arXiv subjects

Daisuke Toyouchi

Publications and source records attributed to Daisuke Toyouchi.

At least 19 recordsLinked to original sources

arXiv2026

CROCODILE-SIDM: Tidal Formation of Dark Matter-Deficient Galaxies as a Test Case

We introduce CROCODILE-SIDM, a framework for treating self-interacting dark matter (SIDM) with the $N$-body part of GADGET4-Osaka code, as part of CROCODILE simulation family. As a test case, we investigate the impact of SIDM on the tidal formation of dark matter-deficient galaxies (DMDGs) with velocity-dependent cross-section models. We demonstrate that our implementation reproduces the analytic scattering rate in isolated halos. Including dynamical friction self-consistently, we evolve a dwarf satellite with $M_*=2\times10^8\,\mathrm{M}_\odot$ in a $\sim10^{11}\,\mathrm{M}_\odot$ halo on a decaying orbit around a massive host, comparing CDM with four SIDM cross sections for two initial satellite density profiles: a cuspy Navarro--Frenk--White (NFW) profile and a cored Burkert profile. We find that self-interactions primarily regulate the amount of DM retained between pericentric passages, and that the sign of this effect depends on the initial profile: a larger cross section retains more DM for the Burkert initial condition but less DM for the NFW initial condition. We show that this opposing behavior reflects the direction of SIDM heat conduction, which is set by the halo's evolutionary state at infall. Core formation in the cuspy profile assists DM stripping, whereas tidally accelerated gravothermal contraction in the cored profile suppresses tidal mass loss. Consequently, SIDM can either assist or hinder DMDG formation, depending on the satellite's inner structure, making DMDGs a potential probe of SIDM cross section.

astro-ph.GA↗

arXiv2026

Grain-size evolution and rapid dust growth in high-redshift galaxies

We present a galaxy evolution model that incorporates grain-size evolution in a multiphase interstellar medium (ISM) to investigate dust attenuation in galaxies at $z \geq 5$. Our fiducial setup assumes a low dust yield of $y_{\rm d} = 10^{-4}~\rm M_\odot$ and a small characteristic size of stellar dust of $a_0 = 0.01~\mu$m, motivated by efficient dust destruction by reverse shocks in dense ISM environments. Our model demonstrates that, even with such low dust yields, massive galaxies with $M_\ast > 10^9~\rm M_\odot$ reach high dust-to-stellar mass ratios of $M_{\rm d}/M_\ast \sim 10^{-2}$ by $z \sim 7$ because small grains supplied by SNe efficiently serve as seeds for metal accretion in the ISM. Because dust growth significantly lags behind star formation, the outer regions beyond the half-star-formation-rate radius remain relatively dust poor, allowing a non-negligible fraction of UV photons to escape without strong attenuation. We further find that dust growth becomes most efficient when the ISM is dominated by cold dense gas but still contains a modest warm component, as the former promotes metal accretion while the latter supplies additional small grains through shattering, thereby further enhancing subsequent grain growth. In particular, with a cold dense gas fraction of $\sim 90~\%$, our model predictions become broadly consistent with the dust-to-stellar mass ratios inferred for dust-rich galaxies at $z \sim 7$, as well as the upper limits for blue galaxies at $z \gtrsim 10$. Self-consistently, the model successfully reproduces the UV luminosity functions observed at both $z = 7$ and $z = 12$. Overall, this study demonstrates that a physically motivated treatment of grain growth in a multiphase ISM is essential for linking the dust content of high-redshift galaxies to their radiative properties during cosmic dawn.

