arXiv subjects

Michiko S. Fujii

Publications and source records attributed to Michiko S. Fujii.

At least 19 recordsLinked to original sources

arXiv2026

Hidden in Pixels. I. Discovery of dual "little red dots" indicates excess clustering on kilo-parsec scales

``Little Red Dots'' (LRDs) are an abundant high-redshift population newly discovered by the James Webb Space Telescope (JWST) and considered to be an early growth phase of supermassive black holes (SMBHs). Using a method of pixel-by-pixel color selection and relaxing the compactness criteria, we identify four dual LRD candidates in the COSMOS-Web survey with projected separations of $0.\!\!^{\prime\prime}2$-$1.\!\!^{\prime\prime}2$. A comparison between existing LRD samples and mock data reveals that the projected separations of these dual LRD candidates are unlikely to result from chance projections of objects at different redshifts. Furthermore, two of the four systems are covered by COSMOS-3D slitless spectroscopy, and a single-line detection at the same observed wavelength for each LRD in a pair strongly supports that they are at identical redshifts. Assuming that the detected lines are H$α$ based on their high equivalent width and broad profile, the spectroscopic redshifts of $z=5.822$ and $5.464$ for the two pairs are consistent with their photometric redshifts, yielding projected separations of $1.64$ and $7.36\,{\rm kpc}$. These discoveries suggest that the angular auto-correlation function (ACF) of LRDs exhibits an excess ($\sim20$-$30$ times) on sub-arcsec (kilo-parsec) separations compared to an extrapolation of a power-law ACF of JWST-found AGNs measured over $10^{\prime\prime}$-$100^{\prime\prime}$. Our sample is likely to represent precursors of mergers between LRDs, and such mergers may be one of the mechanisms that can drive the rapid growth of SMBHs in their early evolutionary stages.

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arXiv2026

SIRIUS Project: Dynamical Evolution of Primordial Binaries during Star Cluster Formation

Binary populations are closely linked to the star formation process; however, their primordial properties can be changed by subsequent dynamical interactions within their natal clusters. The aim of this study is to clarify how different primordial binary populations affect the evolution of multiplicity and the global structure of forming star clusters. We investigate the dynamical evolution of primordial binaries during star cluster formation using self-consistent Nbody/smoothed particle hydrodynamics simulations that follow the collapse of a molecular cloud to a star cluster. We systematically compare three star formation models: close binary (CB), wide binary (WB), and single star (SS) formation model. In CB and WB models, the multiplicity fraction decreases with time due to dynamical interactions. In particular, the fraction in the WB model drops to a level comparable to that in the SS model. The multiplicity fraction of high-mass stars is similarly high in all models, whereas only the CB model shows a relatively high fraction for low-mass stars. Due to the assumption of equal-mass binary formation, the CB and WB models exhibit an excess at q= 1 in the mass-ratio distribution, while the SS model has no clear trend. Frequent few-body interactions generate distinct stellar populations inside and outside the cluster: the multiplicity fraction within the cluster is systematically higher, while mass functions in the outside have a shallower slope. Finally, stellar density profiles in the clusters are broadly similar among all models. The primordial binary population significantly affects the final binary properties, while having only a limited impact on their host cluster structures. Our results suggest that close binaries need to form at the star formation stage to reproduce the observed multiplicity fraction of low-mass stars and the excess of equal-mass binaries.

