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William McClymont

Publications and source records attributed to William McClymont.

At least 19 recordsLinked to original sources
arXiv2026

The THESAN-ZOOM project: clumpiness of high-redshift galaxies and its connection to bursty star formation

Recent JWST observations have revealed diverse high-redshift galaxy morphologies, including a population with irregular and clumpy structures. The physical origin of these structures, and the extent to which observational biases shape their appearance, remain uncertain. We present a power-spectrum-based method for quantifying galaxy clumpiness across spatial scales, using the radiation-hydrodynamic simulation suite THESAN-ZOOM, which employs a state-of-the-art galaxy formation model that resolves the multiphase interstellar medium (ISM). Although the total stellar mass distributions in THESAN-ZOOM galaxies are usually smooth, clumpy structures appear in the H$α$, far-ultraviolet (FUV), and optical light distributions. Tracers sensitive to shorter-timescale star formation exhibit more pronounced small-scale structure ($\sim10^{2}$--$10^{3}{\rm pc}$). The corresponding projected light spectra follow $P(k)\propto k^{-1}$ to $k^{-2}$, with progressively shallower slopes for tracers sensitive to more recent star formation, reflecting enhanced small-scale power and greater spatial intermittency in young stellar populations. This behaviour is consistent with a highly compressible, shock-dominated ISM in which stellar feedback and outflows reorganise dense gas into filamentary and clumpy structures. We also find that galaxy clumpiness depends on the treatment of stellar feedback. Weaker early stellar feedback enhances small-scale power in both the mass and light distributions. Clumpiness also varies strongly over the bursty star formation cycle, implying that observed samples may be biased towards galaxies caught in phases of elevated star formation. Galaxy clumpiness, therefore, could provide a complementary probe of the bursty star formation in the early Universe.

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arXiv2026

Predicting ionised gas emission in 3D with SKIRT. I. Framework and validation

Emission lines from ionised gas are key diagnostics of star formation, metallicity, and ionisation conditions in galaxies. Interpreting spatially resolved observations from integral-field surveys (e.g. MaNGA, MUSE, JWST/NIRSpec) and comparing them with hydrodynamical simulations requires 3D photoionisation models that handle realistic geometries, dust attenuation, and synthetic instrument output. We present a new photoionisation module for the Monte Carlo radiative transfer code SKIRT that predicts emission-line luminosities of ionised gas in 3D, combining pre-computed Cloudy tables for gas temperature and opacity with a direct calculation of ion fractions and line emissivities. The local ionising radiation field (1-6 Ryd) is characterised by log U and four spectral-shape ratios; Cloudy tables map these to temperature and opacity, converging through SKIRT's existing iteration cycle. An inline solver then determines ion fractions from the converged field and temperature and evaluates line emissivities. We validate against Cloudy on 60 spherical shell models and against COLT on a Milky Way-analogue galaxy. On the 1D grid, hydrogen recombination lines agree with Cloudy to within a few per cent (Halpha median ratio 0.97) and the forbidden lines to within ~5%, except [S II] 6717 (1.23), whose offset traces a temperature overestimate near the ionisation front. In 3D, integrated luminosities agree with COLT to within 18% for the hydrogen lines and 2% for [N II], while [O III] and [S II] are elevated by ~70 and ~80%. Pixel-by-pixel correlation coefficients reach r >= 0.92, with luminosity-weighted scatter of 0.14-0.31 dex and broadly consistent BPT ratios. The module enables self-consistent synthetic observations in which ionised-gas emission lines, dust attenuation, and dust re-emission are computed in a single MCRT run, applicable to any hydrodynamical simulation.