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arXiv2026

Cosmic Himalayas in CROCODILE : Probing the Extreme Quasar Overdensities by Count-in-Cells analysis and Nearest Neighbor Distribution

The recently reported Cosmic Himalayas (CH) -- an extreme quasar overdensity at z~2 -- poses an apparent challenge to the Lambda CDM framework, with a reported significance of 16.9-sigma under Gaussian assumptions. Such an event appears improbably rare, with a formal probability of P ~ 10^-68. In this work, we investigate whether CH-like structures can naturally arise in cosmological hydrodynamic simulations. Using the CROCODILE simulation, which self-consistently models galaxy-black hole coevolution, we examine quasar clustering through two complementary approaches: the count-in-cells (CIC) statistic, which probes large-scale overdensities, and the nearest-neighbor distribution (NND), sensitive to small-scale environments. CIC analysis reveals that the underlying distribution is heavy-tailed and non-Gaussian, and that conventional Gaussian-based evaluation substantially overestimates the significance of extreme events. When modeled with an asymmetric generalized normal distribution (AGND), the inferred rarity of the CH is substantially reduced and reconciled with standard Lambda CDM; for instance, regions appearing as 12-sigma outliers under Gaussian assumptions (P ~ 10^-33) are found to occur in the AGND regime with a probability of P ~ 10^-4. NND analysis further demonstrates that extreme overdense regions within the simulation can naturally sustain two-point correlation function values similar to those observed in the CH (r0 ~ 30 Mpc/h), suggesting that the strong clustering stems from sample selection biases and local environmental variations. These two analyses conclusively highlight the importance of adopting non-Gaussian statistics when quantifying extreme overdensities of quasars and establish that the CH is not an anomaly, but a natural outcome of structure formation in the Lambda CDM universe.

astro-ph.CO↗

arXiv2026

Clump-Scale Dust Attenuation in Epoch of Reionization Galaxies: Spatially Resolved Properties from FirstLight Simulations

Understanding dust attenuation in galaxies at both integrated and spatially resolved scales is fundamental for accurately determining the physical properties of galaxies. Recent high-spatial-resolution observations with ALMA and JWST enable investigations of spatially resolved properties in high-redshift galaxies ($z \gtrsim 6$), but spatial variations in dust observables remain poorly constrained. We use cosmological zoom-in simulations combined with post-processing dust radiative transfer calculations for 376 clumpy galaxies at $z=6$-$9$ with stellar masses of $M_* \gtrsim 10^9 \, M_\odot$. For each system, we investigate dust attenuation and re-emission properties for three components: system-integrated, individual clumps, and diffuse regions. We find that system-integrated attenuation curves are grayer than the Calzetti curve, even when assuming MW- or SMC-type dust. Attenuation curves of individual clumps are even grayer, while diffuse regions exhibit steeper curves owing to enhanced scattering in optically thin environments. Since the effects of optical depth and dust-star geometry are intrinsically degenerate in attenuation curves, we introduce a toy model based on the IRX-$\Delta\beta$ plane, where $\Delta\beta$ denotes the difference between attenuated and intrinsic UV slopes. Applying this framework, we find that clumps have dust column densities approximately an order of magnitude higher than system-integrated values and exhibit co-spatial or dust-extended geometries. In contrast, system-integrated attenuation reflects star-extended geometries driven by contributions from optically thin diffuse regions. We apply this framework to REBELS-IFU galaxies at $z \sim 7$ and find good agreement with our simulation predictions.

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arXiv2026

Forecasting Supermassive Black Hole Binary Gravitational Wave Probes: Prospects for Future Pulsar Timing Array and Space-Borne Detectors

We present a comprehensive framework for predicting the detection prospects of supermassive black hole binaries (SMBHBs) by future gravitational wave (GW) observatories, examining both space-borne detectors (LISA, Taiji, TianQin) and next-generation pulsar timing array (PTA) combined with the Square Kilometre Array (SKA-PTA). Leveraging dual active galactic nucleus (AGN) fractions and AGN X-ray luminosity functions, we systematically evaluate the detectable SMBHB populations with a detection threshold of signal-to-noise ratio $\geq 5$ for each GW observatory. Our analysis reveals that space-borne detectors are expected to identify approximately $\sim 1 \text{--} 2$ to $\sim 20$ events per year, depending on the SMBHB orbital evolution prescriptions. On the other hand, SKA-PTA demonstrates the potential to reach the first GW detection from individual SMBHBs within a few years of observation and achieve detectable GW source counts of $10^2 \text{--} 10^3$ after about 10 years, depending on PTA configurations. These facilities will significantly improve SMBHB detectability and enable characterization of their properties across different frequency bands.

