Marc Rafelski

We present a complete and homogeneous analysis of the Ly α emission properties of the cosmic web at 3 ≲ z ≲ 5 as a function of the overdensity of Ly α emitters (LAEs) in the MUSE Ultra Deep Field (MUDF) and the MUSE Extremely Deep Field (MXDF). We identified 41 overdensities, probing environments that are δ ≈ 2 − 10 times denser than the field. We searched for extended emission down to surface brightness (SB) levels of 3 − 5 × 10 ⁻²⁰ erg s ⁻¹ cm ⁻² arcsec ⁻² , revealing filamentary structures beyond the scale of the circumgalactic medium, confirming that LAEs act as signposts of the cosmic web. A trend with overdensity emerges: non-detections are mainly at δ < 2, a ≈1 dex scatter in SB appears for 2 ≲ δ ≲ 3.5, and detections reach a maximum intrinsic SB of ≈2 × 10 ⁻¹⁹ erg s ⁻¹ cm ⁻² arcsec ⁻² for δ ≳ 3.5. Emitting regions occupy a fraction of projected area f _s ≈ 0.20 − 0.25 inside filaments, yielding a cosmological incidence for Ly α emission in the cosmic web ℓ (Ly α ) of ≈1.22, which is similar to the one of partially neutral Lyman limit systems. This analysis indicates that the emitting gas is partially ionized at moderate densities ( n _H ≈ 10 ⁻³ − 10 ⁻¹ cm ⁻³ ), likely tracing the denser spines of intergalactic filaments and embedded substructures. Finally, we forecast how oriented stacking in larger samples of overdensities from shallower observations could yield an expanded view of the cosmic web before the next-generation wide-field spectroscopic instruments become operational.

We present the KCWI Analysis of Gas around Galaxies (KAGG) survey of Ly α emission around damped Ly α absorbers (DLAs) at z ≈ 3–5 with the Keck Cosmic Web Imager (KCWI) integral-field spectrograph. The KAGG survey is characterized by the sample of 36 DLAs spanning a wide metallicity range, the wide search area for associated Ly α emission (impact parameter b ≲ 120 kpc), and the high emission sensitivity, reaching Ly α luminosities ≤10 ⁴² erg s ⁻¹ . We identify seven DLA Ly α emitters that are likely DLA galaxy candidates due to the good match between the Ly α emission and DLA redshifts. Based on a composite sample of 11 DLA Ly α emitters from KAGG and MUSE Analysis of Gas around Galaxies (MAGG), we find that the impact parameters of DLA Ly α emitters decrease with DLA metallicity, from b > 50 kpc at [M/H] ≈ −2 to b < 50 kpc at [M/H] ≈ −1. Including Ly α emitters at larger velocity offsets and at larger impact parameters from the DLAs, we construct samples of 35 “group” Ly α emitters and 17 “large-scale” Ly α emitters. We find that their impact parameters and incidence rates also depend on DLA metallicity. Based on simulations of our survey and comparisons with models of galaxy formation, we conclude that higher DLA metallicities are associated with higher halo masses. The low Ly α emission detection rate at low impact parameters suggests that most DLA galaxies either have low stellar masses ( M ≲ 10 ⁸ M _⊙ ) or have their Ly α emission suppressed due to H i resonant scattering and/or dust absorption.

An accurate determination of dust attenuation within galaxies is essential for deriving key physical properties such as the star formation rate (SFR). We present an analysis using the JWST/NIRISS data from the GLASS-JWST ERS programme to investigate and characterise the stellar and nebular reddening of galaxies at 1.0 < z < 2.4 down to the sub-kiloparsec scale. We used a multiregion fitting method to extract high-quality H α and H β emission line maps for 99 individual galaxies across a stellar mass range 7.0 < log ₁₀ ( M _* /M _⊙ ) ≲ 10.5. We find no evidence for ratios of the Balmer decrement (H α /H β ) below the intrinsic limit for Case B recombination, beyond the expected variation from observational uncertainties. We reproduce the local correlation between the Balmer decrement and total stellar mass, and find no measurable difference when splitting the sample by redshift, with negligible attenuation below log ₁₀ ( M _* /M _⊙ ) ≲ 8.5. Similarly, the best-fit relation between the nebular and continuum reddening follows the same relation as in local starburst galaxies, E ( B − V ) _SED = (0.46 ± 0.02) E(B − V ) _neb , together indicating no significant evolution in the dust geometry within galaxies out to z ≲ 2.4. We derive best-fit linear relations between the differential nebular-stellar reddening and the spectral-energy-distribution-derived SFR and stellar mass, finding statistically significant relations for both quantities. We used our spatially resolved measurements to derive an empirical calibration between the resolved differential reddening, and the SFR surface density. These will enable crucial dust attenuation corrections for spatially resolved science at higher redshifts where the Balmer lines are inaccessible, such as with future Roman grism observations.

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 arcmin ² 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.

