arXiv subjects

C. Kobayashi

Publications and source records attributed to C. Kobayashi.

At least 19 recordsLinked to original sources

arXiv2026

Stardust Galaxies at z>9: A Dust-Origin Transition Behind the Excess of UV-Bright Galaxies

Recent JWST observations suggest that galaxies at z > 9 may be dominated by low-opacity SNe-produced dust before efficient ISM grain growth is established. This transition in dust origin and opacity could explain both the prevalence of galaxies with extremely low dust attenuation and the excess of UV-bright galaxies relative to most pre-JWST predictions. We investigate whether this transition, combined with evolving star-formation efficiency, can reproduce these observed properties. We develop a physically motivated attenuation framework combining (i) extinction laws for reverse-shock-processed SNe dust, (ii) metallicity- and dust-to-metal-dependent opacity scalings, and (iii) porous radiative-transfer geometries allowing partial UV-photon leakage. Unlike outflow-driven scenarios requiring large-scale gas evacuation, our approach preserves gas reservoirs while reducing effective UV opacity through dust composition and geometry. We introduce extinction-based, gas-based, and hybrid attenuation prescriptions linking SNe-dominated and ISM grain-growth dust regimes. We find that the observed A_FUV-M_star relation at z > 9 is best reproduced for an intrinsic FUV dust opacity kappa_UV(dust)=10^3 - 10^4 cm2/g, characteristic of low-opacity SNe dust, naturally producing very low attenuation even in gas-rich galaxies. This regime reproduces galaxies with extremely low dust attenuation (GELDAs), which dominate observed samples at z > 9. Applied to intrinsic UV luminosity function models, our SNe-dominated and hybrid prescriptions mainly suppress the brightest galaxies, bringing predictions into agreement with JWST measurements without requiring extreme star-formation efficiencies or dust-free interstellar media. Our results suggest that the UV-bright galaxy excess at z > 9 reflects a transition in dust origin and opacity during the earliest phases of galaxy evolution.

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arXiv2026

Chemical abundance patterns at z=1-7: Carbon, Nitrogen, Neon, Sulphur, and Argon abundances from JWST spectroscopy

The relative abundances of chemical elements in the interstellar medium (ISM) trace the history of star formation and chemical enrichment in galaxies. We investigate the abundance patterns of carbon (C), nitrogen (N), neon (Ne), sulfur (S), and argon (Ar) in star-forming galaxies at z=1-7, combining ultra-deep JWST/NIRSpec observations from the MARTA survey with archival spectroscopy. We assemble 244 galaxies with [OIII]4363 detections and derive direct Te-based abundances. New empirical strong-line calibrations for N/O, Ne/O, S/O, and Ar/O extend the analysis to an additional 514 galaxies. We find distinct abundance-metallicity trends. N/O is enhanced by 0.18 dex in median relative to local galaxies, with a stronger enhancement at low metallicity. Direct measurements reveal decreasing S/O and Ar/O with increasing metallicity, while strong-line measurements suggest a flattening at higher metallicities, especially for S/O. We also identify a stellar-mass dependence in the O/Ar-Ar/H plane, with lower-mass galaxies exhibiting lower O/Ar at fixed Ar/H, consistent with differences in star-formation efficiency and delayed SNe Ia enrichment. Ne/O remains approximately constant up to 12+log(O/H)=8.2, with a mild increase at higher metallicities and no significant redshift evolution. C/O tentatively increases with metallicity, broadly following the local relation. Finally, a forward model shows that observational selection affects the abundance ratios differently: it mainly restricts the accessible metallicity range for Ne/O and Ar/O, but preferentially selects galaxies with enhanced N/O at low metallicity, with a weaker effect for S/O. These results provide new constraints on chemical enrichment at high redshift and highlight the importance of accounting for selection effects when interpreting abundance-metallicity trends.

