arXiv subjects

Kazutaka Kimura

Publications and source records attributed to Kazutaka Kimura.

6 recordsLinked to original sources

arXiv2026

Blue-tilted spectral running and the JWST early galaxy tension

Recently, the James Webb Space Telescope (JWST) collaboration has found the unexpectedly large abundance of massive galaxies with stellar masses of $\sim 10^{10}~M_{\odot}$ at high redshifts $z \simeq 6.5 - 9$ compared with the prediction of the standard $Λ$CDM model. As a possible solution to the tension, we consider a blue-tilted spectrum of density perturbations with a positive running. We find that, for $α_s \simeq 0.02$ and $β_s \simeq 0.02$, a joint analysis with CMB observations shows that the tension can be resolved at the 1$σ$ confidence level. Such a blue-tilted spectrum is also plausible from the perspective for formations of primordial black holes on much smaller scales in the early Universe.

astro-ph.CO↗

arXiv2026

Force convergence in Monte Carlo Lyman-alpha radiative transfer

Monte Carlo radiative transfer (MCRT) is widely used to model Lyman-alpha (Lya) resonant-line transport, but convergence is difficult to assess in optically thick media where photons undergo many scatterings before escape. This is especially important for internal quantities such as radiative acceleration and the force multiplier, which depend on momentum deposition throughout the gas rather than only on emergent spectra. We study the convergence of Lya MCRT momentum-transfer estimators in static spherical clouds. We first establish diffusion-limit benchmarks for radial acceleration profiles and integrated force multipliers, then develop a moment-based framework for diagnosing convergence from the photon-packet contribution distribution. This framework separates three distinct questions: whether the estimator converges to the correct mean, how large its finite-sampling uncertainty is, and whether the estimated uncertainty is itself stable. We apply this hierarchy to the direct event-based scattering estimator, a gradient-of-energy-density estimator, and a divergence-of-radiation-pressure estimator. Zeroth-order convergence is assessed with profile comparisons, integrated force-multiplier bias, and finite-group relative error. First-order convergence is quantified with fractional error, the photon number required to reach a target precision, and the corresponding runtime requirement. Second-order convergence is tested with the coefficient of variation of variance, which measures the reliability of the variance estimate used in the first-order diagnostics. Core-skipping prescriptions, source geometry, estimator construction, and spatial resolution enter this hierarchy in different ways. Our results provide a practical convergence framework for internal Lya MCRT force calculations and show why statistical precision, computational cost, and physical accuracy must be evaluated separately.

astro-ph.GA↗

arXiv2026

An Explicit M1 Radiation-hydrodynamics Scheme for 3D Protostellar Evolution

We present a radiation-hydrodynamics (RHD) scheme that enables 3D simulations resolving both protostellar interiors and their surrounding accretion flows within a single framework, to clarify how a protostar evolves while interacting with the accretion flow. The method builds on an explicit two-moment M1 closure scheme with a reduced speed of light approximation (RSLA) for massively parallel computation. Our scheme introduces a complementary non-RSLA radiation component that dominates in optically thick regions. This hybrid treatment restores physical energy conservation inside protostars, which would otherwise be violated under the RSLA, while retaining the advantage of large time steps. To overcome the limitation of the conventional M1 closure in solving radiative transfer in extremely optically thick regions inside protostars and across steep optical-depth gradients near their surfaces, we incorporate the optical-depth information of neighboring cells into the radiative transfer calculation. We further evolve photon-number densities in addition to radiation energy densities to reconstruct an effective local spectrum on the fly without resorting to costly multi-frequency transport. We implement this scheme in the adaptive mesh refinement code SFUMATO and verify its validity through a series of test calculations. As an application, we follow the early evolution of a massive protostar formed at high redshift, within a full cosmological context. The results reveal a continuous structure connecting the swollen protostar and its surrounding disk, which cannot be captured in conventional 1D models. This RHD scheme opens a path to studies of protostellar evolution and its interaction with the accretion flow in realistic 3D environments.

