arXiv subjects

Chiaki Kobayashi

Publications and source records attributed to Chiaki Kobayashi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Nucleosynthesis and the chemical enrichment of galaxies

Stars are fossils that retain the history of their host galaxies. Carbon and heavier elements are created inside stars and are ejected when they die. From the spatial distribution of elements in galaxies, it is therefore possible to constrain the physical processes during galaxy formation and evolution. This approach, Galactic archaeology, has been popularly used for our Milky Way Galaxy thanks to a vast amount of data from the Gaia satellite and multi-object spectrographs, and now can also be applied to very distant galaxies with the James Webb Space Telescope (JWST) - extra-galactic archaeology. In these studies the most important factor is the input stellar physics, namely nucleosynthesis yields and binary physics, which predominantly determine the model predictions. In this review I give a summary of stellar nucleosynthesis, and how they are tested with the observations in the Milky Way. Then I show how chemical enrichment of galaxies can be calculated, and show some results with the latest nucleosynthesis yields.

astro-ph.GA↗

arXiv2025

Chemical signature reveals co-spatial dwarf satellite of an edge-on disc galaxy with MUSE

Integral field unit (IFU) spectroscopic observations of resolved galaxies provide an optimal experimental setting for determination of stellar population properties, in particular - age, metallicity and $α$-enhancement, which are key to understanding evolution of galaxies across diverse physical environments. We determine these properties for the edge-on disc galaxy IC 1553, through stellar population models fitted to MUSE IFU observations. From our determined spatial distributions of metallicity and [$α$/Fe], we serendipitiously identify the unique chemical signature of a dwarf galaxy that is co-spatial with the luminous disc of IC 1553. The dwarf galaxy is characterized by the presence of higher [$α$/Fe] and metal-poor stellar populations relative to the disc of IC 1553. The identified dwarf is dynamically cold from its determined kinematics, consistent with being a satellite of IC 1553. From modeling the Spitzer IRAC 3.6 $μm$ image of IC 1553, we confirmed the presence of the dwarf galaxy and calculated its stellar mass to be $\sim1.28\times 10^{9} \rm~M_{\odot}$. This is the first such identification of a dwarf galaxy from its unique chemical signature in such integrated light IFU observations, even though its hidden by the luminous body of its massive host.

astro-ph.GA↗

arXiv2025

Population III Supernovae as a dust factory I --- molecule formation and mixing/fallback in ejecta

Recent observations have revealed the spectral feature of carbonaceous grains even in a very distant galaxy. We develop a state-of-the-art dust synthesis code by self-consistently solving molecule and dust formation in supernova (SN) ejecta that contain various elements in different layers. With a progenitor mass 25 Msun and explosion energy 10^{52} erg, we run the following four test calculations to investigate the impact of input physics. (i) With molecule formation solved, our SN model produces 8.45x10^{-2} Msun carbonaceous grains. (ii) If all available C and Si were initially depleted into CO and SiO molecules, respectively, the C grain mass could be underestimated by ~40%. In these two models producing 0.07 Msun 56Ni without mixing fallback, a large amount of silicates (0.260 Msun) created in O-rich layers are also ejected and likely to hide the spectral feature of carbonaceous grains. We then consider mixing-fallback that can reproduce the observed elemental abundance ratios of C-normal and C-enhanced extremely metal-poor stars in the Milky Way. (iii) In the former, the mass ratio of carbonaceous to silicate grains is still small (~0.3). However, (iv) in the latter (known as a ``faint SN'), while the C grain mass is unchanged (6.78x10^{-2} Msun), the silicate mass is reduced (9.98x10^{-3} Msun). Therefore, we conclude that faint SNe can be a significant carbonaceous dust factory in the early Universe.

astro-ph.GA↗

arXiv2025

Fluorine abundances in CEMP stars at the lowest metallicity: Hints on the nature of the first stars