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arXiv2026

Linking dynamics and chemistry in barred galaxies through action-space mapping

We aim to extend the action-based framework, proposed by a previous study, for assigning chemical information to collisionless $N$-body simulations of barred galaxies, enabling more realistic comparisons with observations. We developed a new method to map chemistry of star-forming $N$-body+hydrodynamical simulations (donor models) onto pure $N$-body simulations (target models) using a matching algorithm. We applied the method to two barred $N$-body galaxies, one Milky Way (MW)-like with a classical bulge and another with a strongly buckled bar, combined with two star-forming donor models exhibiting bimodal and unimodal chemical tracks in the [Fe/H]-[$α/$Fe] plane. Our models reproduce key chemo-dynamical trends observed in the MW and external galaxies. Metal-rich populations are more strongly associated with the X-shaped bulge morphology, leading to vertically pinched metallicity maps consistent with observations and chemo-hydrodynamical simulations. The [Fe/H]-[O/Fe] distributions show latitude-dependent bimodality, and the mock magnitude distributions of red clump stars reveal a stronger X-shape for metal-rich populations. In the model with a classical bulge, metal-poor stars exhibit a more spherical morphology on the longitude-latitude plane, while metal-rich stars show a boxy distribution, consistent with observations of the MW bulge. The proposed action-based chemical assignment provides a computationally efficient and flexible approach to link the dynamical and chemical evolution of barred galaxies. It enables realistic chemo-dynamical modelling of the galactic bulges.

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arXiv2026

Simulating Star Formation and Star Cluster Assembly in the Aquila Rift Using Archival Observations

We simulate star formation and star cluster assembly inside a molecular cloud with parameters we derive directly from observations of the Aquila Rift. We model the evolution of stars and gas together while resolving close encounters between stars, the formation of new stars, and stellar feedback to follow cluster formation up to the expulsion of the surrounding gas. We find that star formation takes place in clumps spaced unevenly along Serpens South and that these clumps accrete surrounding gas to grow and form new stars. Gas flows along the filament promote the merger of these clumps into a star cluster inside the Serpens South filament. The imprints of these mergers are seen in the dynamics of the Serpens South cluster in the form of velocity space anisotropies, cluster rotation, and cluster expansion. Before gas is removed from the simulation, the Serpens South cluster merges with the nearby cluster W40 non-monolithically resulting in a fractal cluster at the end of the simulation. The dynamics inherited from the mergers throughout the simulation are still seen in the final bound stellar system after the gas has been removed. We compare these results with recent observations of Milky Way clusters to comment on their formation histories. We also study how our results change when lowering the mass resolution of our simulation and removing observations of dense gas tracers from our initial condition setup. Each of the three simulations result in different final cluster configurations pointing towards the importance of gas in cluster assembly.

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arXiv2026

GW231123 Formation from Population III Stars: Isolated Binary Evolution

GW231123 is a merger of two black holes (BHs) with estimated masses exceeding $100\;{\rm M}_{\odot}$, making them the most massive BHs discovered to date via gravitational wave (GW) observations. We investigate whether GW231123-like events can originate from isolated Population (Pop) III binary stars using binary population synthesis calculations. Our findings indicate that isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the $^{12}\text{C}(α, γ)^{16}\text{O}$ rate is $2σ$ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521 -- the most massive BH merger in the Gravitational Wave Transient Catalog 3 -- can be formed from isolated Pop III binaries even with the standard $^{12}\text{C}(α, γ)^{16}\text{O}$ rate. We demonstrate that the discovery of GW231123 is increasingly constraining the parameter ranges of single star evolution models, under the assumption that these GW events originate from isolated binary evolution.

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arXiv2026

Nitrogen enhancement of GN-z11 by metal pollution from supermassive stars

Spectroscopic observations by the James Webb Space Telescope (JWST) have revealed young, compact, high-redshift ($z$) galaxies with high nitrogen-to-oxygen (N/O) ratios. GN-z11 at z=10.6 is one of these galaxies. One possible scenario for such a high N/O ratio is pollution from supermassive stars (SMSs), from which stellar winds are expected to be nitrogen-rich. The abundance pattern is determined by both galaxy evolution and SMS pollution, but so far, simple one-zone models have been used. Using a galaxy formation simulation, we tested the SMS scenario. We used a cosmological zoom-in simulation that includes chemical evolution driven by rotating massive stars (Wolf-Rayet stars), supernovae, and asymptotic giant branch stars. As a post-process, we assumed the formation of an SMS with a mass between $10^3$ and $10^5$ $M_\odot$ and investigated the contribution of its ejecta to the abundance pattern. The N/O ratio was enhanced by the SMS ejecta, and the abundance pattern of GN-z11, including carbon-to-oxygen and oxygen-to-hydrogen ratios, was reproduced by our SMS pollution model if the pollution mass fraction ranges within 10-30 per cent. Such a pollution fraction can be realized when the gas ionized by the SMS is polluted, and the gas density is $10^4$-$10^5$ cm$^{-3}$ assuming a Strömgren sphere. We also compared the abundance pattern with those of other N/O-enhanced high-$z$ galaxies. Some of these galaxies can also be explained by SMS pollution.

