Michaela Hirschmann

At low redshift, massive quiescent galaxies (MQGs) are most frequently found in massive, rich galaxy clusters, but at high redshift the trend is less clear. Here, we present spectroscopic evidence of the effects of environment on the formation and assembly of high-redshift MQGs. We identify 26 (9) log ( M * / M ⊙ ) ≥ 10.5 ( 10.0 ≤ log ( M * / M ⊙ ) < 10.5 ) spectroscopically confirmed quiescent galaxies in the UDS and EGS fields at 3 < z < 5 with NIRSpec spectroscopy from the Red Unknowns: Bright Infrared Extragalactic Spectroscopic (RUBIES) survey and other public JWST programs. We measure the density contrast in these fields by applying a Monte Carlo Voronoi tessellation density-mapping technique to photometric and spectroscopic redshifts of m _F444W < 27.5 sources. We robustly detect 19 massive overdense peaks with log ( M Peak / M ⊙ ) ≥ 13 and seven extended massive protoclusters ( log ( M Struct / M ⊙ ) ≥ 13.85 ). We observe that MQGs are preferentially found in these massive peaks and within these massive structures: ≈50% of MQGs are found in massive peaks, compared to ≈25% of massive star-forming galaxies (MSFGs), and quiescent fractions are higher in these regions than in the coeval field. We show that when all stellar mass is assumed to form in situ, the inferred masses of MQG progenitors are significantly higher than those of high- z massive galaxies in observations or simulations. Accounting for ex situ star formation and scaling merger rates by the local galaxy overdensity significantly lessens this tension, which indicates that ex situ star formation plays a key role in MQG mass assembly in overdense environments.

We report on the spectroscopic confirmation of overdense regions of massive quiescent galaxies (QGs) in the early Universe with JWST/NIRSpec. Based on data from the DeepDive NIRSpec program and archival data from the Dawn JWST Archive, we confirm three QGs in the vicinity of Jekyll and Hyde, a pair of massive QG and a dusty star-forming galaxy, at z = 3.71 and two QGs around SXDS-27434 at z = 4.01. According to the analysis of galaxy number density with photometric redshifts, Jekyll and Hyde (SXDS-27434) are in an overdense region, where the number density of galaxies is three (four) times higher than the average in the COSMOS (SXDS) field. SED fitting suggests that most of the QGs follow similar star formation histories and have consistent formation and quenching epochs. The same trend is observed in other protoclusters hosting QGs that were already identified by ground-based telescopes, indicating that the large-scale environment plays an important role in the formation of QGs. In addition, JWST spectra reveal a broad H α emission line from SXDS-27434 and faint emission lines from the other three QGs, which are identified as AGN-driven based on their emission line ratios. The overdensity is also reproduced by the Illustris TNG300 simulation at z = 3.71, in which the member QGs also have similar quenching epochs. These results are consistent with a scenario in which the large-scale structure enhances merger activity and/or gas accretion and triggers AGN feedback, thereby driving simultaneous formation and quenching of QGs in the overdensity.

How Ly α photons escape from galaxies during the first billion years remains a central question to understanding early galaxy evolution and cosmic re-ionisation. While Ly α emitters (LAEs) are known to be metal poor and highly ionised, many high-z studies rely on spatially integrated measurements, leaving the internal connection between Ly α emission, chemical enrichment, and feedback largely unexplored. We present a spatially resolved stacking analysis of 287 LAEs at z > 4 observed with JWST/NIRSpec prism spectroscopy. By constructing a two-dimensional stack from public surveys (CAPERS, CEERS, JADES, and RUBIES), we probed the average internal structure of typical LAEs ( z _median = 5.5, M _UV,median = −18.7, EW(Ly α ) _median = 61 Å) on sub-kiloparsec scales. We find a clear radial decoupling between resonant and non-resonant emission. While EW(H β ) and other optical lines decline with radius, EW(Ly α ) increases towards the outskirts, and the Ly α escape fraction rises from ~16% in the centre to ≳24% at larger radii. This behaviour suggests that resonant scattering redistributes Ly α photons into outer regions, where escape becomes more efficient. Optical diagnostics and T _e measurements in this work reveal low metallicities (12 + log(O/H) ≃ 7.7 ± 0.2), high ionisation parameters, negligible dust attenuation, and systematically elevated N/O ratios (log(N/O) ~ −0.4). The latter place typical LAEs among the growing population of nitrogen-enhanced high-redshift galaxies, pointing to rapid and possibly feedback-driven chemical enrichment. The inferred ionising photon production efficiency, log( ξ _ion /Hz erg ⁻¹ ) ≃ 25.1–25.2, together with the high Ly α escape fractions, suggests that these systems are efficient, though not extreme, contributors to the ionising photon budget. The comparison of the observed radial trends of ultraviolet β slope, EW(Ly α ), Ly α escape fraction, and metallicity in our LAE stack with results from SPICE radiation-hydrodynamic simulations shows that bursty supernova feedback models naturally reproduce the observed data, linking the spatial redistribution of Ly α to stochastic star formation and feedback-driven gas flows. Our results demonstrate that Ly α emission, chemical enrichment, and feedback are tightly connected in typical z > 4 LAEs and highlight the power of spatially resolved stacking with JWST to uncover the internal physical drivers of early galaxy evolution.

