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The James Webb Space Telescope is unveiling a surprising lack of evolution in the number densities of ultraviolet (UV)-selected galaxies at redshift z ≳ 10. At the same time, observations and simulations are providing evidence for highly bursty star formation in high-z galaxies, resulting in significant scatter in their UV luminosities. Galaxies in low-mass dark matter halos are expected to experience most stochasticity due to their shallow potential wells. Here, we explore the impact of a mass-dependent stochasticity using a simple analytical model. We assume that scatter in the M UV–M h relation increases toward lower halo masses, following the decrease in halo escape velocity, σUV∼Mh−1/3 , independent of redshift. Since low-mass halos are more dominant in the early universe, this model naturally predicts an increase in UV luminosity functions (LFs) at high redshifts compared to models without scatter. We make predictions for additional observables, which would be affected by stochasticity and could be used to constrain its amplitude, finding (i) galaxies are less clustered compared to the no-scatter scenario, with the difference increasing at higher-z; (ii) assuming that star-bursting galaxies dominate the ionizing photon budget implies reionization starts earlier and is more gradual compared to the no-scatter case; (iii) at fixed UV magnitude, galaxies should exhibit wide ranges of UV slopes, nebular emission line strengths, and Balmer breaks. Comparing to observations, the mass-dependent stochasticity model successfully reproduces the observed LFs up to z ∼ 12. However, the model cannot match the observed z ∼ 14 LFs, implying additional physical processes enhance star formation efficiency in the earliest galaxies.

The surprising JWST discovery of a quiescent, low-mass (M ⋆ = 108.7 M ⊙) galaxy at redshift z = 7.3 (JADES-GS-z7-01-QU) represents a unique opportunity to study the imprint of feedback processes on early galaxy evolution. We build a sample of 130 low-mass (M ⋆ ≲ 109.5 M ⊙) galaxies from the serra cosmological zoom-in simulations, which show a feedback-regulated, bursty star formation history (SFH). The fraction of time spent in an active phase increases with the stellar mass from f duty ≈ 0.6 at M ⋆ ≈ 107.5 M ⊙ to ≈0.99 at M ⋆ ≥ 109 M ⊙, and it is in agreement with the value f duty ≈ 0.75 estimated for JADES-GS-z7-01-QU. On average, 30% of the galaxies are quiescent in the range 6 < z < 8.4; they become the dominant population at M ⋆ ≲ 108.3 M ⊙. However, none of these quiescent systems matches the spectral energy distribution of JADES-GS-z7-01-QU, unless their SFH is artificially truncated a few Myr after the main star formation peak. As supernova feedback can only act on a longer timescale (≳30 Myr), this implies that the observed abrupt quenching must be caused by a faster physical mechanism, such as radiation-driven winds from young massive stars and/or an active galactic nucleus.

JWST is providing a unique opportunity to directly study the feedback processes regulating star formation (SF) in early galaxies. The two z > 5 quiescent systems (JADES-GS-z7-01-QU and MACS0417-z5BBG) detected so far show a recent starburst after which SF is suppressed. To clarify whether such quenching is due to supernova (SN) feedback, we have developed a minimal physical model. We derive a condition on the minimum star formation rate, SFRmin , lasting for a time interval Δt b , required to quench SF in a galaxy at redshift z, with gas metallicity Z, and hosted by a halo of mass M h . We find that lower (z, Z, M h ) systems are more easily quenched. We then apply the condition to JADES-GS-z7-01-QU (z = 7.3, M ⋆ = 108.6 M ⊙) and MACS0417-z5BBG (z = 5.2, M ⋆ = 107.6 M ⊙) and find that SN feedback largely fails to reproduce the observed quenched SF history. Alternatively, we suggest that SF is rapidly suppressed by radiation-driven dusty outflows sustained by the high specific star formation rates (43 and 25 Gyr−1, respectively) of the two galaxies. Our model provides a simple tool to interpret the SF histories of post-starburst galaxies and unravel quenching mechanisms from incoming JWST data.