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arXiv2026

The Thesan-Zoom project: bursty star formation is incompatible with prolonged dust survival

Cosmic dust is a key regulator of galaxy evolution, but its build-up and survival in the first billion years remain poorly constrained. We present a systematic analysis of dust in the thesan-zoom suite of radiation-hydrodynamical zoom-in simulations, which self-consistently model dust formation, growth, destruction, and its coupling to radiative transfer in galaxies at $z \geq 3$, a multi-phase ISM and bursty star formation histories. The simulated galaxies reproduce the observed trends of dust-to-gas and dust-to-metal ratios with gas metallicity, while showing a dust deficit at high specific star-formation rates. They also broadly match observed dust temperatures and UV-IR spatial offsets. We find that dust and its properties are strongly time-variable and tightly linked to bursty star formation, with short-lived IR-bright phases (median duration of $20.3^{+2.3}_{-2.4}$ Myr) and longer dust-poor phases, naturally producing a correlation between dust temperature and distance from the star-forming main sequence. The predicted attenuation at $1500$ Å is low compared to observations, even when including unresolved dust through post processing, indicating that a mechanism able to shield dust from strong feedback events is necessary to reconcile our galaxy formation model with observations. In our model, bursty star formation prevents the survival of large dust reservoirs ($M_{dust} / M_{star} \geq 10^{-3}$) over a significant fraction of cosmic time. This implies that bursty star formation can produce the observed overabundance of UV-bright galaxies at $z \geq 10$ only if it rapidly settles down by $z \sim 8$ (where large dust reservoirs are detected). It is also possible that our models lack physical ingredients or emergent phenomena that aid the survival of dust. Future observations of high-redshift dust will be key to diagnose the physical mechanism at play in the first galaxies.

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arXiv2026

Globular cluster abundance patterns inherited from giant molecular clouds

Globular clusters exhibit large star-to-star variations and anticorrelations in their light element abundances that are commonly interpreted in terms of in-cluster self-enrichment, in which ejecta from early-forming cluster stars pollute the gas from which later stars form over millions of years. Yet proposed self-enrichment scenarios suffer from a severe mass-budget problem or invoke exotic stellar populations. Using cosmological radiation-hydrodynamic simulations with a standard chemical enrichment model, we identify a population of giant molecular clouds whose internal abundance patterns reproduce several key globular cluster signatures: large light-element abundance spreads and nitrogen-oxygen anticorrelations at nearly constant iron abundance. These clouds form at the restart of star-formation activity after an earlier starburst, where previously ejected oxygen-rich gas collides with nitrogen-rich galactic gas, and are sites of dense star-cluster formation. In this picture, the chemical abundance patterns of globular clusters need not require extended in-cluster star formation, but can be inherited at birth from chemically structured interstellar gas shaped by the baryon cycle. Globular clusters therefore provide a fossil record of chemical enrichment and gas flows in high-redshift galaxies.

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arXiv2026

From THESAN-ZOOM to JWST: Predicting ionizing photon escape and the rise of UV-bright reionization sources

Understanding the sources and evolution of cosmic reionization remains a central challenge in astrophysics, with the escape of ionizing Lyman-continuum (LyC) photons from early galaxies representing a major uncertainty. In this work, we use more than 35,000 galaxy realisations from the THESAN-ZOOM cosmological radiation-hydrodynamic simulations to identify indirect diagnostics of the LyC photon escape fraction ($f_\mathrm{esc}$) and the LyC photon escape rate ($\dot{N}_\mathrm{ion,esc}$) across the redshift range $z=3-16$. We train random forest regression models using these diagnostics to predict both quantities. We present four models: two trained with the full set of simulation-derived indicators to predict $f_\mathrm{esc}$ and $\dot{N}_\mathrm{ion,esc}$, and two restricted to observables accessible to JWST photometric surveys. We find the 10-to-100$\,$Myr star-formation rate ratio ($\mathrm{SFR}_{10} / \mathrm{SFR}_{100}$) and the gas-to-stellar mass ratio ($M_\mathrm{gas} / M_*$) to be the strongest diagnostics of $f_\mathrm{esc}$, suggesting a strong relationship between ionizing photon escape and gas clearing through bursty star formation. In contrast, rest-frame UV ($1500 \, Å$) absolute magnitude ($M_\mathrm{UV}$) dominates $\dot{N}_\mathrm{ion,esc}$ prediction. Motivated by the strong predictive power of $M_\mathrm{UV}$, we combine observed UV luminosity functions with derived $\dot{N}_\mathrm{ion,esc} - M_\mathrm{UV}$ relations to construct histories of reionization. These are consistent with observational constraints, avoiding the recently reported crisis in the ionizing photon budget. Our analysis suggests that the bulk of reionization occurred rapidly after $z \approx 8$, driven by UV-bright galaxies, with the $M_\mathrm{UV} < -17$ populations providing the dominant contribution.

