arXiv subjects

T. M. Stanton

Publications and source records attributed to T. M. Stanton.

At least 19 recordsLinked to original sources

arXiv2026

A search for the first galaxies across $>0.6$ deg$^2$ of JWST imaging: new evidence for a rapid decline in star-formation activity at $z>12$

We present a new determination of the evolving galaxy UV luminosity function (LF) over the extreme redshift range $12.5 $0.6 deg$^2$ of JWST NIRCam imaging containing $>150$ independent sight-lines. We find evidence for an accelerated decline in the UV LF, and hence inferred star-formation rate density ($\rho_{\rm SFR}$), over the $\simeq100\rm{Myr}$ cosmic time interval between $z=11$ and $z=13.5$. Moreover, based on a notable lack of galaxy candidates at $z>14.5$, we find evidence for an even more rapid descent in star-formation activity towards earlier times, with our new measurement of $\rho_{\rm SFR}$ at $z\simeq15.5$ lying significantly below an extrapolation of the log-linear $\rho_{\rm SFR}(\rm z)$ relation inferred from early JWST LF studies. Instead, we find that the evolution in $\rho_{\rm SFR}(\rm z)$ at these very early times is better described by a piece-wise log-linear relation, in which the decline in $\rho_{\rm SFR} (\rm z)$ at $z>12$ is $\simeq4$ times steeper than at redshifts $z < 12$. Our observational results are consistent with a number of theoretical models of galaxy evolution which have incorporated a range of treatments in an attempt to explain the prevalence of UV-bright galaxies at least out to $z \simeq 12$ (e.g., increased star-formation efficiency, stochastic star-formation histories, an evolving stellar initial mass function and/or a shift towards attenuation-free stellar populations). However, our results are also entirely consistent with a relatively simple galaxy evolution model with no such adjustments, in which the rapid evolution of the dark-matter halo mass function at early times is for a while partially masked by progressively younger stellar ages, with the inferred epoch of first galaxy formation lying at $z\simeq15$.

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arXiv2026

Electron temperature relations and the direct N, O, Ne, S and Ar abundances of 49959 star-forming galaxies in DESI Data Release 2

We present the largest direct-method abundance catalogue of galaxies to date, containing measurements of 49959 star-forming galaxies at z < 0.96 from DESI Data Release 2. By directly measuring electron temperatures across multiple ionisation zones, we provide constraints on a number of electron temperature relations. Using the temperature measurements, we derive reliable abundances for N, O, Ne, S and Ar and measure the evolution of abundances and abundance ratios of as a function of metallicity and other galaxy properties. Our measurements include direct oxygen abundances for 49507 galaxies, leading to the discovery of the two most metal-poor galaxies in the nearby Universe, with oxygen abundances of 12+log(O/H) = $\rm 12+\log(O/H) = 6.77_{-0.03}^{+0.03}~\rm dex $ (1.2\% $\rm Z_{\odot}$) and $\rm 12+\log(O/H) = 6.81_{-0.04}^{+0.04}~\rm dex$ (1.3\% $\rm Z_{\odot}$). We identify a rare outlier population of 24 galaxies with high N/O ratios at low metallicity, reminiscent of galaxy abundances observed in the early Universe. We find the Ne/O ratio is constant at low metallicity but increases gradually at $\rm 12+log(O/H) > 8.105\pm0.004$ dex. We show that the S/O and Ar/O abundance ratios are strongly correlated, consistent with the expected additional Type Ia enrichment channel for S and Ar. In this work we present an initial survey of the key properties of the sample, with this dataset serving as a foundation for extensive future work on galaxy abundances at low redshift.

