Galaxy clusters are the most massive, gravitationally bound structures in the Universe. They emerged through hierarchical structure formation of large-scale dark matter and baryon overdensities. Early galaxy ‘proto-clusters’ are believed to have substantially contributed to the cosmic star-formation rate density and served as ‘hotspots’ for the reionization of the intergalactic medium. Our understanding of the formation of these structures at the earliest cosmic epochs is, however, limited to sparse observations of their galaxy members or is based on phenomenological models and cosmological simulations. Here we report the detection of a large and coherent structure of neutral atomic hydrogen gas (H i) extending from a galaxy proto-cluster at redshift z = 5.4, one billion years after the Big Bang. The presence of this H i gas is revealed by strong damped Lyman-α absorption features observed in several background-galaxy spectra. Although the sight lines overall probe a large range in H i column densities, N_HI = 10²⁰ cm⁻² to 10^23.5 cm⁻², they are similar across nearby sight lines, demonstrating that they probe the same dense neutral gas. This observation of a dense large-scale overdensity of cold neutral gas challenges current cosmological simulations and has strong implications for the reionization topology of the Universe.

We report the detection of a previously unknown object with a single emission line within the Hubble Ultra Deep Field, using a new JWST NIRSpec technique called Multiobject Coronagraphy for Slitless Spectroscopy (MOCSS) with the G140M/F100LP observing mode. The MOCSS technique involves opening almost all the shutters on the NIRSpec microshutter array, while simultaneously closing the shutters that lie on top of known low-redshift objects, facilitating a wide-field slitless spectroscopic search without contamination from bright objects. This observational technique has enabled the detection of a strong emission line in four different dither positions and in two position angles, and thus cannot be artificial. This line is detected at 1.60 μm with a line flux of [2.5 ± 0.3] × 10⁻¹⁸ erg s⁻¹cm⁻² and an FWHM of 13.2 ± 2.0 Å, and does not correspond to any known source with some of the deepest Hubble Space Telescope and JWST imaging in existence. Based upon the absence of imaging detection, we place a lower limit on the observed frame equivalent width (EW) of 7620 Å on this object, which strongly limits the number of possible lines that this line can be. Given the wavelength, an EW limit that is near the ceiling of expectation, and the lack of other lines in the bandpass, this line cannot be easily explained by [O ii] at z = 3.29, [O iii] at z = 2.20, Hα at z = 1.44, and Lyα at z = 12.16. We discuss possible active galactic nucleus and transient phenomena that could explain the finding.

The James Webb Space Telescope (JWST) has extended the frontier of galaxy detection to redshifts z > 11, finding a high abundance of UV-bright sources that challenge theoretical models. However, most current results come from just a few fields, introducing uncertainties due to cosmic variance. Here, we constrain z ∼ 7–14 UV luminosity functions (LFs) over ∼400 arcmin² across 36 independent sight lines from Data Release 2 of BEACON, a JWST pure-parallel NIRCam multiband imaging survey. We identify 164 7 < z < 12 galaxy candidates: 150 F090W, 14 F115W, and no robust F150W dropouts. In the 11 pointings overlapping with public JWST spectroscopy, we find no contaminants, indicating a high purity in our sample. Our z ∼ 7.5 UV LF agrees with previous bright-end measurements but yields lower number densities at −21 ≤ M_UV ≤ −19. At z ∼ 10, our measurements are lower than most photometric JWST results but match spectroscopic constraints, consistent with the high purity of our selection. The LFs at z ∼ 7.5 and 10 are consistent with pre-JWST models, while our limits at z > 13 do not rule out a possible excess. We measure significant clustering of bright (M_UV < −20.5) galaxies at 7 < z < 10. Fields hosting such sources are approximately 3 times more likely to be overdense relative to the full survey, implying that UV-bright galaxies preferentially reside in the most massive halos at these redshifts. Comparing with seminumerical simulations, we estimate that M_UV < −20.5 galaxies inhabit halos ∼0.8 dex less massive at z ∼ 11 than at z ∼ 8, consistent with a shift to higher star formation rates. However, their observed clustering exceeds predictions from pre-JWST luminosity–halo mass relations, suggesting these sources reside in more massive halos than previously modelled and/or multiple halo occupation.