In the last decade, the available measurements of fluorine abundance have increased significantly, providing additional information on the chemical evolution of our Galaxy and details on complex stellar nucleosynthesis processes. However, the observational challenges to obtain stellar F abundances favor samples with higher metallicities, resulting in a scarcity of measurements at low-metallicity ([Fe/H] $<-2.0$). We present F abundances and upper limits in 7 carbon enhanced metal-poor (CEMP) stars observed with the Immersion Grating Infrared Spectrometer (IGRINS), at the Gemini-South telescope. These new observations delivered high-resolution, high signal-to-noise ratio, infrared spectra allowing us to probe significantly deeper into the metal-poor regime and the cosmic origin of F. This work presents the results of our observations, including two 2-sigma detections and five upper limits in a variety of CEMP stars. Arguably the most important result is for CS 29498-0043, a CEMP-no star at [Fe/H] = $-3.87$ with a F detection of [F/Fe] = $+2.0\pm 0.4$, the lowest metallicity star (more than a factor of 10 lower in metallicity than the next detection) with observed F abundance to date. This measurement allowed us to differentiate between two zero metallicity Population III (Pop III) progenitors: one involving He-burning with primary N in Wolf-Rayet stars, and the other suggesting H-burning during hypernova explosions. Our measured value is in better agreement with the latter scenario. This detection represents a pilot, and pioneering study demonstrating the power of F to explore the nature and properties of the first chemical enrichment from Pop III stars.

astro-ph.SR↗

arXiv2025

Unveiling galaxy chemical enrichment mechanisms out to z~8 from direct determination of O & Ar abundances from JWST/NIRSPEC spectroscopy

Galaxy chemical enrichment mechanisms have primarily been constrained by [$α$/Fe] and [Fe/H] measurements of individual stars and integrated light from stellar populations. However such measurements are limited at higher redshifts (z>1). Recently, we proposed an analogous diagram of the oxygen-to-argon abundance ratio, log(O/Ar), vs Ar abundance, 12+log(Ar/H), as a new diagnostic window for emission nebulae. In this Letter, using robust line flux measurements including temperature sensitive auroral lines, we present direct determination of O and Ar abundances in nine SFGs from JWST/NIRSPEC spectra at z$\sim$1.3-7.7, and two more with Keck/MOSFIRE spectra at z$\sim$2.2. Utilising their positions on the log(O/Ar) vs 12+log(Ar/H) plane, we present the first inference of galaxy chemical enrichment mechanisms from an ensemble of galaxies. The SFGs at z$\sim$1.3-3.4 are consistent with the solar neighbourhood galactic chemical enrichment models of the Milky Way Galaxy that are driven by core-collapse and Type Ia supernovae. Such enrichment mechanisms thus occur at least out to z$\sim$3.4. However, the highest-redshift SFGs (z$\sim$3.6-7.7) have very low log(O/Ar) values, revealing a different enrichment process at z>3.6. Such low log(O/Ar) values may be caused by a rapid but intermittent star-formation and/or additional sources. The new diagnostic window for SFGs enables us to reveal the unique fingerprints of galaxy chemical enrichment out to cosmic dawn.

astro-ph.GA↗

arXiv2025

JADES: The star-formation and chemical enrichment history of a luminous galaxy at z~9.43 probed by ultra-deep JWST/NIRSpec spectroscopy