Galaxy mergers play a crucial role in shaping the morphology, the star formation, and the mass growth of galaxies across cosmic time. While mergers have been extensively investigated in the local Universe, the evolution of their frequency and physical properties in the early Universe has yet to be fully understood. In this work, we investigate the role of mergers in a large spectroscopic sample of 1233 galaxies in the range 5 < z < 14 with good detection (S∕N _pixel > 3) in the JWST/F444W band, covering six different extragalactic fields (COSMOS, UDS, GOODS-South, GOODS-North, EGS, and Abell2744). We identified mergers from rest-frame optical disturbances in F444W, using a combination of Gini, M ₂₀ , and asymmetry morphological parameters, which trace more advanced mergers (including also minor mergers) with shorter observability timescales compared to the close photometric pair selection. The observed morphological merger fraction, f _m , does not strongly evolve with redshift from z = 0 to z ∼ 8. We find an average merger fraction of f _m ∼ 5% for our golden merger condition (Gini+0.14×M ₂₀ > 0.33, A> 0.35) tracing major mergers with τ _obs,golden 30-10 Myr at z = 5-10. This fraction increases to ∼8% for asymmetry mergers ( A > 0.35) owing a longer observability timescale, and reaches ∼13% when considering Gini-M ₂₀ mergers (Gini+0.14×M ₂₀ > 0.33), which trace major+minor mergers. After accounting for the different merger observability timescales, and considering the shorter merger duration in the early Universe, we find that the merger rate is strongly increasing from z = 1 to z = 7 by more than one order of magnitude, averaging ∼2 merger/galaxy/Gyr at redshifts of 5 < z < 10 for major mergers (in agreement with photometric pair studies), and a factor of ∼3 higher for minor+major mergers. We finally performed spectral energy distribution modeling using available HST+JWST photometry to infer stellar masses and star formation rates (SFRs) averaged over multiple timescales, using a nonparametric star formation history. We find that mergers at this cosmic epoch have a significant impact (although significantly lower than at z = 1) on the SFR of galaxies, with an average enhancement by a factor of ∼1.8 compared to isolated galaxies in the same mass and redshift range. This enhancement is higher in the last 10 Myr and for shorter merger timescale indicators, suggesting that mergers trigger star formation through short-lived powerful bursty episodes. Despite this, mergers contribute only by 5–10% to the mass buildup of galaxies in the redshift range explored, suggesting that in situ star formation is the dominant process in galaxy growth at all cosmic epochs.

The James Webb Space Telecope opened a new window for the study of the highest redshift ( z > 7) Universe. This work presents a theoretical investigation of the very high-redshift Universe using the state-of-the-art GALaxy Evolution and Assembly (GAEA) model, run on merger trees from the Planck-Millennium N -body simulation. We show that GAEA successfully reproduces a wide range of high- z observational estimates including: the galaxy stellar mass function up to z ∼ 13 and the total (galaxies and AGN) UV luminosity function (LF) up to z ∼ 10. We find that the AGN UV emission represents an important contribution at the bright end of the UVLF up to z ∼ 8, but it is negligible at higher redshift. Our model reproduces well the observed mass-metallicity relation (MZR) at z ≤ 4, while it slightly overestimates the normalization of the relation at earlier cosmic epochs. At z ≥ 11, current UVLF estimates are at least one order of magnitude larger than model predictions. We investigate the impact of different physical mechanisms, such as an enhanced star formation efficiency coupled with a reduced stellar feedback or a negligible stellar feedback at z > 10. In the framework of our model, both the galaxy stellar mass and UVLFs at z ≥ 10 can be explained by assuming feedback-free starbursts in high-density molecular clouds. However, we show that this model variant leads to a slight increase of the normalization of the z ≥ 10 MZR, strengthening the tension with available data. A model with negligible stellar feedback at z > 10 also predicts larger numbers of massive and bright galaxies aligning well with observations, but it also overestimates the metallicity of the interstellar medium. We show that these model variants can in principle be discriminated using the relation between the star formation rate and galaxy stellar mass.