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arXiv2026

SIRIUS: Dark matter cusp evolution in dense dwarf galaxies

Dwarf galaxies have a wide variety of structures, such as dark matter (DM) distribution, stellar-to-halo mass ratio, and stellar density. Recent high-resolution simulations have shown a variety of stellar-to-halo mass ratios for dwarf galaxies with a DM halo mass of $\sim 10^9 M_{\odot}$ at $z=0$. In this study, we performed cosmological $N$-body/smoothed-particle hydrodynamic zoom-in simulations of dwarf galaxies with the highest gas and DM particle mass resolutions of 2.37 $M_{\odot}$ and 12.8 $M_{\odot}$, respectively. The stellar-to-DM halo mass ratio of one of our simulated dwarf galaxies was $\sim 10^{-4}$, typical for satellites of the Milky Way. The stellar mass ($10^5 M_{\odot}$) and half-mass radius (68 pc) were also similar to those of the satellites of the Milky Way. The power-law slope of the DM halo was $α= -1.1$. On the other hand, the other simulated galaxy exhibited a stellar-to-halo mass ratio of $\sim 10^{-3}$ and a steeper power-law slope ($α=-1.9$) than the other; the presence of baryonic matter deepened the cusp. The mass of $>10^6 M_{\odot}$ and a half-mass radius of $\sim 36$ pc of this galaxy were similar to those of ultra-compact dwarf galaxies rather than the satellites of the Milky Way. This DM halo grew in mass earlier than the former one, and the central DM density was higher than that of the other even in the DM-only simulations.

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arXiv2025

Bursty star formation, chemical enrichment, and star cluster formation in numerical analogues of GN-z11

The James Webb Space Telescope reveals anomalous nitrogen enrichment (high N/O ratios) in compact, star-forming galaxies, such as GN-z11 at $z\sim10$. The origin of this chemical signature provides an insight into the early star and galaxy formation processes, yet remains unclear. We performed high-resolution cosmological zoom-in simulations of massive galaxies at high redshift ($z\sim10$) in rare density peaks, incorporating various chemical evolution channels including stellar winds, core-collapse, Type Ia supernovae, and asymptotic giant branch stars. Our simulations reproduce several key features of high-redshift galaxies: (1) stars form with high efficiencies ($>0.1$) at the center of rare peak halos, creating very compact galaxies similar to GN-z11; (2) high N/O ratios emerge during the first 10-20 Myr of intense starburst, before being diluted by CCSNe; (3) multiple star clusters form in and around the galaxy with high efficiency ($\sim20\%$), some of which exhibit high N/O ratios and sodium-oxygen anti-correlations similar to those observed in local globular clusters. Although our simulations can reproduce the high log(N/O) values (up to -0.61, exceeding the solar value by 0.25 dex), they remain below the observational lower limits of GN-z11, indicating room for improvement through additional chemical evolution channels, such as supermassive stars.

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arXiv2025

On the Formation of GW231123 in Population III Star Clusters

GW231123 is a binary black hole merger whose primary component lies within or above the pair-instability mass gap, while the secondary component falls within this gap. The standard theory of stellar evolution is significantly challenged by this event. We investigate the formation of candidate progenitors of GW231123 in Population III (Pop III) star clusters. We find that they could form through stellar mergers, binary black hole mergers, and mixed mergers. The mass distribution of these candidate progenitors covers the component masses of GW231123. Under our model assumptions, their predicted merger rate density spans the range of $0.001-0.26{\rm Gpc^{-3}yr^{-1}}$, encompassing that of GW231123. These findings suggest that GW231123 may originate from Pop III star clusters. Furthermore, such candidate progenitors are expected to be detectable by future gravitational wave detectors LISA/Taiji/TianQin/DECIGO/Cosmic Explorer/Einstein Telescope, which would provide valuable insights into the formation scenarios of events like GW231123.