astro-ph.HE↗

arXiv2026

Coherence of Supermassive Black Hole Binary Demographics with the nHz Stochastic Gravitational Wave Background

We present a refined estimation of the stochastic gravitational wave background (SGWB) based on observed dual active galactic nuclei (AGNs) together with AGN X-ray luminosity functions, in light of recent Pulsar Timing Array detections of an nHz SGWB. We identify a characteristic luminosity dependence in dual AGN fractions by compiling recent observational datasets, providing crucial constraints on supermassive black hole binary (SMBHB) populations. Our AGN-based model reproduces the current SGWB measurements within PTA observational uncertainties of $2 - 4 σ$ uncertainties, demonstrating consistency between electromagnetic and gravitational wave observations. These findings establish SMBHBs as the dominant source of the nHz gravitational wave signal, providing crucial insights into their demographics and evolution.

astro-ph.HE↗

arXiv2025

CROCODILE-DWARF: Assembly and Kinematics of Field Dwarf Galaxies with GADGET4-OSAKA

We present results from CROCODILE-DWARF, a new suite of cosmological zoom-in hydrodynamic simulations of isolated field dwarf galaxies with halo masses of $\sim10^{10}\,M_\odot$ at $z=0$, performed with the GADGET4-OSAKA code. The simulations include detailed modeling of star formation, chemical enrichment, and supernova feedback using the CELib and Grackle libraries, achieving baryonic resolutions of $\sim2\times10^3\,M_\odot$. Our study focuses on how assembly history governs the structural and kinematic diversity of dwarf galaxies within the $\Lambda$CDM framework. The simulated galaxies reproduce the observed stellar-to-halo mass, mass--metallicity, and size--mass relations for nearby dwarf galaxies, including those of the Local Group, yielding stellar masses of $\sim10^7\,M_\odot$. The galaxies display a broad range of rotational support, where gas is generally more rotationally supported than stars. Differences in morphology and kinematics primarily reflect variations in halo assembly timescales and merger activity. Early-assembling, high-concentration halos form stars efficiently and become gas-poor by $z=0$, while late-assembling, low-concentration halos remain gas-rich due to delayed star formation and rejuvenated gas accretion. We find a trend between rotational support and the cumulative merger mass fraction, providing tentative evidence that dynamical heating induced by mergers plays a role in shaping the kinematic diversity. In some cases, late-time mergers induce the formation of extended gas disks by delivering fresh gas and angular momentum. These results demonstrate that it is assembly history, rather than halo mass alone, that shapes the present-day kinematic and morphological diversity of dwarf galaxies.

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arXiv2025

Bridging Theory and Observations: Insights into Star Formation Efficiency and Dust Attenuation in $z > 5$ Galaxies

We investigate early galaxy evolution by modeling self-consistently their radially-resolved evolution of gas, stars, heavy elements, and dust content. Our model successfully reproduces various observed properties of JWST-identified galaxies at $z > 5$, including sizes, stellar masses, star formation rates (SFR), metallicities, and dust-to-stellar mass ratios. We show that the star formation efficiency (SFE), $f_\ast \equiv {\rm SFR}/(f_{\rm b} \dot{M}_{\rm h})$, is regulated by the global equilibrium between cosmological gas inflows, star formation, and gas outflows. Our model predicts $f_\ast \lesssim 20~\%$ for galaxies with halo masses of $M_{\rm h} \sim 10^{11-12}\, M_\odot$ down to $z = 5$, allowing them to reach intrinsic UV magnitudes of $M_{\rm UV} \lesssim -22~{\rm mag}$; when dust attenuation is ignored, the predicted UV luminosity function (LF) at $z \sim 12$ agrees well with observations. However, our model also suggests that these galaxies would be heavily obscured by dust, with high optical depths at 1500~Å~of $τ_{1500} \gtrsim 10$, causing the dust-attenuated UV LF to fall significantly below the observed one. This discrepancy highlights the need for mechanisms that mitigate strong dust attenuation, such as dust evacuation from star-forming regions and/or preferential production of large dust grains. Further exploration of these processes is essential for understanding the early stages of galaxy evolution.