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arXiv2026

JADES: the mass-metallicity relation at $z=1-10$. New calibrations, extremely metal-poor galaxies, and chemical diversity

We present gas-phase metallicities of star-forming galaxies at $z=1$-10 with deep JWST/NIRSpec spectra from the JADES full data release, Dark Horse, and OASIS programmes. We stack $\sim$1500 medium-resolution spectra, yielding detections of the [OIII]$λ$4363 auroral line down to $12+\log(\mathrm{O/H})=7.0$ to derive stack-based strong-line calibrations over the metallicity range $12+\log(\mathrm{O/H})=7.0$-8.7. At a fixed metallicity, our stacks exhibit [OIII]$λ$5007/H$β$ and [OIII]$λ$5007/[OII]$λλ$3726,3729 values generally lower than calibrations based on high-$z$ individual auroral-line emitters, suggesting an observational bias towards higher excitation introduced when requiring auroral line detections in individual spectra. Based on our new calibrations, we obtain canonical mass-metallicity relations (MZRs) at z$=$1-10, identifying a decrease in metallicities from $z\sim0$ to z$\sim$4-10, without significant change in slope. Moreover, we identify 50 promising candidates of extremely metal-poor galaxies (EMPGs) with $12+\log(\mathrm{O/H})=6.7$-7.3 (1-4\% solar metallicity) at $z=1.2$-9.1. The MZRs of EMPGs are characterised by a large scatter, with those having lower metallicities generally exhibiting lower sSFRs, opposite of what expected from the local Fundamental Metallicity Relation. These results support a stochastic star-formation history involving gas consumption/ejection and metal-poor inflow, strongly affecting metallicities of low-mass galaxies. Furthermore, we identify two Little Red Dots in our EMPG candidates, both exhibiting broad H$α$ and prominent Ly$α$, offering insights into the early black-hole growth in extremely metal-poor environments.

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arXiv2026

Too many protoclusters? Reconciling the overabundance of cluster progenitors within the first billion years of the Universe

JWST has revealed an overabundance of apparent (`Coma'-like) cluster progenitors at $z>5$ that is $\sim$ two (300) times in excess of, and in $2.3σ$ ($4σ$) tension with, the number density of such objects at present day and theoretical predictions from $Λ$CDM. We present an analysis of protocluster candidates between $5 10^{15}\, {\rm M_{\odot}}$) by $z=0$. Although some of these overdensities may not become clusters, they still trace extreme nodes of the cosmic web that may host early environmental effects, drive the first ionised regions, and contribute significantly to the cosmic star-formation rate density. These results demonstrate the need for more careful comparisons between observations and simulations of high-redshift protoclusters, and that improved selection criteria, potentially using the summation of the halo mass on Lagrangian scales, are vital for high-redshift protocluster science.

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arXiv2026

BlackTHUNDER strikes twice: rest-frame Balmer-line absorption and high Eddington accretion rate in a Little Red Dot at $z=7.04$

JWST has revealed a population of 'Little Red Dots' (LRDs): compact, red objects at redshifts z=2-9 with 'v'-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H$α$ line in the LRD Abell2744-QSO1 at z=7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H$α$ spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H$α$ absorption (EW relative to the continuum $\sim 22_{-7}^{+12} \mathring{\mathrm{A}}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H$α$, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $σ_{\rm abs}=110_{-10}^{+20}$ km s$^{-1}$, consistent with the value inferred from the analysis of the Balmer break. Based on H$α$, we infer a black hole mass of log($M_{\rm BH}/{\rm M_\odot}$)=7.2, smaller but close to the previous estimates based on H$β$. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H$α$ line with the spectral resolution R=3,700 of the G395H grating, we infer a narrow-line intrinsic dispersion $σ_{\rm n}=22_{-6}^{+5}$ km s$^{-1}$, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be $M_{\rm BH}/M_{\rm dyn}$=0.15-1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.