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arXiv2025

The First Star-by-star $N$-body/Hydrodynamics Simulation of Our Galaxy Coupling with a Surrogate Model

A major goal of computational astrophysics is to simulate the Milky Way Galaxy with sufficient resolution down to individual stars. However, the scaling fails due to some small-scale, short-timescale phenomena, such as supernova explosions. We have developed a novel integration scheme of $N$-body/hydrodynamics simulations working with machine learning. This approach bypasses the short timesteps caused by supernova explosions using a surrogate model, thereby improving scalability. With this method, we reached 300 billion particles using 148,900 nodes, equivalent to 7,147,200 CPU cores, breaking through the billion-particle barrier currently faced by state-of-the-art simulations. This resolution allows us to perform the first star-by-star galaxy simulation, which resolves individual stars in the Milky Way Galaxy. The performance scales over $10^4$ CPU cores, an upper limit in the current state-of-the-art simulations using both A64FX and X86-64 processors and NVIDIA CUDA GPUs.

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arXiv2025

Ripples spreading across the Galactic disc. Interplay of direct and indirect effects of the Sagittarius dwarf impact

Gaia data have revealed vertically asymmetric phase-space structures in the Milky Way (MW) disc, such as phase spirals, indicating vertical oscillations. These oscillations exhibit two distinct modes: the bending mode and the breathing mode, associated with one-arm and two-arm phase spirals, respectively. This study aims to explore the excitation mechanisms of the bending and breathing modes and their subsequent evolution in the MW disc, focusing on the interplay between direct perturbations from the Sagittarius dwarf galaxy and indirect contributions from tidally induced spiral arms. We perform high-resolution $N$-body simulations to model the interaction between an MW-like disc galaxy and a Sagittarius dwarf-like satellite. These simulations resolve fine phase-space structures, enabling analysis of the bending and breathing modes at both macroscopic (global bending and breathing waves) and microscopic (local phase spirals) scales. Our simulations demonstrate that the satellite's perturbation directly excites the bending mode and induces spiral arms in the galactic disc. These spiral arms excite the breathing mode, making it an indirect consequence of the satellite interaction. Initially, the bending mode dominates, but it rapidly decays due to horizontal mixing. In contrast, the breathing mode persists for a longer duration, sustained by the spiral arms, leading to a transition from a bending-dominated to a breathing-dominated state. This transition progresses faster in the inner galaxy than in the outer regions. The simulations reproduce the one-arm phase spiral observed in the solar neighbourhood and reveal two-arm phase spirals, particularly in the inner galaxy, associated with spiral arm-induced breathing modes. Our findings highlight the combined effects of direct satellite perturbations and indirect spiral arm dynamics in shaping the vertical structure of the MW disc.

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arXiv2025

Dynamics of Star Cluster Formation: The Effects of Ongoing Star Formation and Stellar Feedback

We perform a high resolution zoom-in simulation of star cluster assembly including the merger of two sub-clusters with initial conditions taken from previous large scale giant molecular cloud (GMC) simulations. We couple hydrodynamics to N-body dynamics to simulate the individual stars themselves, and the gas-rich environment in which they evolve. We include prescriptions for star formation and stellar feedback and compare directly to previous simulations of the same region without these prescriptions to determine their role in shaping the dynamics inherited from the cluster assembly process. The stellar mass of the cluster grows through star formation within the cluster and accretion of new stars and star forming gas from a nearby filament. This growth results in an enhancement in the cluster's rotation and anisotropic expansion compared to simulations without star formation. We also analyze the internal kinematics of the cluster once it has lost most of its gas and find that the rotational velocity and the velocity anisotropy profiles are qualitatively similar to those expected of clusters that have undergone violent relaxation. As well, rotation and anisotropic expansion are still present by the time of gas removal. This implies that evolution within the GMC was unable to completely erase the kinematics inherited by the merger.