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arXiv2024

Radiation hydrodynamical simulations of super-Eddington mass transfer and black hole growth in close binaries

Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. To study the mass transfer process from a massive donor star to a stellar-mass black hole (BH), we perform multi-dimensional radiation-hydrodynamical simulations that follow accretion flows from the first Lagrange point down to about a hundred times the Schwarzschild radius of the accreting BH. Our simulations reveal that rapid mass transfer occurring at over a thousand times the Eddington rate leads to significant mass loss from the accretion disk via radiation-driven outflows. Consequently, the inflow rates at the innermost radius are regulated by two orders of magnitude smaller than the transfer rates. We find that convective motions within the accretion disk drive outward energy and momentum transport, enhancing the radiation pressure in the outskirts of the disk and ultimately generating large-scale outflows with sufficient energy to leave the binary. Furthermore, we observe strong anisotropy in the outflows, which occur preferentially toward both the closest and furthest points from the donor star. However, when averaged over all directions, the specific angular momentum of the outflows is nearly comparable to the value predicted in the isotropic emission case. Based on our simulation results, we propose a formula that quantifies the mass growth rates on BHs and the mass loss rates from binaries due to radiation-driven outflows. This formula provides important implications for the binary evolution and the formation of merging binary BHs.

astro-ph.HE↗

arXiv2023

The Assembly of Black Hole Mass and Luminosity Functions of High-redshift Quasars via Multiple Accretion Episodes

The early evolution of the quasar luminosity function (QLF) and black hole mass function (BHMF) encodes key information on the physics determining the radiative and accretion processes of supermassive black holes (BHs) in high-$z$ quasars. Although the QLF shape has been constrained by recent observations, it remains challenging to develop a theoretical model that explains its redshift evolution associated with BH growth self-consistently. In this study, based on a semi-analytical model for the BH formation and growth, we construct the QLF and BHMF of the early BH population that experiences multiple accretion bursts, in each of which a constant Eddington ratio is assigned following a Schechter distribution function. Our best-fit model to reproduce the observed QLF and BHMF at $z\simeq 6$ suggests that several episodes of moderate super-Eddington accretion occur and each of them lasts for $τ\simeq 20-30$ Myr. The average duty cycle in super-Eddington phases is $\simeq 15\%$ for massive BHs that reach $\gtrsim 10^8~M_\odot$ by $z\simeq 6$, which is nearly twice that of the entire population. We also find that the observed Eddington-ratio distribution function is skewed to a log-normal shape owing to detection limits of quasar surveys. The predicted redshift evolution of the QLF and BHMF suggests a rapid decay of their number and mass density in a cosmic volume toward $z\gtrsim 6$. These results will be unveiled by future deep and wide surveys with the James Webb Space Telescope, Roman Space Telescope, and Euclid.

astro-ph.GA↗

arXiv2022

Radiative feedback on supermassive star formation: the massive end of the Population III initial mass function