We analyse ultra-deep JWST observations of the galaxy JADES-GS-z9-0 at z = 9.4327, and derive detailed stellar and interstellar medium (ISM) properties of this luminous (MUV=-20.43) high-redshift system. Complementary information from NIRCam imaging and NIRSpec (both low- and medium-resolution) spectroscopy reveal a compact system (Re ~110 pc) characterised by a steeply rising star formation history, which is reflected in the inferred young stellar age (t ~ 3 Myr, light-weighted), high star-formation rate surface density (ΣSFR ~ 72 M yr-1 kpc-2), high ionisation parameter (log(U) ~ -1.5), low metallicity (12+log(O/H) ~ 7.5), and low carbon-over-oxygen abundance ([C/O] = -0.64). Leveraging the detection of N iii]1750 we derive nitrogen-over-oxygen abundance ([N/O] ~ 0) higher than the plateau followed by low-redshift galaxies of similar metallicity, possibly revealing the imprint from (very) massive stars on the ISM enrichment and favouring a top-heavy Initial Mass Function (IMF) scenario. Massive stars powering a hard radiation field are also required to explain the rest-frame UV line ratios, though the presence of the high-excitation [Ne v]λ3426 emission line possibly hints at additional ionization from an AGN. We also report the tentative detection of Lyα emission in the G140M spectrum, shifted by ~450 km/s redward of the systemic redshift. Combined with a modelling of the Lyα spectral break, we rule out the presence of very high column densities of neutral gas pertaining to local absorbers, as well as any extended surrounding ionised bubble, suggesting that JADES-GS-z9-0 has not yet significantly contributed to cosmic Reionization.

astro-ph.GA↗

arXiv2025

Core to Cosmic Edge: SIMBA-C's New Take on Abundance Profiles in the Intragroup Medium at z = 0

We employ the SIMBA-C cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo mass ($10^{13} \leq M_{500}/\mathrm{M_\odot} \leq 10^{15}$). Typically, SIMBA-C generates lower-amplitude abundance profiles than SIMBA with flatter cores, in better agreement with observations. For low-mass groups, both simulations over-enrich the IGrM with Si, S, Ca, and Fe compared to observations, a trend likely related to inadequate modeling of metal dispersal and mixing. We analyze the 3D mass-weighted abundance profiles, concluding that the lower SIMBA-C IGrM abundances are primarily a consequence of fewer metals in the IGrM, driven by reduced metal yields in Chem5, and the removal of the instantaneous recycling of metals approximation employed by SIMBA. Additionally, an increased IGrM mass in low-mass SIMBA-C groups is likely triggered by changes to the AGN and stellar feedback models. Our study suggests that a more realistic chemical enrichment model broadly improves agreement with observations, but physically motivated sub-grid models for other key processes, like AGN and stellar feedback and turbulent diffusion, are required to realistically reproduce observed group environments.

astro-ph.GA↗

arXiv2024

Cloud-scale elemental abundance variations and the CO-to-dust-mass conversion factor in M31

From a spectroscopic survey of candidate H II regions in the Andromeda galaxy (M31) with MMT/Hectospec, we have identified 294 H II regions using emission line ratios and calculated elemental abundances from strong-line diagnostics (values ranging from sub-solar to super-solar) producing both Oxygen and Nitrogen radial abundance gradients. The Oxygen gradient is relatively flat, while the Nitrogen gradient is significantly steeper, indicating a higher N/O ratio in M31's inner regions, consistent with recent simulations of galaxy chemical evolution. No strong evidence was found of systematic galaxy-scale trends beyond the radial gradient. After subtracting the radial gradient from abundance values, we find an apparently stochastic and statistically significant scatter of standard deviation 0.06 dex, which exceeds measurement uncertainties. One explanation includes a possible collision with M32 200 - 800 Myrs ago. Using the two-point correlation function of the Oxygen abundance, we find that, similar to other spiral galaxies, M31 is well-mixed on sub-kpc scales but less so on larger (kpc) scales, which could be a result of an exponential decrease in mixing speed with spatial scale, and the aforementioned recent merger. Finally, the MMT spectroscopy is complemented by a dust continuum and CO survey of individual Giant Molecular Clouds, conducted with the Submillimeter Array. By combining the MMT and SMA observations, we obtain a unique direct test of the Oxygen abundance dependence of the $α^{\prime}(^{12}\mathrm{CO})$ factor which is crucial to convert CO emission to dust mass. Our results suggest that within our sample there is no trend of the $α^{\prime}(^{12}\mathrm{CO})$ with Oxygen abundance.