GHZ2 is among the highest-redshift galaxies discovered to date, exhibiting a spectrum rich with prominent emission lines in the rest-frame ultraviolet (UV) and optical. These features raise critical questions about the mechanism powering this nebular emission, in particular the extremely strong C iv λ 1548+1551 emission (rest-frame equivalent width of 45 Å). Here we aim to quantify the active galactic nucleus (AGN) contribution within this system using the BEAGLE -AGN tool to simultaneously fit the spectrum and photometry of GHZ2. We consider a range of models with and without AGN components, allowing us to disentangle the stellar and AGN contribution of GHZ2 for the first time simultaneously using all observed emission lines. We conclude that a partial contribution by an AGN is significantly favored based on χ ² comparisons between models with and without an AGN component, measuring an AGN contribution of 97 − 2 + 1 %, and 64 − 12 + 23 % for the C IV λ 1548+1551 and C III ] λ 1908 emission lines, respectively. We estimate the black hole mass using the accretion luminosity ( L _acc ) from the best-fit BEAGLE -AGN model, inferring a value of log ₁₀ ( M _BH / M _⊙ ) = 7.19 − 0.03 + 0.03 , assuming an Eddington ratio of η = 0.5 (with a much larger systematic uncertainty of ∼1 dex). The inferred black hole mass to stellar mass ratio is 0.06 − 0.02 + 0.02 (statistically), consistent with other high-redshift AGN systems. If the black hole interpretation is confirmed, GHZ2 would represent the most distant black hole identified to date, making it an ideal laboratory to study AGN growth and their role in shaping high-redshift galactic evolution.

Understanding the origin and chemical properties of gas accreted by supermassive black holes (SMBHs) is essential for linking black hole growth to galaxy evolution. Using a suite of 30 high-resolution cosmological zoom-in simulations, we investigate the chemical properties of gas accreted onto SMBHs in massive galaxies with stellar masses of 10 ^10.9−11.9 M _⊙ and black hole masses of 10 ^8.5−9.7 M _⊙ at z = 0. By tracing the full cosmological histories of individual gas particles, we identify their origins and enrichment pathways. The accreted gas is classified into four categories: “early” gas accreted during the early assembly phase of the main halo, “external” gas originating from other galaxies or subhalos, “recycled” gas enriched through stellar evolution processes within the primary galaxy, including asymptotic giant branch (AGB) winds and supernova ejecta, and “smooth” gas accreted from the intergalactic medium. We find that recycled gas dominates the accretion budget and is already metal-rich at early epochs. Gas from other origins typically undergoes gradual chemical enrichment within the galactic environment prior to black hole accretion. The mean abundance ratios show only weak redshift evolution and are broadly compatible with the high metallicities inferred for quasar broad-line regions. Our results suggest that metal-rich gas supply to SMBHs arises naturally from cosmological galaxy evolution and stellar recycling.

The launch of the James Webb Space Telescope (JWST) has enabled the discovery of a small but increasing sample of high-redshift core-collapse supernovae (CC SNe), which provide new tests of massive star evolution in the early Universe. In this study, we report the analysis of SN 2023aeaf discovered in COSMOS-Web survey observations, which at z = 3.195 has one of the highest SN spectroscopic redshifts to date. Using two epochs of JWST photometry separated by ∼1 month in the rest frame, we photometrically classify SN 2023aeaf by comparing the JWST photometry to spectrophotometric CC SN and Type Ia (SN Ia) models and UV observations of SNe from the Swift telescope, finding that SN 2023aeaf is highly likely to be a Type II SN. A spectrum of the SN+host galaxy was also obtained ∼30 rest-frame days after discovery but shows no clearly identifiable SN features, with H α emission from the host potentially masking emission from the SN. Although the limited photometric coverage prevents strong constraints on the explosion properties, we find that the data are most consistent with a ∼12 M _⊙ progenitor with ∼0.5 M _⊙ of circumstellar material. We next use the host-galaxy spectrum and photometry to model the host spectral energy distribution (SED) using the Prospector Bayesian inference framework. We find that the host is a star-forming galaxy with an sSFR of log 10 ( sSFR / yr − 1 ) = − 10.1 7 − 0.10 + 0.13 , a stellar mass of log ( M ⋆ / M ⊙ ) = 9.0 4 − 0.04 + 0.03 , and a gas-phase metallicity of 12 + log 10 ( O / H ) = 7.82 ± 0.02 . SN 2023aeaf joins a growing sample of early Universe CC SNe with high luminosities, dense circumstellar material (CSM), and low-metallicity environments.

The physical processes that led to the formation of billion-solar-mass black holes within the first 700 million years of cosmic time, a period known as cosmic dawn, remain a puzzle ¹ . Several theoretical scenarios have been proposed to seed and rapidly grow black holes ^2–4 , but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that exhibits singular properties: among the largest hydrogen Balmer breaks reported at any redshift, broad multi-peaked Hβ emission, and Balmer line absorption in several transitions. We model this source as an enshrouded black hole in which the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free envelope around a supermassive black hole ^5,6 . This source may provide evidence of an early black hole embedded in dense gas—a theoretical configuration proposed to rapidly grow black holes by super-Eddington accretion ^7,8 . Radiation from the black hole seems to dominate almost all observed light, leaving limited room for contribution from its host galaxy. If the source merged with its brighter neighbour, it would resemble the recently discovered ‘little red dots’ with perplexing spectral energy distributions ^9–11 . The redness of the black hole is due to gas, not dust ^12,13 , and scattering, not kinematics, gives rise to the complex line shapes and luminosities—black hole masses of these sources may therefore be overestimated by orders of magnitude.