astro-ph.SR↗

arXiv2025

Massive Black Hole Seed Formation in Strong X-ray Environments at High Redshift

Direct collapse of pristine gas in early galaxies is a promissing pathway for forming supermassive black holes (BHs) powering active galactic nuclei (AGNs) at the epoch of reionization (EoR). This seeding mechanism requires suppression of molecular hydrogen (H$_2$) cooling during primordial star formation via intense far-ultraviolet radiation from nearby starburst galaxies clustered in overdense regions. However, non-detection of 21 cm signals from the EoR reported by the Hydrogen Epoch of Reionization Array (HERA) experiment suggests that such galaxies may also emit X-rays more efficiently than in the local universe, promoting H$_2$ production and thereby potentially quenching massive BH seed formation. In this study, we examine the thermal and chemical evolution of collapsing gas in dark matter halos using a semi-analytic model incorporating observationally calibrated X-ray intensities. We find that strong X-ray irradiation, as suggested by HERA, significantly suppresses direct collapse and leads most halos to experience H$_2$ cooling. Nevertheless, massive BH seeds with $M_\mathrm{BH} \gtrsim 10^4~M_\odot$ still form by $z\simeq 15$, particularly in regions with baryonic streaming motion, and their abundance reaches $\sim 10^{-4}~\mathrm{Mpc}^{-3}$ sufficient to explain the SMBHs identified by JWST spectroscopy at $3<z<6$. While the formation of highly overmassive BHs with masses comparable to their host galaxies is prohibited by X-ray ionization, our model predicts that BH-to-stellar mass ratios of $\simeq 0.01-0.1$ were already established at seeding.

astro-ph.GA↗

arXiv2023

3D Radiation-Hydrodynamic Simulations Resolving Interior of Rapidly Accreting Primordial Protostar

Direct collapse of supermassive stars is a possible pathway to form supermassive black hole seeds at high redshifts. Whereas previous three-dimensional (3D) simulations demonstrate that supermassive stars form via rapid mass accretion, those resolving the stellar interior have been limited. We here report 3D radiation-hydrodynamic (RHD) simulations following the evolution of rapidly accreting protostars resolving the stellar interior. We use an adaptive mesh refinement code with our newly developed RHD solver employing an explicit M1 closure method. We follow the early evolution until the stellar mass reaches $\sim 10~M_\odot$ from two different initial configurations of spherical and turbulent clouds. We demonstrate that, in both the cases, a swollen protostar whose radius is $100\mathrm{-}1000~R_\odot$ appears, as predicted by the stellar evolution calculations. Its effective temperature remains a few thousand Kelvin, and the radiative feedback by ionizing photons is too weak to disturb the accretion flow up to the epoch examined in this work. In the turbulent case, the protostar rotates rapidly at more than 0.4 times the Keplerian velocity owing to the angular momentum provided by the initial turbulence. The protostar approximates an oblate spheroid, and its equatorial radius is more than twice the polar radius. Our results suggest that we need to consider the rapid stellar rotation to elucidate the realistic 3D protostellar evolution in the supermassive star formation.

astro-ph.GA↗

arXiv2021

Growth of Massive Disk and Early Disk Fragmentation in the Primordial Star Formation

Recent high-resolution simulations demonstrate that disks around primordial protostars easily fragment in the accretion phase before the protostars accrete less than a solar mass. To understand why the gravitational instability generally causes the fragmentation so early, we develop a one-dimensional (1D) non-steady model of the circumstellar disk that takes the mass supply from an accretion envelope into account. We also compare the model results to a three-dimensional (3D) numerical simulation performed with a code employing the adaptive mesh refinement. Our model shows that the self-gravitating disk, through which the Toomre $Q$ parameter is nearly constant at $Q \sim 1$, gradually spreads as the disk is fed by the gas infalling from the envelope. We further find that the accretion rate onto the star is an order of magnitude smaller than the mass supply rate onto the disk. This discrepancy makes the disk more massive than the protostar in an early evolutionary stage. Most of the infalling gas is used to extend the outer part of the self-gravitating disk rather than transferred inward toward the star through the disk. We find that similar evolution also occurs in the 3D simulation, where the disk becomes three times more massive than the star before the first fragmentation occurs. Our 1D disk model well explains the evolution of the disk-to-star mass ratio observed in the simulation. We argue that the formation of such a massive disk leads to the early disk fragmentation.

astro-ph.GA↗