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arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Northern Local Star-forming Dwarf Galaxies. Analogues of the First Galaxies and Probes of the Cosmic Metallicity Scale

Star-forming dwarf galaxies in the local Universe, especially extremely metal-poor ones, can be considered analogous to early galaxies of the Epoch of Reionization (z >= 6). Currently available telescopes cannot adequately detect and measure heavy element recombination lines and certain faint collisionally excited lines, which are essential for exploring the effects and biases that potential inhomogeneities in electron temperature and density of the ionized gas may have on determining the chemical composition of these galaxies. On the other hand, the origin of very high-ionization lines (e.g. He II, [Ne V], C IV]) measured in the spectra of an important fraction of these objects remains unknown and a challenge to current stellar models, suggesting the presence of Population III-like stars and/or the existence of non-conventional ionizing sources. Obtaining very deep spectra for a selected sample of local star-forming dwarf galaxies would provide unprecedented constraints on their nature, ionization and true chemical abundances, and could change the metallicity scale we assume to understand the chemical evolution of galaxies over cosmic time.

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arXiv2025

The JWST EXCELS Survey: gas-phase metallicity evolution at 2 < z < 8

We present an analysis of the gas-phase mass-metallicity relationship (MZR) and fundamental metallicity relationship (FMR) for $65$ star-forming galaxies at $2 < z < 8$ from the JWST/EXCELS survey. We calculate gas-phase metallicities (12 + log(O/H)) using strong-line calibrations explicitly tested against the EXCELS sample, and report direct-method metallicities for $19$ galaxies. Our sample spans $8.1<\log(\rm M_\star/M_\odot)<10.3$ and $0<\log(\rm SFR/M_\odot \, yr^{-1})<2$, consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both $2 3$ studies. We discuss potential drivers of this offset, noting that high-redshift samples have significantly different physical properties compared to local samples used to define the $z=0$ FMR. Our results confirm that low-mass, high specific star-formation rate galaxies common at high redshift are inconsistent with the equilibrium conditions underlying the local FMR, and highlight the rapid chemical enrichment at early cosmic epochs.

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arXiv2025

Extreme equivalent width-selected low-mass starbursts at $z=4-9$: insights into their role in cosmic reionization

We investigate the properties of extreme emission line galaxies (EELGs) at $z=4-9$ and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compile 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+H${\beta}$)=1616\r{A} and EW(H${\alpha}$)=763\r{A}. They are low-mass (median log(M$_{\star}$/M$_{\odot}$)=8.26) with high sSFRs (median 43 Gyr$^{-1}$), above the $z\sim6$ main sequence. UV slopes are diverse, with a mean $\beta=-2.0$, and only 7% have extremely blue continua ($\beta<-2.6$). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% is cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with mean log($\xi_{\rm ion}$ [Hz erg$^{-1}$])=25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at $z<7$ show escape fractions $>$5% for both Ly${\alpha}$ and LyC photons, while 82% lack detectable Ly${\alpha}$ emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR >25 Gyr$^{-1}$. These results indicate that EELGs contribute approximately 16-40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star-formation burst. Our analysis indicates that sSFR and star-formation rate surface density are the primary drivers of their extreme emission line strengths.

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arXiv2025

The JWST EXCELS Survey: A spectroscopic investigation of the ionizing properties of star-forming galaxies at 1<z<8

Charting the Epoch of Reionization demands robust assessments of what drives the production of ionizing photons in high-redshift star-forming galaxies (SFGs), and requires better predictive capabilities from current observations. Using a sample of $N=159$ SFGs at $1<z<8$, observed with deep medium-resolution spectroscopy from the JWST/NIRSpec EXCELS survey, we perform a statistical analysis of their ionizing photon production efficiencies ($\xi_\rm{ion}$). We consider $\xi_\rm{ion}$, measured with Balmer line measurements, in relation to a number of key galaxy properties including; nebular emission line strengths ($W_\lambda(\rm{H\alpha})$ and $W_\lambda$( [OIII])), UV luminosity ($M_\rm{UV}$) and UV slope ($\beta_\rm{UV}$), as well as dust attenuation ($E(B-V)_\rm{neb}$) and redshift. Implementing a Bayesian linear regression methodology, we fit $\xi_\rm{ion}$ against the principal observables while fully marginalising over all measurement uncertainties, mitigating against the impact of outliers and determining the intrinsic scatter. Significant relations between $\xi_\rm{ion}$ and $ W_\lambda(\rm{H\alpha})$, $W_\lambda$([OIII]) and $\beta_\rm{UV}$ are recovered. Moreover, the weak trends with $M_\rm{UV}$ and redshift can be fully explained by the remaining property dependencies. Expanding our analysis to multivariate regression, we determine that $W_\lambda(\rm{H\alpha})$ or $W_\lambda$([OIII]), along with $\beta_\rm{UV}$ and $E(B-V)_\rm{neb}$, are the most important observables for accurately predicting $\xi_\rm{ion,0}$. The latter identifies the most common outliers as SFGs with relatively high $E(B-V)_\rm{neb}\gtrsim0.5$, possibly indicative of obscured star-formation or strong differential attenuation. Combining these properties enable $\xi_\rm{ion,0}$ to be inferred with an accuracy of $\sim0.15\,$dex, with a population intrinsic scatter of $\sigma_\rm{int}\sim0.035\,$dex.