astro-ph.GA↗

arXiv2024

The Lunar Gravitational-wave Antenna: Mission Studies and Science Case

The Lunar Gravitational-wave Antenna (LGWA) is a proposed array of next-generation inertial sensors to monitor the response of the Moon to gravitational waves (GWs). Given the size of the Moon and the expected noise produced by the lunar seismic background, the LGWA would be able to observe GWs from about 1 mHz to 1 Hz. This would make the LGWA the missing link between space-borne detectors like LISA with peak sensitivities around a few millihertz and proposed future terrestrial detectors like Einstein Telescope or Cosmic Explorer. In this article, we provide a first comprehensive analysis of the LGWA science case including its multi-messenger aspects and lunar science with LGWA data. We also describe the scientific analyses of the Moon required to plan the LGWA mission.

gr-qc↗

arXiv2024

\texttt{Simba}-\texttt{C}: the evolution of the thermal and chemical properties in the intragroup medium

The newly updated \texttt{GIZMO} and \texttt{Simba} based simulation, \texttt{Simba-C}, with its new stellar feedback, chemical enrichment, and recalibrated AGN feedback, allows for a detailed study of the intragroup medium X-ray properties. We discuss the impact of various physical mechanisms, e.g. stellar and AGN feedback, and chemical enrichment, on the composition and the global scaling relations of nearby galaxy groups. We also study the evolution ($z=2$ to $0$) of the global properties for the $1\,\mathrm{keV}$ temperature groups. \texttt{Simba-C} shows improved consistent matching with the observations of all X-ray scaling relations compared to \texttt{Simba}. It is well known that AGN feedback has a significant influence on $L_{X,0.5-2.0}-T_{spec,corr}$, $S_{500/2500}-T_{spec,corr}$, and gas mass fractions, with our \texttt{Simba-C} results consistent with it. Our recalibrated AGN feedback strength also showed an additional improvement in gas entropy, which now aligns with CLoGS observations. The updated stellar feedback and chemical enrichment model is shown to play an important role in our understanding of the chemical abundance ratios and their evolution within galaxy groups. In particular, we find that \texttt{Simba-C} produces an increase in the amount of heavier elements (specifically Si and Fe) relative to O, compared to \texttt{Simba}.

astro-ph.GA↗

arXiv2024

Understanding stellar populations in thin & thick discs of edge-on galaxies with MUSE -- I. The case of the reignited S0 galaxy ESO 544-27

Edge-on galaxies act as the best laboratories to understand the origin of thin and thick discs in galaxies. Measurement of spatially resolved stellar population properties in such galaxies, particularly age, metallicity and [$α$/Fe], are crucial to understanding the formation and evolution of disc galaxies. Such measurements are made possible from stellar population model fits to deep integral field spectroscopic (IFU) observations of resolved galaxies. We utilise archival MUSE IFU observations of the edge-on galaxy ESO 544-27 to uncover the formation history of its thin and thick discs through its stellar populations. We find the thin disc of the galaxy is dominated by an old ($>9$ Gyr) low [$α$/Fe] metal-rich stellar population. Its outer thick disc is dominated by an old ($>9$ Gyr) high [$α$/Fe] metal-rich component that should have formed with higher star-formation efficiency than the Milky Way thick disc. We thus find [$α$/Fe] dichotomy in ESO 544-27 with its thin and thick discs dominated by low and high [$α$/Fe] stellar populations respectively. However, we also find a metal-rich younger ($<2$ Gyr old) stellar population in ESO 544-27. The galaxy was nearly quenched until its star-formation was reignited recently first in the outer and inner thick disc ($\sim$1 Gyr ago) and then in the thin disc ($\sim$600 Myr ago). We thus find that both the low [$α$/Fe] thin and high [$α$/Fe] thick discs of ESO 544-27 are inhabited primarily by similarly old metal-rich stellar populations, a contrast to that of other galaxies with known thin and thick disc stellar population properties.