Supermassive stars (SMSs) with masses of $M_\ast \simeq 10^4$--$10^5~{\rm M_\odot}$ are invoked as possible seeds of high-redshift supermassive black holes, but it remains under debate whether their protostar indeed acquires sufficient mass via gas accretion overcoming radiative feedback. We investigate protostellar growth in dynamically heated atomic-cooling haloes (ACHs) found in recent cosmological simulations, performing three-dimensional radiation hydrodynamical (RHD) simulations that consider stellar evolution under variable mass accretion. We find that one of the ACHs feeds the central protostar at rates exceeding a critical value, above which the star evolves in a cool bloating phase and hardly produces ionizing photons. Consequently, the stellar mass reaches $M_\ast \gtrsim 10^4~{\rm M_\odot}$ unimpeded by radiative feedback. In the other ACH, where the mass supply rate is lower, the star spends most of its life as a hot main-sequence star, emitting intense ionizing radiation. Then, the stellar mass growth is terminated around $500~{\rm M_\odot}$ by photoevaporation of the circumstellar disk. A series of our RHD simulations provide a formula of the final stellar mass determined either by stellar feedback or their lifetime as a function of the mass supply rate from the parent cloud in the absence of stellar radiation. Combining the results with the statistical properties of SMS-forming clouds in high-redshift quasar progenitor haloes, we construct a top-heavy mass distribution of primordial stars over $M_\ast \simeq 100$--$10^5~{\rm M_\odot}$, approximately following a power-law spectrum of $\propto M_\ast^{-1.3}$ with a steeper decline at $M_\ast \gtrsim 2 \times 10^4~{\rm M_\odot}$. Their massive BH remnants would be further fed via the dense debris disk, powering "milli-quasars" with a bolometric luminosity of $L_{\rm bol}~\gtrsim~10^{43}~{\rm erg~s^{-1}}$.

astro-ph.GA↗

arXiv2022

Rapid growth of seed black holes during early bulge formation

We study the early growth of massive seed black holes (BHs) via accretion in protogalactic nuclei where the stellar bulge component is assembled, performing axisymmetric two-dimensional radiation hydrodynamical simulations. We find that when a seed BH with $M_\bullet \sim 10^5~M_\odot$ is embedded in dense metal-poor gas ($Z=0.01~Z_\odot$) with a density of $\gtrsim 100~{\rm cm}^{-3}$ and bulge stars with a total mass of $M_\star \gtrsim 100~M_\bullet$, a massive gaseous disk feeds the BH efficiently at rates of $\gtrsim 0.3-1~M_\odot~{\rm yr}^{-1}$ and the BH mass increases nearly tenfold within $\sim 2$ Myr. This rapid accretion phase lasts until a good fraction of the gas bounded within the bulge accretes onto the BH, although the feeding rate is regulated owing to strong outflows driven by ionizing radiation emitted from the accreting BH. The transient growing mode can be triggered for seed BHs formed in massive dark-matter halos with masses of $\gtrsim 10^9~M_\odot$ at $z\sim 15-20$ (the virial temperature is $T_{\rm vir}\simeq 10^5~{\rm K}$). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by $z\simeq 6$. This mechanism naturally yields a mass ratio of $M_\bullet/M_\star >0.01$ higher than the value seen in the local universe and the existence of such overmassive BHs provides us a unique opportunity of detecting highly accreting seed BHs at $z\sim 15$ with AB magnitude of $m_{\rm AB} \sim26 - 29$ mag at $2~μ{\rm m}$ (rest-frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.

astro-ph.GA↗

arXiv2021

Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]

Recent observations of active galactic nuclei (AGNs) have shown a high Fe~II/Mg~II line-flux ratio in their broad-line regions, nearly independent of redshift up to $z \gtrsim 6$. The high flux ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of ${\rm [Fe/Mg]} \gtrsim 0.2$ as high as those observed in matured galaxies in the local universe. We propose a possible explanation of rapid iron enrichment in AGNs by massive star formation that follows a top-heavy initial mass function (IMF) with a power-law index of $Γ$ larger than the canonical value of $Γ=-2.35$ for a Salpeter IMF. Taking into account metal production channels from different types of SNe, we find that the high value of ${\rm [Fe/Mg]} \gtrsim 0.2$ requires the IMF to be characterized with $Γ\gtrsim -1$ ($Γ\gtrsim 0$) and a high-mass cutoff at $M_{\rm max} \simeq 100$--$150~{\rm M_\odot}$ $(M_{\rm max} \gtrsim 250~{\rm M_\odot})$. Given the conditions, core-collapse SNe with $M_\ast \gtrsim 70~{\rm M_\odot}$ and pair-instability SNe give a major contribution for iron enrichment. Such top-heavy stellar IMFs would be a natural consequence from mass growth of stars formed in dense AGN disks under Bondi-like gas accretion that is regulated by feedback at $M_\ast \gtrsim 10~{\rm M_\odot}$. The massive stellar population formed in AGN disks also leave stellar-mass black hole remnants, whose mergers associated with gravitational-wave emission account for at most 10 \% of the merger rate inferred from LIGO/Virgo observations to simultaneously explain the high ${\rm [Fe/Mg]}$ ratio with metal ejection.

