arXiv subjects

Wei-Xiang Feng

Publications and source records attributed to Wei-Xiang Feng.

15 recordsLinked to original sources

arXiv2026

Small-Scale Clustering of Primordial Black Holes: The Little Red Dot Mass Function and the High-Redshift Galaxy Tension

Supermassive black holes (SMBHs) in "little red dots" (LRDs) discovered by the James Webb Space Telescope (JWST) may result from runaway mergers of primordial black holes (PBHs) in clusters---through long-short mode coupling on small scales in the early Universe. In this framework, we derive the SMBH mass function, together with the compactness and overmassive features of LRDs. We also estimate that the dense gas residing in PBH clusters is consistent with LRD observations. In addition, SMBHs formed from PBH clusters can help accelerate galaxy formation at high redshifts, thus alleviating tension with $Λ$CDM cosmology.

astro-ph.GA↗

arXiv2026

Dark Bondi Accretion Aided by Baryons and the Origin of JWST Little Red Dots

The gravothermal core collapse of self-interacting dark matter halos provides a mechanism for seeding supermassive black holes in the early Universe. We show that the collapse of a small fraction of the halo mass can trigger general-relativistic instability and produce black hole seeds in halos with masses $\gtrsim10^{9}{\rm\,M_\odot}$ at high redshift. We investigate whether this process can account for the origin of JWST little red dots at $z\sim4-11$, which host black holes with masses $\sim10^7{\rm\,M_\odot}$. We find that such masses can be reached within $\sim500{\rm\,Myr}$ through rapid core collapse followed by efficient accretion, even for initial seed masses of $1-10^3{\rm\,M_\odot}$. This scenario allows a substantial fraction of the black hole mass to originate from dark matter accretion, potentially offering a viable pathway to forming massive black holes at high redshift.

astro-ph.GA↗

arXiv2026

Smoluchowski Coagulation Equation and the Evolution of Primordial Black Hole Clusters

In arXiv:2507.07171, we demonstrate that the high-redshift supermassive black holes in the so-called "little red dots" discovered by James Webb Space Telescope (JWST) can be explained by the primordial black hole (PBH) clustering on small scales. In this paper, we present a comprehensive simulation of the successive PBH mergers within a cluster by solving the Smoluchowski coagulation equation. We derive the coagulation kernel considering both cases with and without the effects of mass segregation. Then we employ the Monte Carlo method to solve the equation, implementing the full-conditioning scheme using the discrete inverse transformation method. Our simulations determine the runaway timescales of clusters and the mass population evolution of PBHs across a wide range of cosmic redshifts, depending on the number of PBHs within the cluster and the associated density.

astro-ph.CO↗

arXiv2026

Black Hole Cold Brew: Fermi Degeneracy Pressure

We investigate the dynamical instability of a self-gravitating thermal system in the quantum regime, where Fermi degeneracy pressure becomes significant. Using a truncated Fermi-Dirac distribution and solving the Tolman-Oppenheimer-Volkoff equation, we identify marginally stable configurations following Chandrasekhar's criterion. While Fermi pressure stabilizes a system against gravitational collapse in Newtonian gravity, in general relativity it can instead drive the instability, enabling collapse even at low temperatures. In the low-temperature limit, the critical mass is independent of the boundary temperature. We discuss implications for the formation of massive black holes in the early Universe through the gravothermal collapse of dark matter.

gr-qc↗

arXiv2025

Formation of the Little Red Dots from the Core-collapse of Self-interacting Dark Matter Halos

We present a statistical study of black hole (BH) formation and growth seeded by gravothermal core collapse of self-interacting dark matter (SIDM) halos at high redshift, using a cosmological semi-analytical framework based on Monte Carlo merger trees. We demonstrate that gravothermal collapse naturally leads to BH formation in high-concentration halos at a characteristic mass scale set by the SIDM cross section, and occurs predominantly in the early Universe. This mechanism is particularly promising for explaining the abundance of the little red dots (LRDs) -- a population of early, apparently galaxy-less active galactic nuclei hosting supermassive BHs. By incorporating this seeding process with simple models of BH growth and assuming a 100% duty cycle, we reproduce the observed LRD mass function for velocity-dependent cross sections of $σ_{0m} \sim 30\,\mathrm{cm}^2\,\mathrm{g}^{-1}$ and $ω\sim 80\,\mathrm{km}\,\mathrm{s}^{-1}$, which are consistent with independent constraints from local galaxies. While higher values of $σ_{0m}$ (or $ω$) would overpredict the low-mass (or high-mass) end of the BH mass function, such deviations could be reconciled by invoking a reduced duty cycle or lower Eddington ratio. Our results suggest that the demographics of high-redshift BHs can serve as a novel and complementary probe of SIDM physics.