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arXiv2026

Too many or too bright? Status and perspectives in the study of the UV Luminosity Function at z>10

The finding that galaxies in the first 500Myr were much more numerous and/or brighter than expected is often cited as one of the main discoveries of the James Webb Space Telescope in its first four years of operation. This result is now solidly based on an impressive number of surveys that have detected, spectroscopically confirmed, and characterized a relatively large sample of galaxies up to z=14.44, the redshift of the most distant galaxy currently known. This finding has motivated the introduction of additional physical mechanisms into theoretical models to reproduce the observed numbers and luminosities. These results contrast with the failure, despite ongoing efforts, to confirm any galaxy candidate beyond this redshift. This review summarizes the status of the search for and characterization of galaxies at z>9, focusing on the evolution of the UV Luminosity Function (UVLF), which is the main statistical description of the galaxy population that can be obtained at these redshifts. We first provide a comprehensive overview of the full set of JWST data obtained so far. We perform a 'meta--analysis' on the entire data set and provide a simple yet effective description of the global evolution of the UVLF, averaging over the various surveys. We also compare these results extensively with theoretical models that were developed to match the new JWST observations and that explore different physical mechanisms able to drive the observed evolution. We conclude with a summary of the contrasting results regarding the search for galaxy candidates at z>15, and outline the requirements for future surveys aimed at breaking this barrier.

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arXiv2026

Magnesium isotope ratios in Milky Way and dwarf galaxy stars

Under the assumption of hierarchical galaxy formation, dwarf galaxies are the closest existing analogues to the high-redshift protogalaxies that merged to form the Milky Way. These low-mass systems serve as unique laboratories for studying nucleosynthetic channels, given that the chemical compositions of their stars play a pivotal role in constraining their chemical enrichment history. To date, stellar abundances in dwarf galaxies have focused almost exclusively on elemental abundance ratios. While important, elemental abundances omit critical information about the isotopic composition. Here, we compute the Mg isotopic ratios of six accreted dwarf galaxy stars (low-alpha) and seven Milky Way stars (high-alpha) using a set of high-resolution (65000 < R < 160000) and high signal-to-noise ratio (S/N > 250) optical spectra. We show, for the first time, that at a given [Fe/H], stars born in a dwarf galaxy differ in their Mg isotopic ratios from stars born in the Milky Way. However, when comparing isotopic ratios at a given [Mg/H] rather than [Fe/H], a powerful diagnostic emerges that suggests nucleosynthesis processes are consistent across different stellar environments. This universality of Mg isotopic abundances provides additional dimensionality for chemical evolution models and helps to constrain massive-star nucleosynthesis across cosmic time.

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arXiv2026

Tracing nitrogen enrichment across cosmic time with JWST

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in a sample of ~ 660 star-forming galaxies at redshift z ~ 1-6, with a median redshift of z = 3.0, using deep JWST/NIRSpec spectroscopy. Leveraging detections of faint auroral emission lines in 92 galaxies at z>1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derive direct electron temperature-based abundances for nitrogen and oxygen using rest-frame optical lines. We establish the first high-redshift calibrations of strong-line N/O diagnostics based on 'direct' abundance measurements, finding a mild evolution for the N2O2 = log([NII]6585/[OII]3727,3729) diagnostic and no clear evolution for the N2S2 = log([NII]6585/[SII]6717,6731) diagnostic compared to local realisations. We then investigate the N/O-O/H relation across cosmic time using both 'direct' abundances and strong-lines based measurements (additional 535 galaxies). We find evidence for mild but systematic nitrogen enhancement at high redshift: galaxies at z>1 exhibit N/O ratios elevated by ~0.18 dex (median offset) at fixed O/H compared to local trends, with a more pronounced enhancement at low metallicity (i.e. 12 + log(O/H) < 8.1), where the offset reaches up to ~0.4-0.5 dex. We consider several scenarios to explain the observed trends, including bursty star formation, differential metal loading, and inflows of pristine gas. Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics and within a self-consistent frame.

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arXiv2026

The metal-poor tail of the APOGEE survey II. Spectral analysis of Mg and Si in very metal-poor APOGEE spectra