We systematically search for galaxy overdensities using 20 independent fields with a minimum of six filters (F090W, F115W, F150W, F277W, F356W, and F444W) from BEACON, the JWST Cycle 2 NIRCam pure-parallel imaging survey. We apply an adaptive kernel-density estimation method that incorporates the full photometric redshift probability distribution function of each galaxy to map galaxy overdensities, and identify 207 significant (>4σ) overdensities at 1.5 < z < 5. We measure the quenched galaxy fraction, the average specific star formation rate (sSFR), the total dark matter mass, and the local galaxy density of each system. By investigating the correlation among these observables, we find that galaxy quenching proceeds in two paths. (i) Overdensities within more massive halos exhibit higher quenched fractions and lower averaged sSFRs. This trend weakens at z ≳ 2, consistent with cold gas streams penetrating shock-heated massive halos and sustaining star formation activity at early times. (ii) We also find a dependence of the same parameters on local densities at z < 2, where the quenched fraction increases and the sSFR decreases toward higher densities. The environmental trend in sSFR weakens at z ∼ 2–3 and shows tentative evidence for a reversal at z > 3, potentially due to a larger cold gas supply in earlier times. Our study reveals a complex interplay between individual galaxies and large-scale environmental properties, marking the onset of environmental effects on galaxy quenching in massive halos at cosmic noon.

One of the major conundrums in contemporary extragalactic astrophysics is the apparent overabundance of a remarkable population of UV-bright galaxies at redshifts z ≳ 9. We analysed galaxies spectroscopically observed by JWST/NIRSpec PRISM and confirmed to lie at z > 9, with a sufficient signal-to-noise to carefully model their rest-frame UV-to-optical continua and line emission. In particular, we modelled the damped Lyman-α (Lyα) absorption (DLA) features of each galaxy to place observational constraints on the gas assembly of neutral atomic hydrogen (H I) onto the galaxy halos at the onset of cosmic reionisation. Based on the derived H I column densities and star formation rate (SFR) surface densities, we show that all galaxies are highly efficient at forming stars on rapid ∼10 − 100 Myr depletion timescales, greatly in excess compared to the canonical local Universe Kennicutt-Schmidt (KS) relation and predictions from state-of-the-art galaxy formation simulations. The dense H I gas appears to also drive the offset from the fundamental-metallicity relation (FMR) of these galaxies though its dust-to-gas (DTG) ratio is seemingly consistent with values derived for local galaxies except for the lowest metallicity sight lines. Our results provide the first robust observational constraints on the impact of pristine H I gas on early galaxy assembly, and they imply that a combination of highly efficient star formation and low dust obscuration can likely explain the UV brightness of galaxies at cosmic dawn.

The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the Universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ( f esc LyC ) of high-redshift (z ≳ 4) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict f esc LyC for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY. These methods—based on ultraviolet (UV) absorption lines, the UV continuum slope, Lyα kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission-line ratios—enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions >1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent interstellar medium clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.

Star-forming galaxies drive massive outflows that play an important role in galaxy evolution by regulating feedback and influencing the dynamics of surrounding media. Measuring galactic outflow rates is essential for quantifying feedback efficiency and the amount of mass, momentum, and energy deposited into the circumgalactic medium (CGM). In this paper, we examine 17 galactic outflows from the COS Legacy Archive Spectroscopy Survey (CLASSY) survey with radiative transfer modeling of UV absorption lines presented in M. Huberty et al. to study their spatial distributions and kinematic properties. We study the Si ii, Si iii, and Si iv ionization states that trace the cool and warm phases of the outflows and find that Si ii–traced winds generally behave differently than the warmer Si iii– and Si iv–traced winds. We derive the mass, momentum, and energy loading factors, which we find scale inversely proportional to stellar mass. We find that our measurements of the mass and momentum loading factors are in agreement with the hydrodynamic FIRE-2 simulations. We model the velocity profiles of the winds, with profiles reaching a maximum velocity of 620 km s⁻¹ on average, in agreement with hydrodynamic simulations from the Cholla Galactic OutfLow Simulations (CGOLS). We also investigate the relationship between outflow properties and the age of the stellar population from spectral energy distribution (SED) fitting. We find that outflows associated with young (<5 Myr) star-forming regions are more likely to have a column density dominated by cooler gas and have mass outflow rates that decrease with radius.