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arXiv2026

Metal Mayhem at $\rm z \sim 7-10$: Diversity and Evolution of Gas-Phase Metallicity Gradients

We present a JWST/NIRSpec-IFU study of metallicity gradients in seven low-metallicity systems at $z=7.2-9.5$. The main sample spans stellar masses of $\rm \log(M_*/M_{\odot}) \sim 7.8-9.5$, star formation rates (SFRs) of $\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim 0.5-2.5$, and gas-phase metallicities of $4\%-15 \%~Z_\odot$. Within our sample, we also identify three low-metallicity satellite galaxies associated with two of our sources, providing a rare view of early-epoch interactions. The three satellites exhibit even more primordial properties, with metallicity $3\% -4\% ~Z_\odot$ and low star-formation activity ($\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim -0.5$ to $-0.9$). We find that our galaxies, and especially the satellites, are significantly offset from the local Fundamental Metallicity Relation (FMR), with deviations reaching $\Delta \text{FMR} \approx -0.9$ dex. This indicates that these galaxies are likely experiencing strong accretion of pristine gas. Overall, we observe a large scatter in radial metallicity gradients, ranging from positive to negative with an average metallicity gradient of $\rm -0.02 \pm 0.04 \ dex \ kpc^{-1}$. Flat gradients are found in systems with confirmed satellites, suggesting that tidal interactions and mergers drive the radial mixing necessary to homogenise the interstellar medium. The (tentative) presence of an AGN in two of our sources suggests that strong feedback may also be responsible for the observed flat gradients. Conversely, the detection of a positive gradient in one source points toward a direct funnelling of metal-poor gas inflow into the central region of the galaxy. These results show that galaxies in the first billion years grow through diverse, episodic processes, suggesting that early evolution is characterised by structural variety rather than a single, predictable path.

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arXiv2026

Metallicity Gradients in Modern Cosmological Simulations II: The Role of Bursty Versus Smooth Feedback at High-Redshift

The distribution of gas-phase metals within galaxies encodes the impact of stellar feedback on galactic evolution. At high-redshift, when galaxies are rapidly assembling, feedback-driven outflows and turbulence can strongly reshape radial metallicity gradients. In this work, we use the FIRE-2, SPICE, Thesan and Thesan Zoom cosmological simulations -- spanning a range of stellar feedback from bursty (time-variable) to smooth (steady) -- to investigate how these feedback modes shape gas-phase metallicity gradients at $3 10^{9}~{\rm M_\odot}$. These results demonstrate that bursty stellar feedback provides sufficient turbulence to prevent strong negative gradients from forming, while smooth stellar feedback does not generically allow for efficient radial redistribution of metals thereby keeping gradients steep. Finally, we compare with recent observations, finding that the majority -- but, notably, not all -- of the observed gradients may favor a bursty stellar feedback scenario. In all, these results highlight the utility of high-resolution observations of gas-phase metallicity at high-redshift as a key discriminator of these qualitatively different feedback types.

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arXiv2026

Before its time: a remarkably evolved protocluster core at z=7.88

Protoclusters represent the most extreme environments in the very early Universe. They form from large-scale dark matter overdensities, harbouring an overabundance of galaxies fed by large gas reservoirs. Their early and accelerated evolution results in a distinct difference in the properties of galaxies resident in protoclusters versus the field, which is known to be in place by $z\sim 5-6$. We utilise JWST NIRCam observations of the A2744-z7p9OD protocluster at $z=7.88$ to constrain the properties of resident galaxies. We identify seven new protocluster members, bringing the total number to 23 and the total stellar mass of the protocluster to in excess of $10^{10}\ \rm{M_{\odot}}$. These galaxies are remarkably evolved just 650 Myr after the Big Bang, preferentially showing redder UV-slopes and stronger Balmer breaks than is typical of field galaxies. We use the PROSPECTOR spectral energy distribution fitting code to derive key galaxy properties, finding distinct populations in the core versus the outskirts of the protocluster. The core is largely composed of dusty, massive galaxies which can be characterised as undergoing a synchronised lulling phase, while galaxies in the protocluster outskirts are undergoing recent bursts of star formation. Finally, a strong suppression of the continuum around the Ly$α$-break evidences extreme neutral hydrogen column densities in many resident galaxies ($N_{\rm HI}\gtrsim10^{22.5}\ {\rm cm^{-2}}$). The A2744-z7p9OD system is the most extreme, evolved overdensity yet observed at $z>7$, with higher stellar masses, gas densities, and dust attenuation, revealing the intersection of local environment and high-redshift galaxy formation at their extremes.

