the overall conclusion—that distinct channels dominate at different masses—is invalidated, because the same qualitative trends appear across the full resolved mass range, but we will qualify the low-mass results accordingly. revision: yes
Circularity Check
No significant circularity detected
full rationale
The paper uses the L-Galaxies-BH semi-analytical model run on Millennium simulations to trace distinct evolutionary channels (merger histories, secular activity, recoils, environmental stripping) that produce offsets from the M_BH-M_* relation. The relation itself is an observed benchmark against which simulated galaxies are compared; the classification of overmassive/undermassive systems is a post-processing label, not an input that is refitted or redefined inside the derivation. No equation or claim reduces by construction to a fitted parameter or to a self-citation whose content is the target result. The central conclusion—that multiple channels operate with mass- and redshift-dependent weights—follows from the simulation’s merger trees and accretion tracking rather than from any definitional loop or renamed empirical pattern.
Axiom & Free-Parameter Ledger
free parameters (3)
- accretion efficiency and Eddington ratio caps
- merger timescale and dynamical friction parameters
- recoil velocity distribution parameters
axioms (2)
- domain assumption The observed M_BH-M_* relation represents the equilibrium state around which galaxies evolve.
- standard math Gravitational-wave recoils can remove the central black hole from its host galaxy for a cosmologically relevant time.
Lean theorems connected to this paper
-
IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We use the L-Galaxies-BH semi-analytical model and the Millennium suite of simulations to investigate the physical origin of galaxies hosting overmassive and undermassive MBHs with respect to the M_BH-M_* relation
-
IndisputableMonolith/Foundation/BranchSelection.leanbranch_selection unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
Overmassive MBHs are primarily associated with galaxies that experienced enhanced merger history and secular activity... Undermassive MBHs originate from two main channels... gravitational recoil... quiescent evolutionary history
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Forward citations
Cited by 1 Pith paper
-
Black Hole Binary Detection Landscape for the Laser Interferometer Lunar Antenna (LILA): Signal-to-Noise Calculations & Science Cases
LILA can detect IMBH binaries at redshifts 20-30, IMRIs, and provide months-to-years early warnings with high-SNR events for gravity tests.
Reference graph
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discussion (0)
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