astro-ph.GA↗

arXiv2025

Little Red Dots from Small-Scale Primordial Black Hole Clustering

The James Webb Space Telescope (JWST) observations have identified a class of compact galaxies at high redshifts ($4 \lesssim z \lesssim 11$), dubbed "little red dots" (LRDs). The supermassive black holes (SMBHs) of $10^{5-8}{\rm\,M}_{\odot}$ in LRDs favor a heavy-seed origin. We propose a mechanism for their formation: Clusters of primordial black holes, formed through long-short mode coupling on small scales in the early Universe, undergo sequential mergers over extended timescales. This mechanism can evade cosmic microwave background distortions and result in heavy-seed SMBHs via runaway mergers. We employ Monte Carlo simulations to solve the Smoluchowski coagulation equation and determine the runaway merging timescale. The resulting stochastic gravitational wave background offers a distinct signature of this process, and the forming SMBHs can be highly spinning at their formation due to the spin residual of the cluster from tidal fields. This mechanism may explain the rapidly spinning SMBHs in LRDs under the assumption of obscured active galactic nuclei.

astro-ph.CO↗

arXiv2025

Gravitational Waves from Primordial Black Hole Dark Matter Spikes

The origin of the binary black hole mergers observed by LIGO--Virgo--KAGRA remains an open question. We calculate the merger rate from primordial black holes (PBHs) within the density spike around supermassive black holes (SMBHs) at the centers of galaxies. We show that the merger rate within the spike is comparable to that within the wider dark matter halo. We also calculate the extreme mass ratio inspiral (EMRI) signal from PBHs hosted within the density spike spiralling into their host SMBHs due to gravitational wave emission. We predict that LISA may detect $\sim10^4$ of these EMRIs with a signal-to-noise ratio threshold of 20 within a 4 yr observation run, if all dark matter is made up of $\sim30{\rm\,M}_\odot$ PBHs. Uncertainties in our rates come from the uncertain mass fraction of PBHs within the dark matter spike, relative to the host central SMBHs, which defines the parameter space LISA can constrain.

astro-ph.CO↗

arXiv2025

Seeding Supermassive Black Holes with Self-Interacting Dark Matter: A Unified Scenario with Baryons

Observations show that supermassive black holes (SMBHs) with a mass of $\sim10^9 M_\odot$ exist when the Universe is just $6\%$ of its current age. We propose a scenario where a self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole. The presence of baryons in protogalaxies could significantly accelerate the gravothermal evolution of the halo and shorten collapse timescales. The central halo could dissipate its angular momentum remnant via viscosity induced by the self-interactions. The host halo must be on high tails of density fluctuations, implying that high-$z$ SMBHs are expected to be rare in this scenario. We further derive conditions for triggering general relativistic instability of the collapsed region. Our results indicate that self-interacting dark matter can provide a unified explanation for diverse dark matter distributions in galaxies today and the origin of SMBHs at redshifts $z\sim6-7$.

astro-ph.CO↗

arXiv2023

On the Dynamical Instability of Monatomic Fluid Spheres in (N+1)-Dimensional Spacetime