Gas flows between galaxies and the circumgalactic medium (CGM) play a central role in galaxy evolution and can become observable as giant nebulae when illuminated by quasars. We present an ensemble study of nebulae around 30 UV-luminous quasars at z ≈ 0.4–1.4 from the CUBS and MUSEQuBES surveys, 27 of which are detected in extended [O II ] and/or [O III ] emission. Based on a joint analysis of nebular morphology and surrounding galaxy environments, we introduce three morpho-kinematic classifications. We identify eleven irregular, large-scale (>50 kpc) systems, many of which are likely interaction-related; twelve compact host-galaxy-scale nebulae, likely tracing CGM/interstellar medium (ISM) gas; and four systems with complex morphologies of uncertain origin. We introduce a quantitative measure of the spatial and kinematic association between nebulae and quasar-host group galaxies, finding a statistically significant association for ten nebulae, most of which are irregular, large-scale nebulae, consistent with qualitative analysis. Radio jets are detected in six systems, with no strong correlation found between radio activity and nebular emission. The [O II ] nebulae are more asymmetric than their Ly α counterparts at z > 2, but bear more similarity to H I gas observed in 21 cm around local elliptical galaxies. Blueshifted–redshifted patterns, likely tracing gas rotation, are observed in roughly 30% of the systems, though disturbed kinematics suggest that feedback may also be important. These results show that giant quasar nebulae are not a uniform class of objects, but instead arise through multiple pathways shaped by host-galaxy gas, galaxy interactions, group environments, and quasar activity, with the most striking cases associated with galaxy interactions.

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 (>4 σ ) 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.

Assessing the contribution of ionizing sources to cosmic reionization is a central goal of extragalactic astrophysics. Understanding and quantifying ionizing escape remains challenging near the Epoch of Reionization. We present the highest-redshift Lyman continuum (LyC) emitter detected to date, MXDFz4.4, at z = 4.442 in the MUSE eXtremely Deep Field, observed only ∼0.25 Gyr after the end of reionization. A high-confidence Ly α line confirms the redshift. LyC flux is detected at 5.3 σ in the F435W filter with a flux of 4.2 ± 0.8 nJy, corresponding to a flux measurement at 5.2 σ . After correcting for the intrinsic production of LyC photons and the intergalactic medium opacity at z = 4.44, we derive high escape fractions, f _esc , ranging from 50% to 100%. We apply established low-redshift tracers of LyC escape and, for the first time at this redshift, promising Ly α morphological tracers such as the halo fraction (HF). Spectral energy distribution fitting indicates the presence of a recent burst of star formation; we explore its impact on the production and escape of ionizing photons. Ly α -based tracers of f _esc reveal a complex scenario in which the recent burst strongly influences LyC production and escape, combined with a more evolved stellar population. This interpretation is supported by UV diagnostics, including Σ _SFR and sSFR. Our results provide cautious support for the Ly α HF as a LyC escape tracer at high redshift. Considering the burst-driven enhancement in ionizing photon production and escape, we conclude that stochastic star formation in the early Universe likely plays a significant role in the contribution of galaxies to cosmic reionization.

We investigate galaxies in the Galaxies at All Redshifts Deciphered and Explained with the NIRSpec microshutter array (MSA; GARDEN) survey that exhibit auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies in this survey have spectra suitable for this analysis: CANDELS 8005 at z = 3.794 and CANDELS 7986 at z = 4.702. For both galaxies, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spatial pixels (spaxels). These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of log(O/H)+12 = 8.008 − 0.027 + 0.025 for CANDELS 8005 and 7.89 − 0.028 + 0.027 for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances are consistent with strong-line estimates based on our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance indices, electron temperature, and direct abundance maps for both galaxies, thanks to the excellent spatial resolution. From the direct abundance maps, we measure linear radial metallicity gradients of − 0.11 1 − 0.025 + 0.026 dex kpc ⁻¹ for CANDELS 8005 (statistically significant) and −0.0928 ± 0.0880 dex kpc ⁻¹ for CANDELS 7986, where the large uncertainties limit the significance of the result. These results provide a rare direct measurement of a radial metallicity gradient at z > 0 from direct-method abundances, offering key observational support for inside-out galaxy growth with feedback-regulated chemical enrichment.

We leverage the IllustrisTNG cosmological simulations to test how the large-scale cosmic web shapes galaxy morphology and to forecast the early cosmic web structure that the Nancy Grace Roman Space Telescope will reveal. In the hydrodynamic TNG50 and N -body TNG50-Dark runs, we reconstruct the cosmic web at redshifts z = 0, 0.5, 1, 2, 3, and 4 with the Monte Carlo Physarum Machine density estimator and the DisPerSE structure identification framework. We confirm that dark matter halos start out predominantly prolate (elongated), and their shapes are aligned with their nearest filaments; prolate galaxies retain strong shape alignment with their outer halos to later times. At z ≥ 1, the fraction of prolate (spheroidal) halos increases (decreases) toward lower stellar mass, higher redshift, and lower filament density. At z < 1, more spheroidal (oblate) stellar structures preferentially reside in higher-density (lower-density) filaments. We also find that higher-density filaments favor extended rotationally supported disks, whereas lower-density filaments more often host smaller dispersion-supported systems. Then, generating mock galaxy samples from TNG100 and TNG50, we predict the early cosmic web accessible to Roman. We find that the spectroscopic emission-line depth planned for the High-Latitude Wide-Area Survey (HLWAS) yields a highly incomplete galaxy sample that does not accurately trace the z = 1 cosmic web. A survey ≥2.5× deeper over a few square degrees would enable a proper reconstruction and reveal qualitatively correct filament–galaxy morphology relationships. Nevertheless, the planned HLWAS Deep field should still identify most galaxy overdensities; targeted deeper spectroscopy of these regions would efficiently and adequately map the early filamentary structure.