astro-ph.GA↗

arXiv2024

Rapid Chemical Enrichment by Intermittent Star Formation in GN-z11

We interpret the peculiar super-solar nitrogen abundance recently reported by the James Webb Space Telescope observations for GN-z11 ($z=10.6$) using our state-of-the-art chemical evolution models. The observed CNO ratios can be successfully reproduced -- independently of the adopted initial mass function, nucleosynthesis yields, and presence of supermassive ($>$1000$M_\odot$) stars -- if the galaxy has undergone an intermittent star formation history with a quiescent phase lasting $\sim$100 Myr, separating two strong starbursts. Immediately after the second burst, Wolf--Rayet stars (up to $120M_\odot$) become the dominant enrichment source, also temporarily ($<$1 Myr) enhancing particular elements (N, F, Na, and Al) and isotopes ($^{13}$C and $^{18}$O). Alternative explanations involving (i) single burst models, also including very massive stars and/or pair-instability supernovae, or (ii) pre-enrichment scenarios fail to match the data. Feedback-regulated, intermittent star formation might be common in early systems. Elemental abundances can be used to test this hypothesis and to get new insights on nuclear and stellar astrophysics.

astro-ph.GA↗

arXiv2023

The impact of supernova feedback on the mass-metallicity relations

Metallicity is a fundamental physical property that strongly constrains galaxy formation and evolution. The formation of stars in galaxies is suppressed by the energy released from supernova explosions and can be enhanced by metal production. In order to understand the impact of this supernova feedback, we compare four different feedback methods, ejecting energy in thermal, kinetic, stochastic and mechanical forms, into our self-consistent cosmological chemodynamical simulations. To minimise other uncertainties, we use the latest nucleosynthesis yields that can reproduce the observed elemental abundances of stars in the Milky Way. For each method, we predict the evolution of stellar and gas-phase metallicities as a function of galaxy mass, i.e., the mass-metallicity relations. We then find that the mechanical feedback can give the best match to a number of observations up to redshift $z\sim3$, although the predicted gas-phase metallicities seem to be higher than observed at $z\ge 1$. The feedback modelling can be further constrained by the metallicities in distant galaxies with the James Web Space Telescope and those of a large sample with ongoing and future spectroscopic surveys.

astro-ph.GA↗

arXiv2023

On the $α$/Fe bimodality of the M31 disks

An outstanding question is whether the $α$/Fe bimodality exists in disk galaxies other than in the Milky Way. Here we present a bimodality using our state-of-the-art galactic chemical evolution models that can explain various observations in the Andromeda Galaxy (M31) disks, namely, elemental abundances both of planetary nebulae, and of red-giant branch stars recently observed with the James Webb Space Telescope. We find that in M31 a high-$α$ thicker-disk population out to 30 kpc formed by more intense initial star burst than in the Milky Way. We also find a young low-$α$ thin disk within 14 kpc, which is formed by a secondary star formation M31 underwent about 2-4.5 Gyr ago, probably triggered by a wet merger. In the outer disk, however, the planetary nebula observations indicate a slightly higher-$α$ young ($\sim$2.5 Gyr) population at a given metallicity, possibly formed by secondary star formation from almost pristine gas. Therefore, an $α$/Fe bimodality is seen in the inner disk ($<$14 kpc), while only a slight $α$/Fe offset of the young population is seen in the outer disk ($>$18 kpc). The appearance of the $α$/Fe bimodality depends on the merging history at various galactocentric radii, and wide-field multi-object spectroscopy is required for unveiling the history of M31.

astro-ph.GA↗

arXiv2023

SIMBA-C: An updated chemical enrichment model for galactic chemical evolution in the SIMBA simulation