In this note, I derive the Chandrasekhar instability of a fluid sphere in ($N$+1)-dimensional Schwarzschild-Tangherlini spacetime and take the homogeneous (uniform energy density) solution for illustration. Qualitatively, the effect of positive (negative) cosmological constant tends to destabilize (stabilize) the sphere. In the absence of cosmological constant, the privileged position of (3+1)-dimensional spacetime is manifest in its own right. As it is the marginal dimensionality in which a monatomic ideal fluid sphere is stable but not too stable to trigger the onset of gravitational collapse. Furthermore, it is the unique dimensionality that can accommodate stable hydrostatic equilibrium with positive cosmological constant. However, given the current cosmological constant observed no stable configuration can be larger than $10^{21}~{\rm M}_\odot$. On the other hand, in (2+1) dimensions it is too stable either in the context of Newtonian Gravity (NG) or Einstein's General Relativity (GR). In GR, the role of negative cosmological constant is crucial not only to guarantee fluid equilibrium (decreasing monotonicity of pressure) but also to have the Ba{ñ}ados-Teitelboim-Zanelli (BTZ) solution. Owing to the negativeness of the cosmological constant, there is no unstable configuration for a homogeneous fluid disk with mass $0<\mathcal{M}\leq0.5$ to collapse into a naked singularity, which supports the Cosmic Censorship Conjecture. However, the relativistic instability can be triggered for a homogeneous disk with mass $0.5<\mathcal{M}\lesssim0.518$ under causal limit, which implies that BTZ holes of mass $\mathcal{M}_{\rm BTZ}>0$ could emerge from collapsing fluid disks under proper conditions. The implicit assumptions and implications are also discussed.

gr-qc↗

arXiv2022

Self-interacting Dark Scalar Spikes around Black Holes via Relativistic Bondi Accretion

We consider the spike mass density profile in a dark halo by self-consistently solving the relativistic Bondi accretion of dark matter onto a non-spining black hole of mass $M$. We assume that the dominant component of the dark matter in the halo is a Standard model gauge-singlet scalar. Its mass $m\simeq 10^{-5}{\rm eV}$ and quartic self-coupling $λ\lesssim10^{-19}$ are constrained to be compatible with the properties of galactic dark halos. In the hydrodynamic limit, we find that the accretion rate is bounded from below, $\dot{M}_{\rm min}=96πG^2M^2 m^4/λ\hbar^3$. Therefore, for $M=10^6~{\rm M}_\odot$ we have $\dot{M}_{\rm min}\simeq1.41\times 10^{-9}~{\rm M}_\odot~{\rm yr}^{-1}$, which is subdominant compared to the Eddington accretion of baryons. The spike density profile $ρ_0(r)$ within the self-gravitating regime cannot be fitted well by a single-power law but a double-power one. Despite that, we can fit $ρ_0(r)$ piecewise and find that $ρ_0(r) \propto r^{-1.20}$ near the sound horizon, $ρ_0(r) \propto r^{-1.00}$ towards the Bondi radius and $ρ_0(r) \propto r^{-1.08}$ for the region in between. This contrasts with more cuspy $ρ_0(r) \propto r^{-1.75}$ for dark matter with Coulomb-like self-interaction.

astro-ph.HE↗

arXiv2022

Gravothermal Phase Transition, Black Holes and Space Dimensionality

In the framework of gravothermal evolution of an ideal monatomic fluid, I examine the dynamical instability of the fluid sphere in ($N$+1) dimensions by exploiting Chandrasekhar's criterion to each quasistatic equilibrium along the sequence of the evolution. Once the instability is triggered, it would probably collapse into a black hole if no other interaction halts the process. From this viewpoint, the privilege of (3+1)-dimensional spacetime is manifest, as it is the marginal dimensionality in which the ideal monatomic fluid is stable but not too stable. Moreover, it is the unique dimensionality that allows stable hydrostatic equilibrium with positive cosmological constant. While all higher dimensional ($N>3$) spheres are genuinely unstable. In contrast, in (2+1)-dimensional spacetime it is too stable either in the context of Newton's theory of gravity or Einstein's general relativity. It is well known that the role of negative cosmological constant is crucial to have the Bañados-Teitelboim-Zanelli (BTZ) black hole solution and the equilibrium configurations of a fluid disk. Owing to the negativeness of the cosmological constant, there is no unstable configuration for a homogeneous fluid disk to collapse into a naked singularity, which supports the cosmic censorship conjecture. However, BTZ holes of mass $\mathcal{M}_{\rm BTZ}>0$ could emerge from collapsing fluid disks. The implications of spacetime dimensionality are briefly discussed.