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arXiv2026

The THESAN-ZOOM project: Mystery N/O more -- uncovering the origin of peculiar chemical abundances and a not-so-fundamental metallicity relation at $3<z<12$

We present an analysis of metallicities and chemical abundances at $3 -0.6$) without the need for exotic yields in our chemical network. Instead, bursty star formation naturally generates order-of-magnitude excursions in N/O on $\lesssim$100 Myr timescales due to temporally differential galactic winds; after a starburst, stellar feedback expels gas, leaving a large population of asymptotic-giant-branch stars to dominate the enrichment of the relatively low-mass interstellar medium. NRGs lie below the main sequence and typically exhibit $\mathrm{EW}[H$β$]\lesssim40$ Å, in apparent tension with observed high-EW NRGs. This tension is reconciled if observed NRGs are in the initial stages of a subsequent starburst, illuminating previously enriched gas, which is supported by the finding of high SFR surface density nitrogen-rich giant molecular clouds.

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arXiv2026

The THESAN-ZOOM project: Burst, quench, repeat -- unveiling the evolution of high-redshift galaxies along the star-forming main sequence

Characterizing the evolution of the star-forming main sequence (SFMS) at high redshift is crucial to contextualize the observed extreme properties of galaxies in the early Universe. We present an analysis of the SFMS and its scatter in the THESAN-ZOOM simulations, where we find a redshift evolution of the SFMS normalization scaling as $\propto (1+z)^{2.64\pm0.03}$, significantly stronger than is typically inferred from observations. We can reproduce the flatter observed evolution by filtering out weakly star-forming galaxies, implying that current observational fits are biased due to a missing population of lulling galaxies or overestimated star-formation rates. We also explore star-formation variability using the scatter of galaxies around the SFMS ($σ_{\mathrm{MS}}$). At the population level, the scatter around the SFMS increases with cosmic time, driven by the increased importance of long-term environmental effects in regulating star formation at later times. To study short-term star-formation variability, or ''burstiness'', we isolate the scatter on timescales shorter than 50 Myr. The short-term scatter is larger at higher redshift, indicating that star formation is indeed more bursty in the early Universe. We identify two starburst modes: (i) externally driven, where rapid large-scale inflows trigger and fuel prolonged, extreme star formation episodes, and (ii) internally driven, where cyclical ejection and re-accretion of the interstellar medium in low-mass galaxies drive bursts, even under relatively steady large-scale inflow. Both modes occur at all redshifts, but the increased burstiness of galaxies at higher redshift is due to the increasing prevalence of the more extreme external mode of star formation.

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arXiv2026

The THESAN-ZOOM project: central starbursts and inside-out quenching govern galaxy sizes in the early Universe

We explore the evolution of galaxy sizes at high redshift ($3 < z < 13$) using the high-resolution THESAN-ZOOM radiation-hydrodynamics simulations, focusing on the mass range of $10^6\,\mathrm{M}_{\odot} < \mathrm{M}_{\ast} < 10^{10}\,\mathrm{M}_{\odot}$. Our analysis reveals that galaxy size growth is tightly coupled to bursty star formation. Galaxies above the star-forming main sequence tend to form stars in a central starburst, which decreases their radial size. These galaxies quench inside-out, causing spatially extended star formation and increasing their radial size, leading to oscillatory behavior around the size-mass relation. Notably, we find a positive intrinsic size-mass relation at high redshift, consistent with observations but in tension with large-volume simulations. We attribute this discrepancy to the bursty star formation captured by our multi-phase interstellar medium framework, but missing from simulations using the effective equation-of-state approach with hydrodynamically decoupled feedback. We also find that the normalization of the size-mass relation follows a double power law as a function of redshift, with a break at $z\approx6$, because the majority of galaxies at $z > 6$ show rising star-formation histories, and therefore are in a compaction phase. We demonstrate that H$α$ emission is systematically extended relative to the UV continuum by a median factor of 1.7, consistent with recent JWST studies. However, in contrast to previous interpretations that link extended H$α$ sizes to inside-out growth, we find that Lyman-continuum (LyC) emission is spatially disconnected from H$α$. Instead, a simple Strömgren sphere argument reproduces observed trends, suggesting that extreme LyC production during central starbursts is the primary driver of extended nebular emission.