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arXiv2025

Ly$\alpha$ visibility from z = 4.5 to 11 in the UDS field: Evidence for a high neutral hydrogen fraction and small ionized bubbles at z $\sim$ 7

The resonant scattering nature of Ly$\alpha$ photons interacting with neutral hydrogen makes Ly$\alpha$ emitters (LAEs) robust tracers of the intergalactic neutral hydrogen fraction, and thus sensitive probes of cosmic reionization. We present an extensive study of the Ly$\alpha$ evolution from galaxies at 4.5 $\leq$ z $\leq$ 11 in the UDS field, observed as part of the CAPERS survey, and complemented with spectra from the DAWN JWST Archive. The combined sample includes 651 spectroscopically confirmed Ly$\alpha$-break galaxies, among which we find 73 S/N>3 LAEs in JWST-NIRSpec PRISM spectra. We trace the redshift evolution of the LAE fraction with EW$_0$ >25 A (X$_{\mathrm{Ly\alpha}}$) between z = 5 and z = 9, extending such an analysis to the UDS field for the first time. At z = 5 and 6, the UDS results agree with the average JWST X$_{\mathrm{Ly\alpha}}$ values from multiple fields. However, JWST measurements are consistently lower than ground-based results. To investigate this, we compare JWST observations to a population of star-forming galaxies at z$\sim$6 observed with VLT-FORS2. We find that a Ly$\alpha$ slit-loss of 35 $\pm$ 10% in JWST spectra accounts for the offset, as the resonant Ly$\alpha$ emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly$\alpha$ visibility, from which we infer a neutral hydrogen fraction of X$_{\mathrm{HI}}$ = 0.7--0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of $R_{\mathrm{ion}}$ = 0.6 and 0.5 physical Mpc and photometric overdensities of N/$\langle$N$\rangle$ = 3 and 4, based on candidate counts down to the photometric completeness limit. Compared to the large ionized region at z$\sim$7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.

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arXiv2025

Strength in Numbers: Red Galaxies Bolster the Cosmic Star Formation Rate Density at z > 3

A comprehensive account of the cosmic star-formation history demands an accurate census of dust-enshrouded star formation over cosmic time. We provide strong new constraints from a large sample of 777 red galaxies, selected based on their dust-reddened, rest-frame UV-optical emission. This sample of 777 galaxies spans $1 < z < 8$ and is selected from PRIMER JWST NIRCam and HST COSMOS optical data, ensuring robust colour criteria. The SEDs indicate that these dust-reddened galaxies are star-forming, with median $\mathrm{SFR \sim 40M_{\odot}yr^{-1}}$ and stellar mass $\log(M_{*}/M_{\odot}) = 10.3^{+0.6}_{-0.8}$; each exceeds the corresponding medians of the full JWST-detected population by over two dex. Our sample thus clearly shows that red galaxies dominate the high-mass end: they comprise 72 \% of galaxies with $\log(M/M_{\odot}) > 10$ at $z = 3.3$, rising to 91\% by $z \sim 7$ (albeit with large uncertainties at the highest redshifts). Crucially, we find that the number density of massive red star-forming galaxies at $z \sim 6$ is sufficient to explain the abundance of quiescent galaxies at $z > 3$, consistent with typical quenching timescales allowed in the $\mathrm{\sim 1Gyr}$ interval from $z \sim 6$ to $z \sim 3$. This large abundance yields a substantial contribution to the cosmic star-formation rate density: at $z \sim 4$, red galaxies provide $\mathrm {\rho_{SFR} = 3.9^{+0.6}_{-0.5} \times 10^{-2} M_{\odot} yr^{-1}Mpc^{-3}}$, and at $z \sim 5$ they supply nearly 40 \% of the total $\rho_{SFR}$. This exceeds the contribution of bright sub(mm)-selected dusty star-forming galaxies by more than an order of magnitude. Future deeper and wider ALMA surveys will provide further opportunities to strengthen and extend our results in our quest to fully quantify the contribution of dust-obscured activity to $\rho_{\mathrm{SFR}}$ at high redshifts.

