arXiv subjects

Izumi Seno

Publications and source records attributed to Izumi Seno.

4 recordsLinked to original sources

arXiv2026

Modeling the Accretion of High-Velocity Clouds from a Rotating Halo

High-Velocity Clouds (HVCs) are a major fuel reservoir for star formation in the Galactic disk. Determining their origin and kinematics is thus crucial for understanding Galactic evolution. In this paper, we employ simple test-particle simulations to model HVC kinematics, generating line-of-sight velocity maps and probability density functions (PDFs) for comparison with observational results. We find that models assuming low angular momentum and an initial scale of tens of kiloparsecs (kpc) successfully reproduce the observed kinematic trends for both blue-shifted and red-shifted components. This consistency may support the dominance of intermediate-halo dynamics (tens of kpc scale) in regulating Galactic evolution, consistent with HVC formation via thermal instability in metal-polluted gas in the halo. Furthermore, by considering the entire bulk mass involved in the continuous accretion process -- including diffuse or ionized components that often escape direct observation -- our theoretical estimates yield a total mass accretion rate of several solar masses per year. This indicates that HVC accretion has the potential to supply a sufficient amount of gas to the Galactic disk to sustain ongoing star formation over several Gyr. Our findings suggest that the Galactic baryon cycle and disk evolution are governed by dynamics within the intermediate halo, providing key kinematic constraints for future magnetohydrodynamical simulations that resolve spatial structures of high velocity clouds.

astro-ph.GA↗

arXiv2025

Linear Analyses of Thermal Instability in Stratified Medium

Thermal instability in the circum-galactic medium (CGM) can be responsible for the existence of cold clouds (e.g., high-velocity clouds) embedded in a hot diffuse medium (e.g., X-ray emitting gas). While many previous studies have analyzed thermal instability in uniform medium, the instability mechanism in gravitationally stratified medium like CGM has not been fully analyzed. This study investigates how gravity affects the behavior of thermal instability through linear perturbation analyses.We find that in stratified medium, thermal instability can drive over-stable modes, a behavior distinctly different from the monotonic growth of thermal instability in a uniformmedium. Furthermore, we find that the combination of buoyancy and thermal instability drives other two unstable modes. Applying our results to a simplified model of the CGM, we estimate the gas accretion rate from the CGM to the Galactic disk and the typical size of high-velocity cloud driven by thermal instability to be a few solar masses per year. This gas accretion rate is comparable to the observed star formation rate, and hence, the mass in the Galactic disk can be maintained. Our results provide a theoretical framework for understanding the formation of multi-phase gas, particularly in the CGM.

astro-ph.GA↗

arXiv2025

Special Relativistic Smoothed Particle Hydrodynamics Based on Riemann Solver

This paper proposes a novel numerical method based on Godunov Smoothed Particle Hydrodynamics for special relativistic fluid dynamics. Our method utilizes a Riemann solver to describe shock, enhancing accuracy in strong shock waves. The formulation maintains conservation laws and achieves higher accuracy through convolution integrals that define physical quantities for SPH particles. We also propose the number density calculation method that uses a non-equal baryon number in each SPH particle and variable smoothing length in a way different from the conventional method. Numerical experiments demonstrate the method's robustness across one- and two-dimensional relativistic shock tube problems, as well as its ability to simulate Kelvin-Helmholtz instabilities accurately, validating SRGSPH as a reliable approach for high-resolution relativistic simulations.

physics.comp-ph↗

arXiv2025

Analytical and Numerical Linear Analyses of Convection Revisited

We conduct linear analyses of convection in domains larger than the temperature scale height. We employ both analytical and numerical methods in these analyses. In the case excluding all dissipation, the typical time scale of convection is determined by the free fall time over the temperature scale height. We quantitatively show the condition for the Boussinesq and Wentzel-Kramers-Brillouin (WKB) approximations to be applicable. We provide a reassessment of the critical Rayleigh number, a key indicator of convection, and show that WKB approximation tends to underestimate the critical Rayleigh number, particularly when the temperature scale height is comparable to or smaller than the domain height. We show clear explanation why both thermal conduction and viscosity are required for stabilizing negative entropy gradient medium.

physics.flu-dyn↗