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arXiv2025

Compact Binary Formation in Open Star Clusters III: Probability of Binary Black Holes Hidden in Gaia Black Hole Binary

The Gaia mission and its follow-up observations have discovered a few candidates of non-interacting single black holes (BHs) and visible stars, Gaia BH1, BH2, and BH3, collectively called ``astrometric BH binaries''. This paper investigates whether any of these candidates harbor binary BHs (BBHs), namely, whether any such candidates are previously undiscovered ``astrimetric BBH triples''. Focusing on open star clusters, which are promising formation sites of astrometric BH binaries, we estimate the formation rate of astrometric BBH triples through gravitational $N$-body simulations. We find a competitively high formation efficiency of astrometric BBH triples ($\sim 10^{-6} {\rm M}_\odot^{-1}$ or $\sim 10$\% of astrometric BH binaries) in low-metallicity environments but no astrometric BBH triples in solar-metallicity environments. Most of the astrometric BBH triples in our simulations were dynamically stable for $10$ Gyrs, indicating that $\sim10$\% of astrometric BH binary candidates may indeed harbor inner BBHs if they originate from open star clusters in low-metallicity environments. Astrometric BBH triples can be distinguished from astrometric BH binaries through radial velocity follow-up of the tertiary star. According to the statistics of our simulated samples, a small percent of astrometric BH binary candidates should exhibit detectable radial-velocity modulations generated by inner BBHs. Such candidates preferentially exhibit ``outer'' orbital periods of $\gtrsim 10^3$ days and moderately high ``outer'' orbital eccentricities ($\gtrsim 0.7$). Our current result will strongly motivate the search for astrometric BBH triples in the upcoming Gaia Data Release 4 and Gaia Final Data Release.

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arXiv2025

ASURA-FDPS-ML: Star-by-star Galaxy Simulations Accelerated by Surrogate Modeling for Supernova Feedback

We introduce new high-resolution galaxy simulations accelerated by a surrogate model that reduces the computation cost by approximately 75 percent. Massive stars with a Zero Age Main Sequence mass of more than about 10 $\mathrm{M_\odot}$ explode as core-collapse supernovae (CCSNe), which play a critical role in galaxy formation. The energy released by CCSNe is essential for regulating star formation and driving feedback processes in the interstellar medium (ISM). However, the short integration timesteps required for SNe feedback have presented significant bottlenecks in astrophysical simulations across various scales. Overcoming this challenge is crucial for enabling star-by-star galaxy simulations, which aim to capture the dynamics of individual stars and the inhomogeneous shell's expansion within the turbulent ISM. To address this, our new framework combines direct numerical simulations and surrogate modeling, including machine learning and Gibbs sampling. The star formation history and the time evolution of outflow rates in the galaxy match those obtained from resolved direct numerical simulations. Our new approach achieves high-resolution fidelity while reducing computational costs, effectively bridging the physical scale gap and enabling multi-scale simulations.

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arXiv2025

SIRIUS: Identifying Metal-poor Stars Enriched by a Single Supernova in a Dwarf Galaxy Cosmological Zoom-in Simulation Resolving Individual Massive Stars

Metal-poor stars enriched by a single supernova (mono-enriched stars) are direct proof (and provide valuable probes) of supernova nucleosynthesis. Photometric and spectroscopic observations have shown that metal-poor stars have a wide variety of chemical compositions; the star's chemical composition reflects the nucleosynthesis process(es) that occurred before the star's formation. While the identification of mono-enriched stars enables us to study the ejecta properties of a single supernova, the fraction of mono-enriched stars among metal-poor stars remains unknown. Here we identify mono-enriched stars in a dwarf galaxy cosmological zoom-in simulation resolving individual massive stars. We find that the fraction of mono-enriched stars is higher for lower metallicity, stars with [Fe/H] $< -2.5$. The percentages of mono-enriched stars are 11% at [Fe/H] = $-$5.0 and 1% at [Fe/H] = $-$2.5, suggesting that most metal-poor stars are affected by multiple supernovae. We also find that mono-enriched stars tend to be located near the center of the simulated dwarf. Such regions will be explored in detail in upcoming surveys such as the Prime Focus Spectrograph (PFS) on the Subaru telescope.

