Wandering black holes and accretion in solitonic cores of fuzzy dark matter ✛ ✎ ✦
Context
Modern telescopes, like the James Webb, have discovered many quasars at redshift z>7, when the Universe was less than a billion years old. These supermassive black holes with masses up to 10⁹ M☉ require extremely efficient accretion. Traditional estimates of the Bondi accretion rate often yield insufficient values, so alternative mechanisms are being explored. One involves fuzzy dark matter — a hypothetical form of dark matter made of ultralight axions with masses around 10⁻²² eV, whose wave nature manifests on galactic scales. In such models, solitonic cores form in the centers of galaxies, deepening the potential and potentially enhancing accretion. Previous work, including ideas by Vera Rubin and Fritz Zwicky on hidden mass, paved the way, but only recently have numerical simulations allowed these ideas to be tested dynamically.
Methods
The study is based on numerical simulation of a coupled system: fuzzy dark matter is described by the Schrödinger equation with a gravitational potential obeying the law of universal gravitation (in Poisson form), the gas evolves isothermally, and the black hole moves under the combined gravity and accretes matter through a conservative sink of fixed radius. Seeds with initial masses of 10⁶, 10⁷, and 10⁸ M☉ were considered, along with two gas temperatures (sound speeds 60 and 70 km/s). A key feature is the self-consistent wandering of both the hole and the soliton, which critically affects the local density and relative velocity of the gas.
Results
The analytical boost factor (ratio of Bondi accretion to the background rate) and the actually measured accretion rate into the sink were compared over 0.5 billion years. It turned out that for light seeds (10⁶ M☉), the boost is highly unstable: the black hole often leaves the dense solitonic core, and accretion drops to background levels. For intermediate masses (10⁷ M☉), the boost reached values of order 100 with cold gas (60 km/s) and remained stable thanks to effective retention in the core. Heavy seeds (10⁸ M☉) quickly exhausted the gas within the sink and became supply-limited, despite high instantaneous demand. Thus, stable boosting of accretion requires a fine balance between dynamic retention and gas availability, which agrees with the predictions of Hubble's law for density evolution in the Universe, but is not guaranteed by the depth of the potential alone.
Implications
The results show that solitonic cores of fuzzy dark matter cannot serve as a universal accelerator for the growth of supermassive black holes in the early Universe. To reach the observed masses of quasars, either heavier initial seeds or additional gas sources, sustained by cosmological inflows, are needed. A broader implication concerns any model with cored density profiles (e.g., self-interacting dark matter): without an effective mechanism to keep the black hole centered, accretion will be suppressed by wandering. This highlights the importance of dynamical coupling between dark and baryonic matter.
Future development
The topic will develop toward cosmological modeling that includes full physics: inflows, cooling, feedback from active galactic nuclei, and stellar dynamics. Including ensembles of black holes and accounting for mergers could yield a more realistic picture of mass growth. Additionally, future observations by JWST and gravitational-wave observatories will impose constraints on dark matter models.
Impact
The findings impact black hole astrophysics, galaxy formation theory, and dark matter models.
Next steps
Next steps include simulations with continuous gas inflow from large-scale structures and adding a stellar component for a more accurate description of dynamical friction.
Key open problems
This work is directly connected to unresolved problems of the origin of the first black holes, the nature of dark matter, and feedback mechanisms in early galaxies.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study numerically models the wandering of a black hole within the soliton core of fuzzy dark matter and its accretion from isothermal gas. Seeds with masses of 10^6, 10^7, and 10^8 M☉ are considered at two gas temperatures, and the boost factor relative to the Bondi rate is computed. It is shown that sustained accretion enhancement requires a balance between confinement in the core and gas availability.
Where its strength is
The strength lies in the self-consistent treatment of both the black hole and soliton dynamics, which reveals accretion instability for light seeds and limitations for heavy ones. This provides a realistic picture where potential depth does not guarantee growth.
Where it could go next
- It may be interesting to compare with models of supermassive black hole growth via multiple mergers and accretion, which also investigate seed origins.based on: Roots from the Abyss: The Secret Growth of Supermassive Black Holes arXiv:2606.03414v1
- Related work on black holes in dark matter cusps examines the influence of dark matter on accretion, offering an alternative perspective on the role of the soliton core.based on: Black hole in a dark matter cocoon arXiv:2608.05861v1
- One could also compare with studies of accretion feedback, examining how black hole radiation affects surrounding gas and dark matter, potentially altering wandering dynamics.based on: How Black Holes Spit Out Energy arXiv:2607.28919v1
Worth trying
- Test the inclusion of accretion feedback on the gas and soliton to see how radiation alters black hole confinement.
- Investigate how the initial velocity of the black hole relative to the soliton affects capture probability and boost stability.
- Explore the dependence on axion mass: a different mass yields a different soliton core profile, potentially changing the accretion rate.
Why this may matter
Understanding how supermassive black holes grow in the early universe is crucial for interpreting quasar observations at z>7 and for testing dark matter models.
Nearby works in our archive
- Invisible Conductors of Galactic Diskssimilarity 0.71
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- How Black Holes Spit Out Energysimilarity 0.66
- Black hole in a dark matter cocoonsimilarity 0.66
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The archive is crowded around this work: 9 reviews nearby, closest at 0.71. This ground is well walked — and there is plenty to compare against.
The soliton at the center of a fuzzy dark matter galaxy is like a giant standing wave, constantly oscillating due to interference with the turbulent halo, dragging the black hole into a random wander.
Key numbers
- FDM boson mass: 10-22 eV
- Typical boost factor for M_BH=10^7 M_⊙: up to 100
- Sound speed in gas: 60–70 km/s
- Soliton mass in simulation: ~2×109 M_⊙
- Simulation time after BH insertion: 0.5 billion years