We introduce a new chemical enrichment and stellar feedback model into GIZMO, using the SIMBA sub-grid models as a base. Based on the state-of-the-art chemical evolution model of Kobayashi et al., SIMBA-C tracks 34 elements from H$\rightarrow$Ge and removes SIMBA's instantaneous recycling approximation. Furthermore, we make some minor improvements to SIMBA's base feedback models. SIMBA-C provides significant improvements on key diagnostics such as the knee of the $z=0$ galaxy stellar mass function, the faint end of the main sequence, and the ability to track black holes in dwarf galaxies. SIMBA-C also matches better with recent observations of the mass-metallicity relation at $z=0,2$. By not assuming instantaneous recycling, SIMBA-C provides a much better match to galactic abundance ratio measures such as [O/Fe] and [N/O]. SIMBA-C thus opens up new avenues to constrain feedback models using detailed chemical abundance measures across cosmic time.

astro-ph.GA↗

arXiv2023

Roman Early-Definition Astrophysics Survey Opportunity: Galactic Roman Infrared Plane Survey (GRIPS)

A wide-field near-infrared survey of the Galactic disk and bulge/bar(s) is supported by a large representation of the community of Galactic astronomers. The combination of sensitivity, angular resolution and large field of view make Roman uniquely able to study the crowded and highly extincted lines of sight in the Galactic plane. A ~1000 deg2 survey of the bulge and inner Galactic disk would yield an impressive dataset of ~120 billion sources and map the structure of our Galaxy. The effort would foster subsequent expansions in numerous dimensions (spatial, depth, wavelengths, epochs). Importantly, the survey would benefit from early defintion by the community, namely because the Galactic disk is a complex environment, and different science goals will require trade offs.

astro-ph.GA↗

arXiv2023

Chemical Evolution with Radial Mixing Redux: Extending beyond the Solar Neighborhood

We present a multi-zone galactic chemical evolution (GCE) model for the Milky Way that takes the most recently updated yields of major nucleosynthesis channels into account. It incorporates physical processes commonly found in previous GCE models like supernova and star formation feedback, the radial flow of gas in the disk, and the infall of fresh gas, along with stellar scattering processes like radial migration. We individually analyse the effect of different physical processes present in our model on the observed properties of the Galaxy. The radial flow of gas in the disk plays an important role in establishing the radial gradient for \feh{} in the low-\alphafe{} sequence. Our model with one episode of smooth gas infall and constant star formation efficiency is capable of reproducing the observed \fehalpha{} distribution of stars at different ($R$, $|z|$) positions in the Milky Way. Our results point to the rapid evolution of \alphafe{} after the onset of Type Ia supernovae and a high star formation rate during the formation of the high-\alphafe{} sequence as the origin of dual peaks in \alphafe{}. A secondary infall is unnecessary to reproduce the \alphafe{}-gap and chemical spread in the disk in our model. We additionally compare the median age for various mono-abundance populations and the age-metallicity relation at different ($R$, $|z|$) positions from our fiducial model to observations. We discuss our results in relation to other related work in detail.

astro-ph.GA↗

arXiv2023

The Andromeda Galaxy's Last Major Merger: Constraints from the survey of Planetary Nebulae

The Andromeda galaxy (M 31) has experienced a tumultuous merger history as evidenced by the many substructures present in its inner halo. We use planetary nebulae (PNe) as chemodynamic tracers to shed light on the recent merger history of M 31. We identify the older dynamically hotter thicker disc in M 31 and a distinct younger dynamically colder thin disc. The two discs are also chemically distinct with the PN chemodynamics implying their formation in a `wet' major merger (mass ratio ~1:5) ~2.5-4 Gyr ago. From comparison of PN line-of-sight velocities in the inner halo substructures with predictions of a major-merger model in M 31, we find that the same merger event that formed the M 31 thick and thin disc is also responsible for forming these substructures. We thereby obtain constraints on the recent formation history of M 31 and the properties of its cannibalized satellite.

astro-ph.GA↗