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arXiv2026

Overmassive black holes in the early Universe can be explained by gas-rich, dark matter-dominated galaxies

JWST has revealed the apparent evolution of the black hole (BH)-stellar mass ($M_\mathrm{BH}$-$M_\rm{\ast}$) relation in the early Universe, while remaining consistent with the BH-dynamical mass ($M_\mathrm{BH}$-$M_\mathrm{dyn}$) relation. We predict BH masses for $z>3$ galaxies in the high-resolution THESAN-ZOOM simulations by assuming the $M_\mathrm{BH}$-$M_\mathrm{dyn}$ relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed $M_\mathrm{BH}$ distribution, including overmassive BHs relative to the local $M_\mathrm{BH}$-$M_\mathrm{\ast}$ relation. We find that $M_\mathrm{BH}/M_\mathrm{\ast}$ declines with $M_\mathrm{\ast}$, evolving from $\sim$0.1 at $M_\mathrm{\ast}=10^6\,\mathrm{M_\odot}$ to $\sim$0.01 at $M_\mathrm{\ast}=10^{10.5}\,\mathrm{M_\odot}$. This trend reflects the dark matter ($f_\mathrm{DM}$) and gas fractions ($f_\mathrm{gas}$), which decrease with $M_\mathrm{\ast}$ but show little redshift evolution down to $z=3$, resulting in small $M_\mathrm{\ast}/M_\mathrm{dyn}$ ratios and thus overmassive BHs in low-mass galaxies. We use $\texttt{Prospector}$-derived stellar masses and star-formation rates to infer $f_\mathrm{gas}$ across 48,022 galaxies in JADES at $3<z<9$, finding excellent agreement with our simulation. Our results demonstrate that overmassive BHs would naturally result from a fundamental $M_\mathrm{BH}$-$M_\mathrm{dyn}$ relation and be typical of the gas-rich, dark matter-dominated nature of low-mass, high-redshift galaxies. Such overmassive BHs may strongly influence early galaxy formation, and we caution that our approach does not include the self-consistent BH-galaxy co-evolution required for a complete understanding.

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arXiv2026

The density-bounded twilight of starbursts in the early Universe

The peculiar nebular emission displayed by galaxies in the early Universe presents a unique opportunity to gain insight into the regulation of star formation in extreme environments. We investigate 500 (109) galaxies with deep NIRSpec/PRISM observations from the JADES survey at $z>2$ ($z>5.3$), finding 52 (26) galaxies with Balmer line ratios more than $1σ$ inconsistent with Case B recombination. These anomalous Balmer emitters (ABEs) cannot be explained by dust attenuation, indicating a departure from Case B recombination. To address this discrepancy, we model density-bounded nebulae with the photoionisation code CLOUDY. Density-bounded nebulae show anomalous Balmer line ratios due to Lyman line pumping and a transition from the nebulae being optically thin to optically thick for Lyman lines with increasing cloud depth. The H$α$/H$β$ versus H$γ$/H$β$ trend of density-bounded models is robust to changes in stellar age of the ionising source, gas density, and ionisation parameter; however, increasing the stellar metallicity drives a turnover in the trend. This is due to stronger stellar absorption features around Ly$γ$ reducing H$β$ fluorescence, allowing density-bounded models to account for all observed Balmer line ratios. ABEs show higher [OIII]/[OII], have steeper ultra-violet slopes, are fainter, and are more preferentially Ly$α$ emitters than galaxies which are consistent with Case B and little dust. These findings suggest that ABEs are galaxies that have become density bounded during extreme quenching events, representing a transient phase of $\sim$20 Myr during a fast breathing mode of star formation.