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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 JWST EXCELS survey: Probing strong-line diagnostics and the chemical evolution of galaxies over cosmic time using Te-metallicities

We present an analysis of the rest-frame optical spectra of 22 [OIII]$λ$4363 detected galaxies in the redshift range $1.65 < z < 7.92$ (with $\langle z \rangle$ = 4.05) from JWST/NIRSpec medium-resolution observations taken as part of the EXCELS survey. To supplement these high-redshift sources, we also consider a sample of 782 local [OIII]$λ$4363 detected galaxies from the DESI Early Data Release. Our analysis demonstrates that many strong-line calibrations are biased in the early Universe due to the systematic evolution in ionization conditions with redshift. However, the recently introduced $\widehat{R}$ calibration mostly removes the dependence on ionization state and can be considered a largely redshift-independent calibration. In a similar spirit, we introduce a new strong-line diagnostic, $\widehat{RNe}$, which can be used to robustly estimate metallicities when the [OIII]$λ$5007 is redshifted out of the wavelength range of JWST/NIRSpec at $z > 9.5$. We also show that strong-line diagnostics using the [NII]$λ$6584 emission line are likely to be biased at high-redshift due to a moderate enhancement in the average N/O abundance ratios (at fixed O/H) in these sources. Finally, we discuss the location of our new [OIII]$λ$4363 detected galaxies at $z \simeq 4$ on the mass-metallicity plane and investigate the redshift evolution of the fundamental metallicity relation (FMR). We find tentative evidence for an increasing deviation from the FMR at $z > 4$ which might indicate fundamental differences in the baryon cycle at these redshifts. However, more data are required as our high-redshift constraints are still based on a relatively small sample of galaxies and the significance of the deviation is strongly dependent on the assumed form of the fundamental metallicity relation.

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arXiv2025

The evolution of [OIII]$+\rm{H}β$ equivalent width from $\mathbf{z\simeq3-8}$: implications for the production and escape of ionizing photons during reionization

Accurately quantifying the ionizing photon production efficiency ($ξ_\rm{ion}$) of $z>6$ star-forming galaxies (SFGs) is necessary to understand their contribution to reionization. We investigate the ionizing properties of N=279 SFGs selected at $z=6.9-7.6$ from the JWST Cycle-1 imaging programmes; PRIMER and JADES. We use BAGPIPES to consistently infer the equivalent widths of their [OIII]+$\rm{Hβ}$ emission lines ($W_λ$) and their physical properties. To supplement our high-redshift galaxies, we measure $W_λ$ photometrically for a sample of N=253 $z=3.2-3.6$ SFGs selected from the VANDELS spectroscopic survey. Comparing these samples, we find a strong apparent redshift evolution in their median $W_λ$, increasing from $W_λ=310\pm25\,Å$ in VANDELS to $W_λ=540\pm25\,Å$ in our JWST-based sample. In the JWST sample at $z>7$, we find that $W_λ$ correlates with both stellar mass and UV luminosity, with high-mass, $M_{ UV}-$faint galaxies producing systematically weaker emission lines. Moreover, we discover a departure from the standard log-normal shape of the $W_λ$ distribution, characterised by a more pronounced tail at lower $W_λ$, consistent with increasingly bursty star formation. Using $W_λ$ as a proxy for $ξ_\rm{ion}$, and UV spectral slope as a proxy for LyC escape fraction ($f_\rm{esc}$), we find a minority of galaxies with high $ξ_\rm{ion}$ and $f_\rm{esc}$ (e.g., $\rm{log(ξ_{ion}/erg^{-1}Hz})\sim25.6$ and $f_\rm{esc}\sim0.15$). However, we find that the LyC photon budget at $z>7$ is dominated by galaxies with more moderate output, close to the sample median of $\rm{log(ξ_{ion}/erg^{-1}Hz})\sim25.3$ and $f_\rm{esc}\sim0.05$. This is consistent with estimates for the number of LyC photons required to power reionization at $z>7$, with no evidence for over or under-production.

