Andrea Ferrara

The discovery of numerous luminous, super-early galaxies at z > 10 by JWST has revealed a striking diversity in their ultraviolet (UV) properties, ranging from extremely blue, dust-poor systems to a smaller population of significantly reddened sources. We investigate the physical origin of this diversity within the framework of the Attenuation-Free Model (AFM), in which radiation-driven outflows redistribute dust to large galactic radii, reducing the effective attenuation. Starting from a sample of 32 spectroscopically confirmed super-early galaxies, we apply the detailed AFM analysis to a subset of 17 systems with suitable size constraints and derive their key physical properties, including halo mass, star formation efficiency, metallicity, and outflow extent. We find that these systems reside in massive halos ( log M / M ⊙ ∼ 10.7 ) and exhibit moderate ( 0.01 ≲ ϵ * ≲ 0.05 ) star formation efficiencies, while frequently reaching super-Eddington conditions that trigger powerful outflows. Within the AFM framework, we propose a possible evolutionary sequence in which galaxies transition from a dust-obscured “Red Monster’’ phase to a UV-bright”Blue Monster’’ phase as outflows clear their central regions. The recently confirmed red galaxy EGS-z11-R0 at z = 11.45 is naturally interpreted as a system observed during this obscured phase. Compact ( r e ≲ 150 pc) sources are instead difficult to reconcile within AFM; we speculate that their emission is dominated by an AGN. Our results provide a coherent interpretation of super-early galaxy properties and highlight the key role of radiation-driven outflows in shaping galaxy evolution at cosmic dawn. Future observations with JWST and ALMA will be essential to test these predictions and further constrain the nature of the earliest galaxies.

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts z > 10 and as high as z = 14. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at z > 15 may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright z ∼ 14 galaxies should be visible at redshifts as high as z ∼ 20–30, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 pc. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr ⁻¹ and (24 ± 3) mas yr ⁻¹ , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.

Finding the first (Population III or Pop III) stars is one of the fundamental quests of astronomy, aiming to deliver the missing link in how stars form at early cosmic times. Yet their initial mass function, formation sites and feedback remain highly uncertain, as well as the timing and topology of the transition to metal-enriched star formation. The observability of their peculiar spectral features is also debated, due to their short lifetime and faintness. This review summarizes current theoretical expectations for Pop III star formation, and the main observational strategies that have been adopted to constrain their properties across cosmic time, including near-field cosmology studies, direct searches for extremely metal-poor star-forming complexes and/or hard-ionizing spectral signatures at high and intermediate redshifts, and prospects for identifying Pop III activity up to Cosmic Dawn. The combination of JWST spectroscopy, time-domain searches, lensing surveys, stellar archaeology, absorption-line studies, as well as improved simulations, is yielding a growing number of observational candidates and narrowing the allowed parameter space for the first stars, setting the stage for a ‘’golden era’’ of Pop III searches.

Star-forming galaxies are generally believed to be the main drivers of cosmic reionization. However, the relative contributions of bright and faint galaxies to this process remain unclear. As the most luminous transient phenomena in the universe, long gamma-ray bursts (LGRBs) provide a unique opportunity to probe star formation occurring in both detectable and undetectable galaxies. In this Letter, we present new estimates of the cosmic star formation rate density (SFRD) at 4 < z < 10 using Swift LGRBs detected over the past two decades, by considering LGRBs as unbiased tracers of total star formation at high redshifts. Crucially, we find that the new LGRB-inferred SFRD can naturally explain current measurements of hydrogen reionization without invoking extreme ionizing photon production efficiencies or escape fractions from galaxies. Using these LGRB-inferred SFRD values, we further investigate the faintest magnitude limits of high-redshift galaxies, finding a redshift evolution of the limiting magnitudes from M lim ∼ − 14 to −15 at z ∼ 6, to M lim ∼ − 10 to −11 at z ∼ 10. This result provides one independent piece of evidence for the presence of a large population of faint galaxies at redshifts z ≳ 6, as an important complement to our understanding of the ionizing photon budget in the early universe.