H-band spectra contain very limited spectral information for stars at the most metal-poor tail ( Fe/H < -2.5) because the available Fe lines in FGK stars in this wavelength range are weak. The first paper in this series successfully identified a sample of 327 very metal-poor stars (with [Fe/H] < -2) from the APOGEE database, 289 of which are on the red giant branch. The spectra of these stars were not properly analysed by the APOGEE main pipeline because they are very metal poor. In this work, we measure metallicities for these stars using the abundances of the elements Mg and Si. We demonstrate that the absorption lines of the elements Mg and Si are of good quality despite the challenging combination of (low) metallicity, wavelength regime, spectral resolution, and signal-to-noise ratios available for these spectra. A specialised pipeline was designed to measure the abundance of Mg and Si in APOGEE spectra and yielded a robust estimate of the overall metallicity. In order to provide reliable measurements, we tested three different sets of assumptions for Mg and Si enhancement. We present Mg and Si abundances as well as overall metallicities for 327 stars, all of which had previously gotten null values from the main APOGEE pipeline for either the calibrated M/H or [Fe/H] . The typical uncertainties for our measurements are 0.2 dex. We found five stars in our sample with unusual [Si/Mg] abundances higher than 0.5, and we connect this signature to globular cluster stars, and this might be related to specific supernova events. Our data suggest a concentration of high [Si/Mg] stars in the Sextans dwarf galaxy. Other dwarf galaxies are found to agree well with results in the literature. Our derived metallicities range between -3.1 $\leq$ [M/H] $\leq$ -2.25, thereby pushing the metal-poor tail of APOGEE results down by 0.6 dex.

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arXiv2025

MARTA: Temperature-temperature relationships and strong-line metallicity calibrations from multiple auroral-line detections at cosmic noon

We present the first results from MARTA (Measuring Abundances at high Redshift with the T$_e$ Approach), a programme leveraging ultra-deep, medium-resolution JWST/NIRSpec spectroscopy to probe the interstellar medium (ISM) of star-forming galaxies at $z \sim 2 - 3$. We report detections of one or more auroral lines, including [O III]$\lambda4363$, [O II]$λ\lambda7320,7330$, [S II] $\lambda4068$, and [S III] $\lambda6312$ for 16 galaxies in the sample, providing measurements of multiple ionic temperatures. We tested the validity of the T[O II]-T[O III] relation at high redshift considering a total sample of 21 objects including literature data, and obtained a shallower slope than in the low-$z$ literature. However, such a slope is consistent with low-redshift data when ultra-low metallicity objects are considered. We assessed the correlation of the T[O II]-T[O III] relationship and its scatter on different physical parameters, finding a mild correlation with the ionisation parameter and radiation field hardness, while no significant correlation with gas density. The location of high-redshift data is also consistent with the low-$z$ literature in the T[O II]-T[S II], and T[S III]-T[O III] relations, although this conclusion is limited with low-number statistics. Finally, we leveraged our sample together with a comprehensive compilation of galaxies with [O III]$\lambda4363$ detections from the literature to recalibrate classical strong-line diagnostics at high redshift. MARTA represents a key addition in this space because it provides direct metallicities at moderately high oxygen abundances (12+log(O/H) $\sim8.0-8.4$).

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arXiv2025

Potassium abundances in extremely metal poor stars: Implications for nucleosynthesis in the final stages of massive star evolution

We present a potassium (K) abundance analysis in extremely metal-poor (EMP) stars based on high-resolution ($R\sim 60000$) spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope, covering the K I resonance lines at 766 and 769 nm. One-dimensional local thermodynamic equilibrium (LTE) abundances of K and other elements, including Na, Mg, Ca, Ti, Cr, and Ni, were derived using spectral synthesis. Non-local thermodynamic equilibrium (NLTE) corrections were applied to the K abundances by interpolating a precomputed grid of corrections based on stellar parameters and the LTE K abundance. We detected K I lines in seven stars with [Fe/H]$< -3.0$ and derived upper limits for other stars in the same metallicity regime, making this sample well-suited for investigating the nucleosynthesis origins of K in the early universe. We found that the [K/Fe] and [K/Ca] ratios of the seven stars are enhanced relative to the solar value, with a scatter of approximately 0.1 dex, as small as the typical measurement uncertainty. Under the assumption that each star formed from gas purely enriched by a single or a few massive stars' supernovae, the small scatter in [K/Fe] and [K/Ca], contrasted with the $\sim$0.7 dex scatter in [Na/Mg] ratios (after NLTE correction), suggests that the production of K in massive stars or their supernovae is independent of the processes that drive Na/Mg variation. These findings demonstrate that K abundances in EMP stars, and their correlations with other elemental abundances, can serve as sensitive tracers of the physical mechanisms governing the final evolutionary stages of massive stars and their supernova explosions.