The bursty, time-variable nature of star formation in the first billion years, as revealed by JWST, drives phases of temporary quiescence in low-mass galaxies that quench after starbursts. These galaxies provide unique probes of the burstiness of early star formation and its underlying physical processes. Using the serra cosmological zoom-in simulations, we analyze over 200 galaxies with M⋆ < 109.5M⊙at z ∼ 6–8, finding that most experience quiescent phases driven by stellar feedback, with minimal influence from environmental effects. The fraction of temporarily quiescent galaxies increases with decreasing mass and luminosity, representing the dominant population at M⋆ < 108M⊙ and MUV > −17. By forward modeling their spectral energy distributions, we show that they are faint (〈MUV〉 = −15.6 for M⋆ = 108M⊙), have strong Balmer breaks (>0.5), and no emission lines. Comparing our predicted fractions with JWST results, we find similar luminosity-dependent trends; however, the observed fractions of temporarily quiescent galaxies at MUV ∼ −20 to −19 are higher, suggesting that stronger feedback or additional mechanisms beyond supernovae may be at play. We propose searching for F200W dropouts and satellites in the proximity (<5″) of massive (>1010M⊙) galaxies as effective strategies to uncover the hidden majority of faint (MUV > −17), temporarily quiescent systems, crucial for constraining early feedback processes in low-mass galaxies.

The recent JWST observation of the Firefly Sparkle at z = 8.3 offers a unique opportunity to link the high- and the low-z Universe. Indeed, the claim of it being a Milky Way (MW) type of assembly at the cosmic dawn opens the possibility of interpreting the observation with locally calibrated galaxy-formation models. Here, we use the a state-of-the-art MW-evolution model to perform forward modeling of our Galaxy's progenitors at high-z. We build a set of mock spectra for the MW building blocks to make predictions for JWST and to interpret the Firefly Sparkle observation. First, we find that the most massive MW progenitor becomes detectable in a deep survey like JADES from z ≈ 8.2, meaning that we could have already observed MW analogs that still need interpretation. Second, we provide predictions for the number of detectable MW progenitors in lensed surveys like the CAnadian NIRISS Unbiased Cluster Survey, and interpret the Firefly Sparkle as a group of MW building blocks. Both the number of detections and the observed NIRCam photometry are consistent with our predictions. By identifying the MW progenitors whose mock photometry best fits the data, we find bursty and extended star formation histories, lasting >150–300 Myr, and estimate their properties: M h ≈ 108−9 M ⊙, M ⋆ ≈ 106.2−7.5 M ⊙, SFR ≈ 0.04–0.20 M ⊙ yr−1, and Z gas ≈ 0.04–0.24 Z ⊙. Uncovering the properties of MW analogs at cosmic dawn by combining JWST observations and locally constrained models will allow us to understand our Galaxy's formation, linking the high- and low-z perspectives.

Observing Population III (hereafter PopIII) galaxies, the hosts of first-generation stars, remains challenging even with the JWST. The current few candidates have been identified through the combination of a prominent He II emission and the absence of metal lines, a well-known but extremely brief signature of metal-free systems. Here, we accurately model the evolution of the emission from PopIII galaxies to increase the number of candidates in JWST observations. To achieve this, we employ a locally calibrated galaxy-formation model that self-consistently follows the star formation and chemical evolution initiated by the first stars. We find that PopIII galaxies can emit metal lines in their “self-polluted” phase, while galaxies host only metal-free stars, but the gas has been chemically enriched by the first supernovae. In this phase, PopIII galaxies have [O III]/Hβ ≈ 1, which opens the pool of candidates to more easily detectable sources. We predict that the high He II emission of PopIII galaxies can last up to ≈20 Myr and that it is partly maintained in the “hybrid” phase, when PopIII and Population II stars coexist in the host galaxy. We propose novel diagnostics involving ultraviolet metal lines to select PopIII candidates in high-z JWST surveys. In JADES, we identify nine candidate galaxies with >25% of their stellar mass in metal-free stars, showcasing the effectiveness of our method. Ultimately, the key to discovering PopIII galaxies could be to catch them during their first episodes of chemical enrichment.