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z > 10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the different proposed scenarios and assessing their implications for the galaxy formation and evolution picture. We identified and analysed a sample of 2420 robust candidates at z ∼ 7 − 14 drawn from the ASTRODEEP-JWST dataset over ∼0.2 deg ² , and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in the current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most (though not all) theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z ∼ 10, indicative of already-evolved galaxies only a few hundred megayears after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z ∼ 7 − 8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z > 20, and the full sample of z > 11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (∼50 Myr old) episodes of major growth.

We present results of new observations of [O i ] 145 μ m and [N ii ] 205 μ m emission lines from four star-forming galaxies at redshifts between z = 6.58 and 7.68 that have previous detections of [C ii ] 158 μ m and dust continua. Using the Atacama Large Millimeter/submillimeter Array, we successfully detect [O i ] 145 μ m emission from all targets at >4 σ significance. However, [N ii ] 205 μ m emission is undetected in all galaxies (signal-to-noise ratio < 3.5 σ ) except for a tentative detection from A1689-zD1. From the observed high [C ii ]/[N ii ] emission line ratios (≳20−80), we find that most of the [C ii ] 158 μ m emission arises from neutral gas regions (3 σ lower limits of ≳74%−96%). From [O i ] 145 μ m, [C ii ] 158 μ m lines, and IR luminosities, we estimate the neutral gas densities of n _H = 10 ^3.5 –10 ⁶ cm ⁻³ and the far-UV (FUV) radiation strengths of G ₀ ∼ 10 ^2.5 –10 ³ . While the neutral gas densities are similar to those of high-redshift starburst galaxies, the FUV strengths are lower compared to both local and high-redshift starbursts. Finally, we estimate atomic hydrogen masses using [O i ] 145 μ m emission lines and the oxygen abundances measured from recent JWST observations. We find gas mass ratios of f _gas ∼ 0.3–0.8, which are similar to earlier studies using [C ii ] 158 μ m. Starting from this pilot observation, future large [O i ] 145 μ m emission line surveys will provide us with currently little-known neutral gas properties of star-forming galaxies in the early Universe.

We present the stellar mass–metallicity relation (MZR) and mass–metallicity–star formation relation (“fundamental metallicity relation”; FMR) of 18 massive ( log ( M ⋆ / M ⊙ ) = 9.5 − 11 ) main-sequence galaxies at z ∼ 5 from the ALPINE–CRISTAL–JWST sample. This sample complements recent studies by JWST at up to 2 orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z ∼ 5 and cosmic noon ( z ∼ 2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z = 5 exhibits a 5× larger scatter (preferentially to lower metallicities) compared to the local FMR relation. This scatter can be explained by a bursty star formation and the direct buildup of metals in early galaxies, as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over 3 orders of stellar mass is generally in good agreement with results from cosmological simulations, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE–CRISTAL–JWST galaxies likely joined the current FMR at z ∼ 10 and will evolve into massive ( log ( M ⋆ / M ⊙ ) ∼ 11.4 ) galaxies with supersolar metallicities by z = 0.

Recent JWST observations have revealed a population of unexpectedly bright sources at ultra-high redshift ( z > 15 ), challenging current models of early galaxy formation. One extreme example is , an F356W-dropout identified in the CEERS survey and initially interpreted as a luminous galaxy at z ∼ 30 , but subsequently found to be variable over an ∼ 800 day baseline. Motivated by this variability, we explore the alternative hypothesis that is a pair-instability supernova (PISN) originating from a massive ( ∼ 250 – 260 M ⊙ ), metal-free star. Using state-of-the-art PISN light curves, spectral energy distributions, and synthetic spectra, we show that a PISN at z ≃ 15 can plausibly reproduce the observed brightness, temporal evolution, photometry, and NIRSpec spectrum. We compare this scenario with alternative interpretations, including a local Y0 brown dwarf, and discuss observational tests to discriminate among them. If confirmed, this event would provide a rare window onto Population~III stars, and highlights the importance of transient contamination in ultra-high redshift galaxy samples.