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arXiv2025

The JWST EXCELS survey: direct estimates of C, N, and O abundances in two relatively metal-rich galaxies at $\mathbf{z\simeq5}$

We present a spectroscopic analysis of two star-forming galaxies at $z\simeq5$ observed with JWST/NIRSpec as part of the EXCELS survey. The detection of the CIII]~$λλ$1906,09, [OII] $λλ$3726,29, [OIII] $λλ$4363,5007, and [NII] $λ$6584 emission lines enables an investigation of the $\mathrm{C/O}$, $\mathrm{N/O}$, and $\mathrm{C/N}$ abundance ratios using the temperature-sensitive method. The galaxies have stellar masses of ${\mathrm{log}(M_{\star}/\mathrm{M}_{\odot}) = 8.09^{+\, 0.24}_{-0.15}}$ and ${\mathrm{log}(M_{\star}/\mathrm{M}_{\odot}) = 8.02^{+\, 0.06}_{-0.08}}$ with metallicities of $Z \simeq 0.2 \, \rm{Z_{\odot}}$ and $Z \simeq 0.3 \, \rm{Z_{\odot}}$. These metallicities are somewhat higher than is typical for other $z\gtrsim 5$ galaxies with similar stellar mass and are comparable to $z \simeq 0$ analogues. Both galaxies display evidence for elevated N/O ratios with respect to the typical star-forming galaxies at $z\simeq0$, with ${\mathrm{log(N/O)} = -1.07^{+\,0.17}_{-0.13}}$ and ${\mathrm{log(N/O)} = -0.86^{+\,0.15}_{-0.11}}$ respectively. In contrast, we find low C abundances, with ${\mathrm{log(C/O)}=-0.82\pm0.22}$ and ${\mathrm{log(C/O)}=-1.02\pm0.22}$, consistent with the predicted yields of core-collapse supernovae. Following the trend observed in other high-redshift sources, we find that the $\mathrm{C/N}$ ratios are lower at fixed $\mathrm{O/H}$ compared to the majority of local galaxies. Via a comparison to detailed chemical evolution models, we find that a standard or bottom-heavy IMF can explain the observed abundance ratios where the N-enrichment comes from intermediate mass ($\simeq 4-7 \, \mathrm{M}_{\odot}$) stars. Our results demonstrate that robust measurements of CNO abundances with \emph{JWST} can reveal unique enrichment pathways in galaxies as a function of both metallicity and redshift.

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arXiv2025

The aluminium-26 distribution in a cosmological simulation of a Milky Way-type Galaxy

Context. The 1.8 MeV gamma-rays corresponding to the decay of the radioactive isotope Al-26 (with a half-life of 0.72 Myr) have been observed by the SPI detector on the INTEGRAL spacecraft and extensively used as a tracer of star formation and current nucleosynthetic activity in the Milky Way Galaxy. Further information is encoded in the observation related to the higher Al-26 content found in regions of the Galaxy with the highest line-of-sight (LoS) velocity relative to an observer located in the Solar System. However, this feature remains unexplained. Aims. We ran a cosmological "zoom-in" chemodynamical simulation of a Milky Way-type galaxy, including the production and decays of radioactive nuclei in a fully self-consistent way. We then analyzed the results to follow the evolution of Al-26 throughout the lifetime of the simulated galaxy to provide a new method for interpreting the Al-26 observations. Methods. We included the massive star sources of Al-26 in the Galaxy and its radioactive decay into a state-of-the-art galactic chemical evolution model, coupled with cosmological growth and hydrodynamics. This approach allowed us to follow the spatial and temporal evolution of the Al-26 content in the simulated galaxy. Results. Our results are in agreement with the observations with respect to the fact that gas particles in the simulation with relatively higher Al-26 content also have the highest LoS velocities. On the other hand, gas particles with relatively lower Al-26 content (i.e., not bright enough to be observed) generally display the lowest LoS velocities. However, this result is not conclusive because the overall rotational velocity of our simulated galaxy is higher than that observed for cold CO gas in the Milky Way Galaxy. Furthermore, we found no significant correlation between gas temperature, rotational velocity, and Al-26 content at any given radius. We also [...]