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arXiv2024

Simulations predict intermediate-mass black hole formation in globular clusters

Intermediate-mass black holes (IMBHs) are those between 100 and 10$^5$ solar masses ($M_{\odot}$); their formation process is debated. One possible origin is the growth of less massive black holes (BHs) via mergers with stars and compact objects within globular clusters (GCs). However, previous simulations have indicated that this process only produces IMBHs $<500 M_{\odot}$ because the gravitational wave recoil ejects them when they merge with other BHs. We perform star-by-star simulations of GC formation, finding that high-density star formation in a GC's parent giant molecular cloud can produce sufficient mergers of massive stars to overcome that mass threshold. We conclude that GCs can form with IMBHs $\gtrsim 10^3 M_{\odot}$, which is sufficiently massive to be retained within the GC even with the expected gravitational wave recoil.

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arXiv2024

Compact Binary Formation in Open Star Clusters II: Difficulty of Gaia NS formation in low-mass star clusters

Gaia mission offers opportunities to search for compact binaries not involved in binary interactions (hereafter inert compact binaries), and results in the discoveries of binaries containing one black hole (BH) or one neutron star (NS), called "Gaia BHs" and "Gaia NSs", respectively. We have assessed if Gaia BHs and NSs can be formed in open clusters through dynamical interactions. In order to obtain a large number of inert compact binaries similar to Gaia BHs and NSs, we have performed gravitational $N$-body simulations for a large number of open clusters whose total mass is $1.2 \times 10^8 M_\odot$. These clusters have various masses, metallicities, densities, and binary fractions. We have found that open clusters form Gaia BHs ($10^{-6}$-$10^{-5} M_\odot^{-1}$) much more efficiently than Gaia NSs ($\lesssim 10^{-7} M_\odot^{-1}$) for any cluster parameters. This is quite inconsistent with observational results, because the reported numbers of Gaia BHs and NSs are $3$ and $21$, respectively. Additionally, we have switched off NS natal kicks for $10^4$ open clusters each weighing $10^3 M_\odot$ in order to retain a large number of NSs in open clusters. Then, open clusters form inert NS binaries originating from primordial binaries rather than formed through dynamical interactions. This means that Gaia NSs are formed dominantly on isolated fields, not in open clusters, if there is no NS natal kick. We have concluded that Gaia BHs can be dominantly formed in open clusters, however Gaia NSs cannot.

astro-ph.SR↗

arXiv2023

Growing Local arm inferred by the breathing motion

Theoretical models of spiral arms suggest that the spiral arms provoke a vertical bulk motion in disc stars. By analysing the breathing motion, a coherent asymmetric vertical motion around the mid-plane of the Milky Way disc, with $\textit{Gaia}$ DR3, we found that a compressing breathing motion presents along the Local arm. On the other hand, with an $N$-body simulation of an isolated Milky Way-like disc galaxy, we found that the transient and dynamic spiral arms induce compressing breathing motions when the arms are in the growth phase, while the expanding breathing motion appears in the disruption phase. The observed clear alignment of the compressing breathing motion with the Local arm is similar to what is seen in the growth phase of the simulated spiral arms. Hence, we suggest that the Local arm's compressing breathing motion can be explained by the Local arm being in the growth phase of a transient and dynamic spiral arm. We also identified the tentative signatures of the expanding breathing motion associated with the Perseus arm and also the Outer arm coinciding with the compressing breathing motion. This may infer that the Perseus and Outer arms are in the disruption and growth phases, respectively.

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