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arXiv2026

When relics were made: vigorous stellar rotation and low dark matter content in the massive ultra-compact galaxy GS-9209 at z=4.66

JWST uncovered a large number of massive quiescent galaxies (MQGs) at $z>3$, which theoretical models struggle to reproduce. Explaining the number density of such objects requires extremely high conversion efficiency of baryons into stars in early dark matter halos. Using stellar kinematics, we can investigate the processes shaping the mass assembly histories of MQGs. We present high-resolution JWST/NIRSpec integral field spectroscopy of GS-9209, a massive, compact quiescent galaxy at $z=4.66$ ($\log (M_{\ast}/M_{\odot})=10.52 \pm 0.06$, $R_{eff}=220 \pm 20$ pc). Full spectral fitting of the spatially resolved stellar continuum reveals a clear rotational pattern, yielding a spin parameter of $λ_{2R_{eff}}=0.85 \pm 0.10$. This study suggests that at least a fraction of the earliest quiescent galaxies were fast rotators and that quenching was a dynamically gentle process, preserving the stellar disc even in highly compact objects. Using Jeans anisotropic modelling and assuming a NFW profile, we measure a dark matter fraction of $f_{DM} (<2R_{eff}) = 14.5^{+6.0}_{-4.2} \% $. Our findings use stellar kinematics to confirm the massive nature of early quiescent galaxies, previously inferred from stellar population modelling. We suggest that GS-9209 has a similar structure to low-redshift `relic' galaxies. However, unlike relic galaxies which have bottom-heavy initial mass functions (IMF), the dynamically inferred mass-to-light ratio of GS-9209 is consistent with a Milky-Way like IMF. The kinematical properties of GS-9209 are different from those of $z<1$ early-type galaxies and more similar to those of recently quenched post-starburst galaxies at $z>2$.

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arXiv2026

JADES: the chemical enrichment pattern of distant galaxies -- $α$ enhancement, silicon depletion, and iron enhancement

We present gas-phase abundances of carbon (C), $α$-elements (O, Ne, Si, and Ar) and iron (Fe) obtained from stacked spectra of high-$z$ star-forming galaxies with the deep Near Infrared Spectrograph medium-resolution data from the James Webb Space Telescope Advanced Deep Extragalactic Survey. Our 564 sources at $z=4$--7 have a median stellar mass of $\log(M_{*}/M_{\odot})=8.46$ and a median star-formation rate of $\log(\mathrm{SFR}/M_{\odot}\,\mathrm{yr^{-1}})=0.30$, placing them close to the star-formation main sequence. We find that the stacked spectrum of all our 564 sources has relatively low [C/O]$=-0.70$, moderate [Ne/O]$=-0.09$, and low [Ar/O]$=-0.28$ values at a low gas-phase metallicity of $12+\log(\mathrm{O/H})=7.71$ ($Z\sim 0.1~Z_\odot$), suggesting dominant yields of core-collapse supernovae evolved from massive stars. The detection of a weak SiIII] emission line in our stacked spectrum provides a silicon-to-oxygen abundance ratio of [Si/O]$=-0.63$, which is lower than that of stars in the Milky Way disc and lower than expected by chemical evolution models, suggesting silicon depletion onto dust grains. Likewise, this Si/O value is lower than that we newly derive for two individual $z>6$ galaxies (GN-z11 and RXCJ2248) with negligible dust attenuation. By performing spectral stacking in bins of $M_{*}$, SFR, specific SFR (sSFR), and ultra-violet (UV) continuum slope $β_{\mathrm{UV}}$, we identify [FeIII] line detections in the high-sSFR bin and the blue-$β_{\mathrm{UV}}$ bin, both of which exhibit supersolar Fe/O ratios, while their C/O, Ar/O, and Si/O ratios are comparable to those of the all-sources stack. Our findings support a chemically young gas composition with rapid dust depletion in the general population of high-$z$ star-forming galaxies, while raising the possibility of anomalous, selective Fe/O enhancement at the very early epoch of star formation.

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