Ryan Begley

We present JWST/NIRSpec PRISM observations of a robust galaxy candidate at z ≃ 14, selected from pure-parallel NIRCam imaging: PAN-z14-1. The NIRSpec spectrum allows confirmation of this source at z spec = 13.5 3 − 0.06 + 0.05 through modeling of the Ly α break. PAN-z14-1 is the fourth most distant galaxy known to date and is extremely luminous ( M _UV = −20.6 ± 0.2), with a blue UV-continuum slope ( β = −2.26 ± 0.08) and a large physical size ( r _c = 233 ± 10 pc). We fail to detect any rest-frame UV emission lines at ≥2 σ significance, with upper limits sufficiently constraining to exclude the possibility of strong line emission. In terms of its physical properties, PAN-z14-1 is remarkably similar to the previously confirmed z _spec = 14.18 galaxy GS-z14-0. The lack of strong emission lines and large physical size is consistent with an emerging picture of two potentially distinct galaxy populations at z > 10, distinguished by star formation rate surface density. In this scenario, PAN-z14-1 is a second example of a “normal” extended luminous star-forming galaxy at z ≃ 14, and differs markedly from the other class of extremely compact galaxies with strong emission lines recently uncovered at extreme redshifts with JWST. These results highlight the importance of further spectroscopic confirmation of z > 10 galaxy candidates in order to fully understand the diversity of properties displayed by the first galaxies.

We present deep, medium-resolution λ = 1 − 5 μm JWST/NIRSpec spectroscopy for 14 quiescent galaxies at 3 < z < 5 with log10(M*/M⊙) > 10, obtained as part of the EXCELS survey. We perform a complete re-reduction of these data, including a custom optimal-extraction approach to combat the spectral “wiggles” that result from undersampling of the NIRSpec spatial PSF. We constrain the star-formation histories and stellar metallicities of these objects via full-spectral fitting, finding a clear stellar age vs stellar mass correlation, in which more massive galaxies assembled their stellar mass at earlier times. This confirms spectroscopically that the archaeological “downsizing” trend was already in place by z ≃ 4. The slope of our measured relation (≃ 2 Gyr per dex in stellar mass) is consistent with literature results at 0 < z < 3. We do not observe objects with log10(M*/M⊙) ≲ 10.5 and ages of more than a few hundred Myr at this epoch, suggesting that recently reported examples of higher-redshift quiescent galaxies at these masses are likely to soon rejuvenate. We measure relatively high stellar metallicities for the majority of our sample, consistent with similar objects at 0 < z < 3. Finally, we explore evidence for α-enhancement in six older and more luminous galaxies within our sample, finding considerable disagreements in the chemical abundances measured using different stellar population models, different fitted rest-frame wavelength ranges, star-formation history models and fitting codes. We therefore conclude that inferring detailed stellar chemical abundances for the earliest quiescent galaxies remains challenging, and higher signal-to-noise spectra are required (observed frame SNR per resolution element ≃ 100 for R ≃ 1000).

The resonant scattering nature of Ly α photons interacting with neutral hydrogen makes Ly α 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 α evolution from galaxies at 4.5 ≤ z ≤ 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 α -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 Å (X _{Ly α} ) 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 _{Ly α} 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 ∼ 6 observed with VLT-FORS2. We find that a Ly α slit-loss of 35 ± 10% in JWST spectra accounts for the offset, as the resonant Ly α emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly α visibility, from which we infer a neutral hydrogen fraction of X _HI = 0.7–0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of R _ion = 0.6 and 0.5 physical Mpc and photometric overdensities of N/⟨N⟩ = 3 and 4, based on candidate counts down to the photometric completeness limit. Compared to the large ionized region at z ∼ 7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.

We study 24 massive quiescent galaxies with log M * / M ⊙ > 10 at 1 < z < 3 with JWST/NIRSpec medium-resolution observations from the Early Extragalactic Continuum and Emission Line Survey (EXCELS). We reconstruct their star formation histories and find that they have large bursts (100–1000 M _⊙ yr ⁻¹ ), followed by a rapid truncation of star formation. The number densities of the quenched galaxies in our sample that we predict underwent a submillimeter phase are consistent with submillimeter galaxies being the progenitors of our quenched population. The median post-starburst visibility time is ∼600 Myr, with more massive galaxies ( log M * / M ⊙ > 10.7 ) exhibiting shorter visibility times than lower-mass galaxies. The range of quenching times—defined as the time from the peak starburst to the time of quiescence—found in this sample (0.06–1.75 Gyr) suggests multiple quenching pathways, consistent with previous studies. We do not see evidence for quenching mechanisms varying with redshift between 1 < z < 3. We detect evidence for weak active galactic nucleus activity in four out of the eight galaxies with robust emission line detections, based on line ratio diagnostics. Our findings suggest that there is a diverse range of quenching mechanisms at Cosmic Noon, and support a scenario in which the primary quenching mechanisms are rapid (<500 Myr) following a starburst.

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 ) and |$0<\log (\rm SFR/M_\odot \, yr^{-1}), consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both 2 < z < 4 (〈z〉 = 3.2) and 4 < z < 8 (〈z〉 = 5.5), with consistent slopes and mild evolution in normalization of ≃ 0.1 dex, matching trends from simulations and recent observations. Our results demonstrate rapid gas-phase enrichment in the early Universe; galaxies at z ≃ 3 (within the first ≃ 15 per cent of cosmic time) are enriched to ≃ 40 per cent of the metallicity of equivalent mass galaxies at z = 0. We find tentative evidence for SFR-dependence in the MZR scatter, though results remain inconclusive and highlight the need for larger high-redshift samples. Comparison with locally derived FMRs reveals a clear offset consistent with other z > 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.