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arXiv2022

Direct measurements of carbon and sulfur isotope ratios in the Milky Way

With the IRAM 30 meter telescope, we performed observations of the $J$ = 2-1 transitions of CS, C$^{33}$S, C$^{34}$S, C$^{36}$S, $^{13}$CS, $^{13}$C$^{33}$S, and $^{13}$C$^{34}$S as well as the $J$ = 3-2 transitions of C$^{33}$S, C$^{34}$S, C$^{36}$S, and $^{13}$CS toward a large sample of 110 HMSFRs. We measured the $^{12}$C/$^{13}$C, $^{32}$S/$^{34}$S, $^{32}$S/$^{33}$S, $^{32}$S/$^{36}$S, $^{34}$S/$^{33}$S, $^{34}$S/$^{36}$S, and $^{33}$S/$^{36}$S abundance ratios with rare isotopologs of CS, thus avoiding significant saturation effects. With accurate distances obtained from parallax data, we confirm previously identified $^{12}$C/$^{13}$C and $^{32}$S/$^{34}$S gradients as a function of galactocentric distance (RGC). In the CMZ, $^{12}$C/$^{13}$C ratios are higher than suggested by a linear fit to the disk values as a function of RGC. While $^{32}$S/$^{34}$S ratios near the Galactic center and in the inner disk are similar, this is not the case for $^{12}$C/$^{13}$C, when comparing central values with those near RGC of 5 kpc. As was already known, there is no $^{34}$S/$^{33}$S gradient but the average ratio of 4.35~$\pm$~0.44 derived from the $J$ = 2-1 transition lines of C$^{34}$S and C$^{33}$S is well below previously reported values. A comparison between solar and local interstellar $^{32}$S/$^{34}$S and $^{34}$S/$^{33}$S ratios suggests that the Solar System may have been formed from gas with a particularly high $^{34}$S abundance. For the first time, we report positive gradients of $^{32}$S/$^{33}$S, $^{34}$S/$^{36}$S, $^{33}$S/$^{36}$S, and $^{32}{\rm S}/^{36}{\rm S}$ in our Galaxy. The predicted $^{12}$C/$^{13}$C ratios from the latest GCE models are in good agreement with our results. While $^{32}$S/$^{34}$S and $^{32}$S/$^{36}$S ratios show larger differences at larger RGC, $^{32}$S/$^{33}$S ratios show an offset across the entire inner 12 kpc of the Milky Way.

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arXiv2022

Non-LTE abundance corrections for late-type stars from 2000Å to 3μm: I. Na, Mg, and Al

It is well known that cool star atmospheres depart from local thermodynamic equilibrium (LTE). Accurate abundance determination requires taking those effects into account, but the necessary non-LTE calculations are often lacking. Our goal is to provide detailed estimates of NLTE effects for FGK type stars for all spectral lines from the ultraviolet to the infrared that are potentially useful as abundance diagnostics. The first paper in this series focusses on the light elements Na, Mg and Al. The code PySME is used to compute curves-of-growth for 2158 MARCS model atmospheres in a wide parameter range. Nine abundance points are used to construct individual line curves-of-growth by calculating the equivalent widths of 35 Na lines, 134 Mg lines, and 34 Al lines. The lines are selected from the ultra-violet to the near infrared wavelength range. We demonstrate the power of the new grids with LTE and NLTE abundance analysis by means of equivalent width measurements of five benchmark stars; the Sun, Arcturus, HD84937, HD140283 and HD122563. For Na, the NLTE abundances are lower than in LTE and show markedly reduced line-to-line scatter in the metal-poor stars. For Mg, we confirm previous reports of a significant 0.25 dex LTE ionization imbalance in metal-poor stars that is only slightly improved in NLTE (0.18 dex). LTE abundances based on Mg II lines agree better with models of Galactic chemical evolution. For Al, NLTE calculations strongly reduce a 0.6 dex ionization imbalance seen in LTE for the metal-poor stars. The abundance corrections presented in this work are in good agreement with previous studies for the subset of lines that overlap, except for strongly saturated lines.