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arXiv2025

The JWST EXCELS survey: tracing the chemical enrichment pathways of high-redshift star-forming galaxies with O, Ar and Ne abundances

We present an analysis of eight star-forming galaxies with $\langle z \rangle = 4.0$ from the JWST EXCELS survey for which we obtain robust chemical abundance estimates for the $α$-elements O, Ne and Ar. The $α$-elements are primarily produced via core-collapse supernovae (CCSNe) which should result in $α$-element abundance ratios that do not vary significantly across cosmic time. However, Type Ia supernovae (SNe Ia) models predict an excess production of Ar relative to O and Ne. The Ar/O abundance ratio can therefore be used as a tracer of the relative enrichment of CCSNe and SNe Ia in galaxies. Our sample significantly increases the number of sources with measurements of ${\rm Ar/O}$ at $z > 2$, and we find that our sample exhibits sub-solar Ar/O ratios on average, with $\rm{Ar/O} = 0.65 \pm 0.10 \, (\rm{Ar/O})_{\odot}$. In contrast, the average Ne/O abundance is fully consistent with the solar ratio, with $\rm{Ne/O} = 1.07 \pm 0.12 \, (\rm{Ne/O})_{\odot}$. Our results support a scenario in which Ar has not had time to build up in the interstellar medium of young high-redshift galaxies, which are dominated by CCSNe enrichment. We show that these abundance estimates are in good agreement with recent Milky Way chemical evolution models, and with Ar/O trends observed for planetary nebulae in the Andromeda galaxy. These results highlight the potential for using multiple element abundance ratios to constrain the chemical enrichment pathways of early galaxies with JWST.

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arXiv2024

JWST PRIMER: strong evidence for the environmental quenching of low-mass galaxies out to $\mathbf{\textit{z} \simeq 2}$

We present the results of a study investigating the galaxy stellar-mass function (GSMF), size-mass relations and morphological properties of star-forming and quiescent galaxies over the redshift range $0.25<z<2.25$, using the JWST PRIMER survey. The depth of the PRIMER near-IR imaging allows us to confirm the double Schechter function shape of the quiescent GSMF out to $z\simeq2.0$, via a clear detection of the upturn at $\mathrm{log}_{10}(M_{\star}/ M_{\odot}) \leq 10$ thought to be induced by environmental quenching. In addition to the GSMF, we confirm that quiescent galaxies can be split into separate populations at $\mathrm{log}_{10}(M_{\star}/M_{\odot}) \simeq 10$, based on their size-mass relations and morphologies. We find that low-mass quiescent galaxies have more disk-like morphologies (based on Sérsic index, Gini coefficient and $M_{20}$ metrics) and follow a shallower size-mass relation than their high-mass counterparts. Indeed, the slope of the size-mass relation followed by low-mass quiescent galaxies is indistinguishable from that followed by star-forming galaxies, albeit with a lower normalization. Moreover, within the errors, the evolution in the median size of low-mass quiescent galaxies is indistinguishable from that followed by star-forming galaxies ($R_{e}\propto(1+z)^{-0.25\pm0.03})$, and significantly less rapid than that displayed by high-mass quiescent galaxies ($R_{e}\propto (1+z)^{-1.14\pm 0.03})$. Overall, our results are consistent with low and high-mass quiescent galaxies following different quenching pathways. The evolution of low-mass quiescent galaxies is qualitatively consistent with the expectations of external/environmental quenching (e.g. ram-pressure stripping). In contrast, the evolution of high-mass quiescent galaxies is consistent with internal/mass quenching (e.g. AGN feedback) followed by size growth driven by minor mergers.