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arXiv2021

Hunting isolated neutron stars with proper motions from wide-area optical surveys

High-velocity neutron stars (HVNSs) that were kicked out from their birth location can be potentially identified with their large proper motions, and possibly with large parallax, when they come across the solar neighborhood. In this paper, we study the feasibility of hunting isolated HVNSs in wide-area optical surveys by modeling the evolution of NS luminosity taking into account spin-down and thermal radiation. Assuming the upcoming 10-year VRO LSST observation, our model calculations predict that about 10 HVNSs mainly consisting of pulsars with ages of $10^4$--$10^5$ yr and thermally emitting NSs with $10^5$--$10^6$ yr are detectable. We find that a few NSs with effective temperature $< 5 \times 10^5$ K, which are likely missed in the current and future X-ray surveys, are also detectable. In addition to the standard neutron star cooling models, we consider a dark matter heating model. If such a strong heating exists we find that the detectable HVNSs would be significantly cooler, i.e., $\lesssim 5\times 10^5$ K. Thus, the future optical observation will give an unique NS sample, which can provide essential constraints on the NS cooling and heating mechanisms. Moreover, we suggest that providing HVNS samples with optical surveys is helpful for understanding the intrinsic kick-velocity distribution of NSs.

astro-ph.HE↗

arXiv2020

Super-Eddington mass growth of intermediate-mass black holes embedded in dusty circumnuclear disks

We perform the first three-dimensional radiation hydrodynamical simulations that investigate the growth of intermediate-mass BHs (IMBHs) embedded in massive self-gravitating, dusty nuclear accretion disks. We explore the dependence of mass accretion efficiency on the gas metallicity $Z$ and mass injection at super-Eddington accretion rates from the outer galactic disk $\dot{M}_{\rm in}$, and find that the central BH can be fed at rates exceeding the Eddington rate only when the dusty disk becomes sufficiently optically thick to ionizing radiation. In this case, mass outflows from the disk owing to photoevaporation is suppressed and thus a large fraction ($\gtrsim 40\%$) of the mass injection rate can feed the central BH. The conditions are expressed as $\dot{M}_{\rm in} > 2.2\times 10^{-1}~M_\odot ~{\rm yr}^{-1} (1+Z/10^{-2}~Z_\odot)^{-1}(c_{\rm s}/10~{\rm km~s}^{-1})$, where $c_{\rm s}$ is the sound speed in the gaseous disk. With increasing numerical resolution, vigorous disk fragmentation reduces the disk surface density and dynamical heating by formed clumps makes the disk thickness higher. As a result, the photoevaorative mass-loss rate rises and thus the critical injection rate increases for fixed metallicity. This process enables super-Eddington growth of BHs until the BH mass reaches $M_{\rm BH} \sim 10^{7-8}~M_\odot$, depending on the properties of the host dark-matter halo and metal-enrichment history. In the assembly of protogalaxies, seed BHs that form in overdense regions with a mass variance of 3-4$σ$ at $z\sim 15-20$ are able to undergo short periods of their rapid growth and transits into the Eddington-limited growth phase afterwards to be supermassive BHs observed at $z>6-7$.

astro-ph.GA↗

arXiv2020

Gaseous dynamical friction under radiative feedback: do intermediate-mass black holes speed up or down?