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arXiv2022

The ALMA REBELS Survey: the dust content of $z \sim 7$ Lyman Break Galaxies

We include a fully coupled treatment of metal and dust enrichment into the Delphi semi-analytic model of galaxy formation to explain the dust content of 13 Lyman Break Galaxies (LBGs) detected by the Atacama Large millimetre Array (ALMA) REBELS Large Program at $z\simeq 7$. We find that the galaxy dust mass, $M_d$, is regulated by the combination of SNII dust production, astration, shock destruction, and ejection in outflows; grain growth (with a standard timescale $τ_0= 30$ Myr) plays a negligible role. The model predicts a dust-to-stellar mass ratio of $\sim 0.07-0.1\%$ and a UV-to-total star formation rate relation such that $log (ψ_{\rm UV}) = -0.05 ~[log (ψ)]^{2} + 0.86 ~log(ψ) -0.05$ (implying that 55-80\% of the star formation is obscured) for REBELS galaxies with stellar mass $M_* = 10^{9-10} M_\odot$. This relation reconciles the intrinsic UV luminosity of LBGs with their observed luminosity function at $z=7$. However, 2 out of the 13 systems show dust-to-stellar mass ratios ($\sim 0.94-1.1\%$) that are up to $18\times$ larger than expected from the fiducial relation. Due to the physical coupling between dust and metal enrichment, even decreasing $τ_0$ to very low values (0.3 Myr) only increases the dust-to-stellar mass ratio by a factor $ \sim 2$. Given that grain growth is not a viable explanation for such high observed ratios of the dust-to-stellar mass, we propose alternative solutions.

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arXiv2021

The ramp-up of interstellar medium enrichment at z>4

Fluorine is one of the most interesting elements for nuclear and stellar astrophysics. Fluorine abundance was first measured for stars other than the Sun in 1992, then for a handful metal-poor stars, which are likely to have formed in the early Universe. The main production sites of fluorine are under debate and include asymptotic giant branch (AGB) stars, $ν$-process in core-collapse supernovae, and Wolf-Rayet (WR) stars. Due to the difference in the mass and lifetime of progenitor stars, high redshift observations of fluorine can help constrain the mechanism of fluorine production in massive galaxies. Here, we report the detection of HF (S/N = 8) in absorption in a gravitationally lensed dusty star-forming galaxy at redshift z=4.4 with $N_{\rm HF}$/$N_{\rm{H_2}}$ as high as $\sim2\times10^{-9}$, indicating a very quick ramp-up of the chemical enrichment in this high-z galaxy. At z=4.4, AGB stars of a few solar masses are very unlikely to dominate the enrichment. Instead, we show that WR stars are required to produce the observed fluorine abundance at this time, with other production mechanisms becoming important at later times. These observations therefore provide an insight into the underlying processes driving the `ramp-up' phase of chemical enrichment alongside rapid stellar mass assembly in a young massive galaxy.

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arXiv2021

Barium lines in high-quality spectra of two metal-poor giants in the Galactic halo

Context. Theoretical results showed the possibility that neutron capture elements were produced in the early Universe by two different sources: a frequent s-process source hosted by rotating massive stars, and a rare r-process source hosted most likely by neutron star mergers. The two sources produce barium with different isotopic compositions. Aims. We aim to investigate the lines of barium in two halo stars, HD 6268 and HD 4306. The spectra present an exquisite quality, both in terms of resolution (R > 100'000) and signal-to-noise (400). Due to hyperfine splitting (hfs) effects, barium lines are expected to show slightly different profiles depending on the barium isotopic fraction. Methods. We applied a standard local thermodynamic equilibrium synthesis of the barium lines. We compared the synthetic results assuming an s-process isotopic pattern or an r-process isotopic pattern for the two barium lines for each star that exhibited hfs. We also applied a methodology, less dependent on the accuracy of the theoretical Ba hfs structure, that transforms the lines of HD 4306 into those we would observe if its atmospheric parameter values (i.e. Teff, log g, micro- and macro-turbulence, Vsin i, and Ba abundance) were the same as those of HD 6268. Results. With both methods, our results show that the barium lines with hfs effects of HD 4306 are in agreement with an s-process composition and the lines in HD 6268 have a different profile, which is most likely linked to the presence of an r-process isotopic pattern. Conclusions. Two lines of barium of HD 6268 and HD 4306 seem to confirm the theoretical expectation that both r-process events and also s-process contribution by rotating massive stars have polluted the ancient halo of our Galaxy.