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arXiv2024

The JWST EXCELS survey: Too much, too young, too fast? Ultra-massive quiescent galaxies at 3 < z < 5

We report ultra-deep, medium-resolution spectroscopic observations for 4 quiescent galaxies with log$_{10}(M_*/\mathrm{M_\odot})>11$ at $3 < z < 5$. These data were obtained with JWST NIRSpec as part of the Early eXtragalactic Continuum and Emission Line Science (EXCELS) survey, which we introduce in this work. The first two galaxies are newly selected from PRIMER UDS imaging, both at $z=4.62$ and separated by $860$ pkpc on the sky, within a larger structure for which we confirm several other members. Both formed at $z\simeq8-10$. These systems could plausibly merge by the present day to produce a local massive elliptical galaxy. The other two ultra-massive quiescent galaxies are previously known at $z=3.99$ and $3.19$, with the latter (ZF-UDS-7329) having been the subject of debate as potentially too old and too massive to be accommodated by the $Λ$-CDM halo-mass function. Both exhibit high stellar metallicities, and for ZF-UDS-7329 we are able to measure the $α-$enhancement, obtaining [Mg/Fe] = $0.42^{+0.19}_{-0.17}$. We finally evaluate whether these 4 galaxies are consistent with the $Λ$-CDM halo-mass function using an extreme value statistics approach. We find that the $z=4.62$ objects and the $z=3.19$ object are unlikely within our area under the assumption of standard stellar fractions ($f_*\simeq0.1-0.2$). However, these objects roughly align with the most massive galaxies expected under the assumption of 100 per cent conversion of baryons to stars ($f_*$=1). Our results suggest extreme galaxy formation physics during the first billion years, but no conflict with $Λ$-CDM cosmology.

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arXiv2024

JWST PRIMER: A new multi-field determination of the evolving galaxy UV luminosity function at redshifts $\mathbf{z \simeq 9-15}$

We present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range $8.5 8.5)>0.05$) to undertake a statistical calculation of the UV LF. Our new measurements span $\simeq4$ magnitudes in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. Our measurements yield a new estimate of the early evolution of cosmic star-formation rate density ($ρ_{\rm{SFR}}$) confirming the gradual decline deduced from early JWST studies, at least out to $z \simeq 12$. Finally we show that the observed early evolution of the galaxy UV LF (and $ρ_{\rm{SFR}}$) can be reproduced in a ${\rm Λ}$CDM Universe, with no change in dust properties or star-formation efficiency required out to $z \simeq 12$. Instead, a progressive trend towards younger stellar population ages can reproduce the observations, and the typical ages required at $z \simeq$ 8, 9, 10, and 11 all converge on $\simeq 380-330$ Myr after the Big Bang, indicative of a rapid emergence of early galaxies at $z \simeq 12 - 13$. This is consistent with the first indications of a steeper drop-off in $ρ_{\rm{SFR}}$ we find beyond $z \simeq 13$, possibly reflecting the rapid evolution of the halo mass function at earlier times.

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arXiv2024

The NIRVANDELS Survey: the stellar and gas-phase mass-metallicity relations of star-forming galaxies at z = 3.5