Coalescence of intermediate-mass black holes (IMBHs) as a result of the migration toward galactic centers via dynamical friction may contribute to the formation of supermassive BHs. Here we reinvestigate the gaseous dynamical friction, which was claimed to be inefficient with radiative feedback from BHs in literature, by performing 3D radiation-hydrodynamics simulations that solve the flow structure in the vicinity of BHs. We consider a $10^4~M_\odot$ BH moving at the velocity $V_{\rm flow}$ through the homogeneous medium with metallicity $Z$ in the range of $0-0.1~Z_\odot$ and density $n_{\infty}$. We show that, if $n_{\infty} \lesssim 10^{6}~{\rm cm^{-3}}$ and $V_{\rm flow} \lesssim 60~{\rm km~s^{-1}}$, the BH is accelerated forward because of the gravitational pull from a dense shell ahead of an ionized bubble around the BH, regardless of the value of $Z$. If $n_{\infty} \gtrsim 10^{6}~{\rm cm^{-3}}$, however, our simulation shows the opposite result. The ionized bubble and associating shell temporarily appear, but immediately go downstream with significant ram pressure of the flow. They eventually converge into a massive downstream wake, which gravitationally drags the BH backward. The BH decelerates over the timescale of $\sim 0.01$~Myr, much shorter than the dynamical timescale in galactic disks. Our results suggest that IMBHs that encounter the dense clouds rapidly migrate toward galactic centers, where they possibly coalescence with others.

astro-ph.GA↗

arXiv2019

Super-Eddington accretion of dusty gas onto seed black holes: metallicity-dependent efficiency of mass growth

The super-Eddington accretion onto intermediate seed BHs is a potential formation mode of supermassive black holes exceeding $10^9~M_\odot$ in the early universe. We here investigate how such rapid accretion may occur with finite amounts of heavy elements contained in the gas and dust. In our 1D radiation-hydrodynamics simulations, the radiative transfer is solved for both the direct UV lights emitted by an accretion disk and the diffuse IR lights thermally emitted by dust grains. Our results show that the radiative force by the IR lights causes a strong feedback to regulate the mass accretion. The resulting mean accretion rate is lower with the higher metallicity, and there is the critical metallicity $Z \sim 10^{-2}~Z_\odot$, above which the super-Eddington accretion is prevented by the radiation pressure of the IR lights. With this taken into account, we examine if the dusty super-Eddington accretion occurs in young galaxies using a simple model. We show that a sufficient number of galaxies at $z \gtrsim 10$ can be such potential sites if BHs accrete the cold dense gas with $T \sim 10^2$ K, approximately the thermal equilibrium value at $Z = 10^{-2}~Z_\odot$. We argue that the efficiency of the BH growth via the rapid accretion depends on the metallicity, and that the metallicity slightly lower than $10^{-2}~Z_\odot$ provides a chance for the most efficient growth.

astro-ph.GA↗

arXiv2018

Metallicity Distribution of Disk Stars and the Formation History of the Milky Way

We investigate the formation history of the stellar disk component in the Milky Way (MW) based on our new chemical evolution model. Our model considers several fundamental baryonic processes, including gas infall, re-accretion of outflowing gas, and radial migration of disk stars. Each of these baryonic processes in the disk evolution is characterized by model parameters, which are determined by fitting to various observational data of the stellar disk in the MW, including the radial dependence of the metallicity distribution function (MDF) of the disk stars, which has recently been derived in the APOGEE survey. We succeeded to obtain the best set of model parameters, which well reproduces the observed radial dependences of the mean, standard deviation, skewness, and kurtosis of the MDFs for the disk stars. We analyze the basic properties of our model results in detail to get new insights into the important baryonic processes in the formation history of the MW. One of the remarkable findings is that outflowing gas, containing much heavy elements, preferentially re-accretes onto the outer disk parts, and this recycling process of metal-enriched gas is a key ingredient to reproduce the observed narrower MDFs at larger radii. Moreover, important implications for the radial dependence of gas infall and the influence of radial migration on the MDFs are also inferred from our model calculation. Thus, the MDF of disk stars is a useful clue for studying the formation history of the MW.

astro-ph.GA↗