We present the results from performing spectral energy distribution (SED) fitting on 121 variable active galactic nuclei (AGN) candidates in the Hubble Ultra Deep Field using photometry from both the Hubble Space Telescope and the James Webb Space Telescope (JWST) covering 0.2–4.8 μm. We designed a bespoke SED fitting code, which decomposes the total SED into its stellar and AGN contributions. Our SED fitting retrieves a significant contribution to the total SED from an AGN template for 26 of our variable sources with 0 < z < 7. We leverage the model AGN spectrum to estimate black hole masses (M_BH) using the measured luminosity at 5100 Å and local empirical calibrations. Common with recently discovered JWST broad-line AGN, we observe a trend in the M_BH–M_* plane where low-redshift sources have M_BH that agrees with local relations while high-redshift sources have increasingly overmassive black holes with respect to the stellar mass (M_*) of their host galaxies. Within our sample, we identify two IMBH candidates hosted by dwarf galaxies at z < 1 featuring overmassive black holes in the M_BH–M_* plane, similarly to our high-redshift sources. Finally, our SED fitter successfully retrieves the AGN nature of one source at z > 6. This object has z_phot = 6.74, and we estimate a lower limit on its black hole mass of log₁₀(M_BH/M_☉) > 7.36.

Polycyclic aromatic hydrocarbons (PAHs) are key diagnostics of the physical conditions in the interstellar medium and are widely used to trace star formation in the mid-infrared (mid-IR). The relative strengths of mid-IR PAH features (e.g., 6.2, 7.7, and 11.3 μm) are sensitive to both the size and ionization state of the molecules and can be strongly influenced by the local radiation field. However, at low metallicities (Z < 0.2Z_⊙), detecting PAHs remains notoriously difficult, likely reflecting a combination of suppressed formation and enhanced destruction mechanisms. We present new JWST/MIRI Medium Resolution Spectroscopy (MRS) observations of the metal-poor (Z ∼ 0.1Z_⊙) dwarf galaxy CGCG 007-025. We confirm the tentative PAH detection previously reported from Spitzer data and, for the first time, identify a compact (∼50 pc) PAH-emitting region nearly cospatial with the newly detected [Ne v] (Ionization potential (I.P.) ∼ 97 eV) emission and the galaxy’s most metal-poor, strongly star-forming region. The PAH_11.3μm and PAH _12.7μm features are detected, while no emission is found from other typically brighter features, suggesting a PAH population dominated by large, neutral molecules resilient to hard ionizing fields. When compared with models, mid-IR line ratios involving [Ne iii], [O iv], and [Ne v] can only be reproduced by a combination of star formation and AGN ionization, with the latter contributing 4%–8%. The [O iv] and [Ne v] luminosities exceed what massive stars or shocks can produce, highlighting a puzzling scenario in line with recent JWST observations of similar galaxies. This work provides a crucial reference for studying the physical conditions of the dust and star formation in low-metallicity starburst regions, environments typical of the early Universe.

We present an analysis of archival JWST NIRSpec IFS and HST imaging observations of the z = 3 Lyman continuum emitter (LCE) candidate LACES104037. We show that a nearby galaxy, denoted LACES104037-S, has a redshift offset from the main galaxy by only ∼ 450 km s⁻¹. Together with the identification of a tidal bridge between the galaxies, this indicates that the galaxies are interacting and most likely in the early stages of a merger. We show that the rest-frame LCE cluster sits ∼ 2.7 kpc from the galaxy core in the tidal bridge. It is faint in the nonionizing stellar continuum and shows faint but nonnegligible Hα and [O III] emission, suggesting that much of the gas surrounding the LCE cluster has been dispersed by feedback in the shallower gravitational potential of the tidal bridge. Comparing the direct LyC escape and the local Hα emission, we find a total ionizing escape fraction of f_esc^LyC =57 ± 8% from the LCE cluster. We estimate the age of the LCE cluster to ≲ 6.5 Myr, indicating that the cluster must have formed in situ in the tidal bridge well after the time of closest interaction. LyC escape from tidal stripping or in situ formed stars in tidal features would depend less on intrinsic galaxy properties than typically observed in low-z LCE surveys and could help explain the higher cosmic escape fraction and enhanced diversity of LCE galaxy properties observed at cosmic noon.