gr-qc↗

arXiv2022

Dynamical Instability of Collapsed Dark Matter Halos

A self-interacting dark matter halo can experience gravothermal collapse, resulting in a central core with an ultrahigh density. It can further contract and collapse into a black hole, a mechanism proposed to explain the origin of supermassive black holes. We study dynamical instability of the core in general relativity. We use a truncated Maxwell-Boltzmann distribution to model the dark matter distribution and solve the Tolman-Oppenheimer-Volkoff equation. For given model parameters, we obtain a series of equilibrium configurations and examine their dynamical instability based on considerations of total energy, binding energy, fractional binding energy, and adiabatic index. Our numerical results indicate that the core can collapse into a black hole when the fractional binding energy reaches $0.035$ with a central gravitational redshift of $0.5$. We further show for the instability to occur in the classical regime, the boundary temperature of the core should be at least $10\%$ of the mass of dark matter particles; for a $10^9~{\rm M_\odot}$ seed black hole, the particle mass needs to be larger than a few keV. These results can be used to constrain different collapse models, in particular, those with dissipative dark matter interactions.

astro-ph.CO↗

arXiv2020

Astrophysical Evidence of Wakefield Acceleration in Galactic and Extragalactic Jets via Gamma Rays and UHECRs

We present six case studies from a broad mass range ($1 - 10^9$ $M_\odot$) of astrophysical objects, each of which exhibit signs of jets and emit intense high energy gamma rays ($>10$ GeV). Many of these objects also emit spatially identifiable ultra high energy cosmic rays (UHECRs). In all cases it is found that wakefield acceleration (WFA) explains both the global properties and details. For blazars, we also explain the temporal structure of these signals, which includes neutrinos, and the correlations in their "bursts" and anti-correlation in flux and index. Blazars ($\sim 10^9$ $M_\odot$), radio galaxies ($\sim 10^8\, M_{\odot}$), Seyfert galaxies ($\sim 10^6 \,M_{\odot}$), starburst galaxies ($\sim 10^{3}\, M_{\odot}$), down to microquasars ($1 \sim 10$ $M_\odot$) interestingly exhibit the same physics since the nature of the accretion and acceleration is independent of the mass, aside from maximum values. It is possible to accelerate electrons to energies much greater than $10$ GeV, and protons beyond $10^{20}$ eV with WFA. We compare observational values with theoretical ones to illustrate they are in good agreement. This mechanism is also accompanied by related emissions, such as high-energy pin-pointed neutrinos, time varying radio, optical, and X-ray emissions, opening an opportunity to characterize these astrophysical objects via multi-messenger approaches.

astro-ph.HE↗

arXiv2019

The Buchdahl Stability Bound in Eddington-inspired Born-Infeld Gravity

We give the Buchdahl stability bound in Eddington-inspired Born-Infeld (EiBI) gravity. We show that this bound depends on an energy condition controlled by the model parameter $κ$. From this bound, we can constrain $κ\lesssim 10^{8}\text{m}^2$ if a neutron star with a mass around $3M_{\odot}$ is observed in the future. In addition, to avoid the potential pathologies in EiBI, a \emph{Hagedorn-like} equation of state associated with $κ$ at the center of a compact star is inevitable, which is similar to the Hagedorn temperature in string theory.

gr-qc↗

arXiv2017

Equation of State of Neutron Stars with Junction Conditions in the Starobinsky Model

We study the Starobinsky or $R^2$ model of $f(R)=R+αR^2$ for neutron stars with the structure equations represented by the coupled differential equations and the \emph{polytropic} type of the matter equation of state. The junction conditions of $f(R)$ gravity are used as the boundary conditions to match the Schwarschild solution at the surface of the star. Based on these the conditions, we demonstrate that the coupled differential equations can be solved \emph{directly}. In particular, from the dimensionless equation of state $\barρ = \bar{k}\, \bar{p}^{\,γ}$ with $\bar{k}\sim5.0$ and $γ\sim0.75$ and the constraint of $α\lesssim {1.47722}\times 10^{7}\, \text{m}^2$, we obtain the \emph{minimal} mass of the NS to be around 1.44 $M_{\odot}$. In addition, if $\bar{k}$ is larger than 5.0, the mass and radius of the NS would be smaller.

gr-qc↗