We characterize the JWST spectra of 61 galaxies at z = 9−14, including 30 newly confirmed galaxies. We directly compare the z > 9 spectroscopic properties against 401 galaxies at 6 < z < 9, with the goal of identifying evolution in the star formation histories and interstellar medium. We measure rest-UV emission-line properties and UV continuum slopes, while also investigating the rest-optical emission lines for the subset of galaxies at 9.0 < z < 9.6. With these spectra, we constrain the stellar masses, specific star formation rates (sSFRs), dust attenuation, and the average metallicity and abundance pattern of z > 9 galaxies. Our dataset indicates that the emission lines undergo a marked change at z > 9, with extremely large C III], Hβ, and Hγ equivalent widths becoming 2 and 3× more common at z > 9 relative to 6 < z < 9. Using the spectra, we infer the distribution of star formation rates (SFRs) on short (SFR3Myr) and medium (SFR3−50Myr) timescales, finding that rapid SFR upturns (large SFR3Myr/SFR3−50Myr ratios) are significantly more likely among z > 9 galaxies. These results may reflect a larger dispersion in UV luminosity at fixed halo mass and larger baryon accretion rates at z > 9, although other physical effects may also contribute. We suggest that the shift in star formation conditions explains the prevalence of extreme nebular spectra that have been detected at z > 9, with hard ionizing sources and nitrogen enhancements becoming more typical at the highest redshifts. Finally, we identify five z > 9 spectroscopically confirmed galaxies with red UV colors (β ≳ −1.5), revealing either a small population with moderate dust attenuation (τV = 0.23−0.35) or very high density nebular-dominated galaxies with hot stellar populations.

We introduce the Bias-free Extragalactic Analysis for Cosmic Origins with NIRCam (BEACON) survey, a JWST Cycle 2 program allocated up to 600 pure-parallel hours of observations. BEACON explores high-latitude areas of the sky with JWST/NIRCam over ∼100 independent sight lines, totaling ∼0.3 deg2, reaching a median F444W depth of ≈28.2 AB mag (5σ). Based on existing JWST observations in legacy fields, we estimate that BEACON will photometrically identify 25–150 galaxies at z > 10 and 500–1000 at z ∼ 7–10 uniquely enabled by an efficient multiple filter configuration spanning 0.9–5.0 μm. The expected sample size of z > 10 galaxies will allow us to obtain robust number density estimates and to discriminate between different models of early star formation. In this paper, we present an overview of the survey design and initial results using the first 19 fields. We present 129 galaxy candidates at z ≳7 identified in those fields, including 11 galaxies at z ≳10 and several UV-luminous (MUV < −21 mag) galaxies at z ∼ 8. The number densities of z < 13 galaxies inferred from the initial fields are overall consistent with those in the literature. Despite reaching a considerably large volume (∼105 Mpc3), however, we find no galaxy candidates at z > 13, providing us with a complimentary insight into early galaxy evolution with minimal cosmic variance. We publish imaging and catalog data products for these initial fields. Upon survey completion, all BEACON data will be coherently processed and distributed to the community along with catalogs for redshift and other physical quantities.

The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their initial mass function (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the He II emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (Population III, Pop III) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with >50% of their stellar mass in Pop III stars. We find that C1 is consistent with a purely Pop III system and adopt a simple parametric approach to infer the implications for the Pop III IMF and stellar mass. The observed He II/Hγ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages (≤1 Myr). Combined with the He II luminosity, this implies a total Pop III stellar mass of 2 · 104 < M⋆/M⊙ < 6 · 105. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations (M⋆ < 105 M⊙) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable Pop III IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-z Pop III systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties.

Il telescopio spaziale piü grande mai costruito, il James Webb Space Telescope (JWST), sará lanciato a ottobre 2021 e ci permetterá di osservare in profonditá l'Universo lontano. Grazie a sofistícate simulazioni cosmologiche ad alta risoluzione, abbiamo dimostrato che JWST potra osservare, per la prima volta, centinaia di sorgenti poco luminose ma fondamentali per la storia cósmica: le galassie nane satelliti. Parole chiave. Galassie nane, prime galassie, JWST.