Measuring the ionizing photon production efficiency ξ _ion,0 —the ratio of the ionizing photon output rate Q H 0 to the UV continuum luminosity L _UV —in galaxies at z > 6 is crucial for constraining their contribution to cosmic reionization. We present integrated and spatially resolved measurements of ξ _ion,0 for 12 exceptionally bright ( M _UV ∼ −22 mag) star-forming galaxies at z ∼ 7 from the Reionization Era Bright Emission Line Survey (REBELS). These measurements are based on JWST NIRSpec/Integral Field Unit PRISM spectroscopy, probing the rest-frame UV and optical regime. Notably, in 8 of the 12 galaxies, the spectral coverage includes H α , enabling self-consistent dust attenuation estimates in both the ionized gas and stellar continuum via the Balmer decrement and rest-UV slope, respectively. We find global log ( ξ ion,0 ) values ranging from 25.19 ± 0.11 to 25.61 ± 0.11, with a weighted mean of 25.44 ± 0.15, consistent with the canonical value of ∼25.3. Using a sample of 25 star-forming clumps within these galaxies, we explore local variations in the ionizing photon production efficiency, finding a broader range, from 24.52 ± 0.21 to 26.18 ± 0.61. We identify strong correlations between ξ _ion,0 and the specific star formation rate, star formation surface density, H β equivalent width, and stellar mass. Clumps with the highest ξ _ion,0 exhibit EW ₀ (H β ) ≥ 150 Å, consistent with young stellar ages. From previous Ly α measurements in three galaxies, we estimate a typical Ly α escape fraction of f _{esc,Ly α} ∼ 2%, suggesting similar or lower escape fractions for LyC photons. Combining this with our H α measurements, we infer ionized bubble sizes ∼1 pMpc, aligned with expectations from Ly α -detected systems and reionization models.

We present Keck/MOSFIRE H -band spectroscopic measurements covering the [C iii ] λ 1907, C iii ] λ 1909 doublet for a sample of eight z ∼ 7 spectroscopically confirmed star-forming galaxies drawn from the Reionization Era Bright Emission Line Survey (REBELS). This REBELS–MOSFIRE sample is notable for its bright median UV luminosity ( M UV = −22.5 AB) and large median stellar mass ( log M * / M ⊙ = 9.2 ). Although three sources show tentative evidence of a C iii ] detection, we obtain no confident detections for any of the eight REBELS–MOSFIRE sources. The median [C iii ] λ 1907+C iii ] λ 1909 3 σ upper limit in equivalent width (EW) for the REBELS–MOSFIRE sample is 6.5 Å, and a stack of their H -band MOSFIRE spectra yields a nondetection with an associated 3 σ upper limit of 2.6 Å. These upper limits fall significantly below the C iii ] EW measured in a composite spectrum of representative z ∼ 7 star-forming galaxies, as well as those measured for notable early star-forming galaxies such as GN-z11, GHZ2, GS-z12, and RXCJ2248-ID. The lack of strong C iii ] emission can be understood within the context of the stellar populations of the REBELS galaxies, as well as the ionization conditions and gas-phase metallicity implied by rest-frame optical spectroscopic properties ([O iii ]+H β EWs, and [O iii ] λ 5007/[O ii ] λ 3727 and [Ne iii ] λ 3869/[O ii ] λ 3727 line ratios). The REBELS–MOSFIRE sample represents the higher-mass, higher-metallicity, lower-excitation tail of the z ∼ 7 galaxy population, whose ionizing properties must be fully characterized to constrain the role of star-forming galaxies during cosmic reionization.