We present determinations of the gas-phase and stellar metallicities of a sample of 65 star-forming galaxies at $z \simeq 3.5$ using rest-frame far-ultraviolet (FUV) spectroscopy from the VANDELS survey in combination with follow-up rest-frame optical spectroscopy from VLT/KMOS and Keck/MOSFIRE. We infer gas-phase oxygen abundances ($Z_{\mathrm{g}}$; tracing O/H) via strong optical nebular lines and stellar iron abundances ($Z_{\star}$; tracing Fe/H) from full spectral fitting to the FUV continuum. Our sample spans the stellar mass range $8.5 < \mathrm{log}(M_{\star}/\mathrm{M}_{\odot}) < 10.5$ and shows clear evidence for both a stellar and gas-phase mass-metallicity relation (MZR). We find that our O and Fe abundance estimates both exhibit a similar mass-dependence, such that $\mathrm{Fe/H}\propto M_{\star}^{0.30\pm0.11}$ and $\mathrm{O/H}\propto M_{\star}^{0.32\pm0.09}$. At fixed $M_{\star}$ we find that, relative to their solar values, O abundances are systematically larger than Fe abundances (i.e., $α$-enhancement).We estimate an average enhancement of $\mathrm{(O/Fe)} = 2.65 \pm 0.16 \times \mathrm{(O/Fe)_\odot}$ which appears to be independent of $M_{\star}$. We employ analytic chemical evolution models to place a constraint on the strength of galactic-level outflows via the mass-outflow factor ($η$). We show that outflow efficiencies that scale as $η\propto M_{\star}^{-0.32}$ can simultaneously explain the functional form of of the stellar and gas-phase MZR, as well as the degree of $α$-enhancement at fixed Fe/H. Our results add further evidence to support a picture in which $α$-enhanced abundance ratios are ubiquitous in high-redshift star-forming galaxies, as expected for young systems whose interstellar medium is primarily enriched by core-collapse supernovae.

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arXiv2024

The ultraviolet continuum slopes of high-redshift galaxies: evidence for the emergence of dust-free stellar populations at z > 10

We present an analysis of the ultraviolet (UV) continuum slopes ($β$) for a sample of $172$ galaxy candidates at $8 < z_{\mathrm{phot}} < 16$ selected from a combination of JWST NIRCam imaging and COSMOS/UltraVISTA ground-based near-infrared imaging. Focusing primarily on a new sample of $121$ galaxies at $\langle z \rangle \simeq 11$ selected from $\simeq 320$ arcmin$^2$ of public JWST imaging data across $15$ independent data sets, we investigate the evolution of $β$ in the galaxy population at $z \geq 9$. We find a significant trend between $β$ and redshift, with the inverse-variance weighted mean UV slope evolving from $\langle β\rangle = -2.17 \pm 0.06$ at $z = 9.5$ to $\langle β\rangle = -2.59 \pm 0.06$ at $z = 11.5$. Based on a comparison with stellar population models including nebular continuum emission, we find that at $z>10.5$ the average UV continuum slope is consistent with the intrinsic blue limit of dust-free stellar populations $(β_{\mathrm{int}} \simeq -2.6)$. These results suggest that the moderately dust-reddened galaxy population at $z < 10$ was essentially unattenuated at $z \simeq 11$. The extremely blue galaxies being uncovered at $z>10$ place important constraints on dust attenuation in galaxies in the early Universe, and imply that the already observed galaxy population is likely supplying an ionising photon budget capable of maintaining ionised IGM fractions of $\gtrsim 5$ per cent at $z\simeq11$.

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arXiv2023

The galaxy UV luminosity function at $\mathbf{z \simeq 11}$ from a suite of public JWST ERS, ERO and Cycle-1 programs

We present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range $9.5 250$ arcmin$^2$) data set of JWST NIRCam near-infrared imaging assembled from thirteen public JWST surveys. Our relatively large-area search allows us to uncover a sample of 61 robust $z>9.5$ candidates detected at $\geq 8σ$, and hence place new constraints on the intermediate-to-bright end of the UV LF. When combined with our previous JWST+UltraVISTA results, this allows us to measure the form of the LF over a luminosity range corresponding to four magnitudes ($M_{1500}$). At these early times we find that the galaxy UV LF is best described by a double power-law function, consistent with results obtained from recent ground-based and early JWST studies at similar redshifts. Our measurements provide further evidence for a relative lack of evolution at the bright-end of the UV LF at $z=9-11$, but do favour a steep faint-end slope ($α\leq-2$). The luminosity-weighted integral of our evolving UV LF provides further evidence for a gradual, smooth (exponential) decline in co-moving star-formation rate density ($ρ_{\mathrm{SFR}}$) at least out to $z\simeq12$, with our determination of $ρ_{\mathrm{SFR}}(z=11)$ lying significantly above the predictions of many theoretical models of galaxy evolution.

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