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arXiv2021

High resolution spectroscopic follow-up of the most metal-poor candidates from SkyMapper DR1.1

We present chemical abundances for 21 elements (from Li to Eu) in 150 metal-poor Galactic stars spanning $-$4.1 $<$ [Fe/H] $<$ $-$2.1. The targets were selected from the SkyMapper survey and include 90 objects with [Fe/H] $\le$ $-$3 of which some 15 have [Fe/H] $\le$ $-$3.5. When combining the sample with our previous studies, we find that the metallicity distribution function has a power-law slope of $Δ$(log N)/$Δ$[Fe/H] = 1.51 $\pm$ 0.01 dex per dex over the range $-$4 $\le$ [Fe/H] $\le$ $-$3. With only seven carbon-enhanced metal-poor stars in the sample, we again find that the selection of metal-poor stars based on SkyMapper filters is biased against highly carbon rich stars for [Fe/H] $>$ $-$3.5. Of the 20 objects for which we could measure nitrogen, 11 are nitrogen-enhanced metal-poor stars. Within our sample, the high NEMP fraction (55\% $\pm$ 21\%) is compatible with the upper range of predicted values (between 12\% and 35\%). The chemical abundance ratios [X/Fe] versus [Fe/H] exhibit similar trends to previous studies of metal-poor stars and Galactic chemical evolution models. We report the discovery of nine new r-I stars, four new r-II stars, one of which is the most metal-poor known, nine low-$α$ stars with [$α$/Fe] $\le$ 0.15 as well as one unusual star with [Zn/Fe] = +1.4 and [Sr/Fe] = +1.2 but with normal [Ba/Fe]. Finally, we combine our sample with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy.

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arXiv2021

R-Process elements from magnetorotational hypernovae

Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis. However, in Galactic chemical evolution models, neutron-star mergers alone cannot reproduce the observed element abundance patterns of extremely metal-poor stars, which indicates the existence of other sites of r-process nucleosynthesis. These sites may be investigated by studying the element abundance patterns of chemically primitive stars in the halo of the Milky Way, because these objects retain the nucleosynthetic signatures of the earliest generation of stars. Here we report the element abundance pattern of the extremely metal-poor star SMSS J200322.54-114203.3. We observe a large enhancement in r-process elements, with very low overall metallicity. The element abundance pattern is well matched by the yields of a single 25-solar-mass magnetorotational hypernova. Such a hypernova could produce not only the r-process elements, but also light elements during stellar evolution, and iron-peak elements during explosive nuclear burning. Hypernovae are often associated with long-duration gamma-ray bursts in the nearby Universe. This connection indicates that similar explosions of fast-spinning strongly magnetized stars occurred during the earliest epochs of star formation in our Galaxy.

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arXiv2021

The MAGPI Survey -- science goals, design, observing strategy, early results and theoretical framework

We present an overview of the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey, a Large Program on ESO/VLT. MAGPI is designed to study the physical drivers of galaxy transformation at a lookback time of 3-4 Gyr, during which the dynamical, morphological, and chemical properties of galaxies are predicted to evolve significantly. The survey uses new medium-deep adaptive optics aided MUSE observations of fields selected from the GAMA survey, providing a wealth of publicly available ancillary multi-wavelength data. With these data, MAGPI will map the kinematic and chemical properties of stars and ionised gas for a sample of 60 massive (> 7 x 10^10 M_Sun) central galaxies at 0.25 < z <0.35 in a representative range of environments (isolated, groups and clusters). The spatial resolution delivered by MUSE with Ground Layer Adaptive Optics (GLAO, 0.6-0.8 arcsec FWHM) will facilitate a direct comparison with Integral Field Spectroscopy surveys of the nearby Universe, such as SAMI and MaNGA, and at higher redshifts using adaptive optics, e.g. SINS. In addition to the primary (central) galaxy sample, MAGPI will deliver resolved and unresolved spectra for as many as 150 satellite galaxies at 0.25 < z <0.35, as well as hundreds of emission-line sources at z < 6. This paper outlines the science goals, survey design, and observing strategy of MAGPI. We also present a first look at the MAGPI data, and the theoretical framework to which MAGPI data will be compared using the current generation of cosmological hydrodynamical simulations including EAGLE, Magneticum, HORIZON-AGN, and Illustris-TNG. Our results show that cosmological hydrodynamical simulations make discrepant predictions in the spatially resolved properties of galaxies at z ~ 0.3. MAGPI observations will place new constraints and allow for tangible improvements in galaxy formation theory.

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