The tremendous new capabilities of JWST already enable great discoveries across broad areas of astrophysics, spanning scales from exoplanets to cosmology. We have also found some puzzling results that require re-examination of our prior assumptions and are a rich source for future investigations. JWST is remarkably complex, and the mission is possible because of the work of thousands of people around the world. Despite the complexity, it remains an open mission, with opportunity for researchers at all career stages, across a variety of institutions, and around the world. We hope to apply the lessons of our experiences to future ambitious astrophysics projects that we will pursue together.

Massive quiescent galaxies (MQGs) at redshifts z > 3 present a profound challenge to our understanding of galaxy formation and evolution. These galaxies undergo rapid and intense star formation followed by swift quenching very early in cosmic history, resulting in the formation of compact galaxies with half-light radii smaller than 1 kpc. New observations, particularly from the James Webb Space Telescope (JWST), have rapidly advanced our knowledge of their abundance, possible formation pathways, and physical properties. However, current theoretical models still struggle to replicate the observed number densities and characteristics of these galaxies. This review attempts to consolidate recent observational breakthroughs, discusses the comparison with theoretical models, and highlights key unresolved questions. An especially interesting development is the discovery of sub-populations of even older and early forming (at z > 7) quiescent galaxies which pose particular challenges and add to the ledger of the ‘too much too soon’ galaxy and AGN problems revealed by JWST. I outline my own views on key future research directions to unravel the mysteries of MQGs and their role in the broader context of cosmic evolution.

Through the discovery and characterization of three mini-quenched galaxies at z = 4 − 6 in the UNCOVER survey (JWST observations of Abell 2744; PIs Bezanson, Labbe), we discuss the diversity of star formation histories (SFHs) and future prospects for the study of “napping” systems. We find these galaxies with their characteristic features – bright rest-frame UV flux, a weak Balmer/4000Å break, and weak or absent emission lines – corresponding to residual star formation and presence of old stellar populations. With the use of flexible non-parametric SFH models via Prospector, the specific star formation rates of these mini-quenched galaxies place them below the star formation main sequence. The posterior distributions of the time-averaged SFRs show that these systems are outliers in the SFR10/SFR100 parameter space. We demonstrate that this class of galaxies can best be identified and characterized spectroscopically due to the challenge of photometrically capturing the UV flux, weak Balmer/4000Å break, and weak emission lines simultaneously.

We report spectroscopic confirmation of a z = 3.757 ± 0.0011 massive $({\rm{log}}({{\rm{M}}_*}/{M_ \odot }) = 10.86_{ - 0.03}^{ + 0.09})$ quiescent galaxy with 11 hours of Keck/MOSFIRE K-band observations. This galaxy was selected from the FENIKS survey, which uses deep Gemini/Flamingos-2 Kb Kr imaging optimized for increased sensitivity to galaxies with strong Balmer/4000 Å breaks. Its rest-frame colors are consistent with a ∼1 Gyr old stellar population. This places it as one of the oldest objects at these redshifts, and challenges the notion that quiescent galaxies at z > 3 are all young post-starbursts. The star formation history suggests that its progenitor was in place by z ∼4 −7, reaching 1010.6 Mȯ by z ∼ 5, which we have confirmed using ultra-deep medium resolution spectroscopy from JWST. These results are in line with the rapid formation and quenching timescales reported for similar massive quiescent galaxies at z > 3, implying extreme baryonic physics within the first billion years of cosmic history.

Using the superb capabilities of JWST, we investigate the structure of thin and thick disks in galaxies beyond the local universe for the first time. We found evidence of sequential formation, where most galaxies initially form a thick disk, followed by the formation of a thin disk. Thin disk formation occurred around 8 Gyr ago in high-mass galaxies, earlier than 4 Gyr ago in low-mass galaxies, aligning with the onset of thin disk formation in the Milky Way. We propose that this downsizing thin disk formation—delayed in low-mass galaxies—can be naturally explained by stability-regulated disk formation by evolving gas fractions. As gas fractions decrease over time, turbulence in the gas disk declines, allowing a thin disk to form. High-mass galaxies, which efficiently convert gas into stars, achieve lower gas fractions, supporting earlier thin disk formation.

The REBELS ALMA large program has obtained, to date, the largest number of [C ii] and dust continuum observations of massive galaxies at z 6-8. We present results from a now complete JWST NIRSpec/IFU program (GO-1626, PI Stefanon) targeting 12 of these galaxies at 0.6-5.3μm. Due to the incredible sensitivity of the NIRSpec prism, key optical emission lines between [O ii] and [S ii] are detected for the majority of the sample, enabling metallicity constraints from a range of different calibrations. One surprising finding of this dataset is the near-solar oxygen abundances for several of these galaxies. We focus on an extension of the mass-metallicity relations at high-z to higher stellar masses, in comparison to the typical galaxies observed in e.g., JADES and CEERS. These relations provide fundamental information on how stellar mass is assembled in galaxies, and how star formation enriches and dilutes the interstellar medium.

JWST is at the forefront of revolutionizing our comprehension of the high-redshift Universe, stretching the boundaries of galaxy detection. Yet, as we strive for a more comprehensive understanding of the high-redshift Universe, concerns arise about the consistency of early estimates of galaxy number density, their stellar masses, and physical properties.

Our study delves into the synergy of NIRCAM and MIRI in the Extended Groth Strip (EGS) field, unveiling insights into the characterization of approximately 300 galaxies at z > 3, including 30 MIRI-detected red massive galaxies selected as HST-dark/faint and Little Red Dots (LRDs). By providing constraints on rest-frame optical and near-infrared emission, MIRI allows for more accurate measurements of stellar mass and helps disentangle the contribution from star formation and AGN activity. Although MIRI data has minimal impact on overall photometric redshift accuracy, it is critical for reliable stellar mass determination, especially at the highest redshifts (z > 7). Excluding MIRI data can lead to overestimating stellar mass by up to 0.15 dex for the general population and 0.4-1 dex for LRDs. Besides, MIRI data significantly improve our ability to constrain the contribution of Active Galactic Nuclei (AGN) to the observed light. For red massive galaxies, or those with the longest NIRCam filters contaminated by strong emission lines, excluding MIRI data can lead to a substantial overestimation of the AGN fraction.

The discovery in JWST data of numerous bright galaxies at Cosmic Dawn, along with the identification of potentially “Universe-breaking” massive galaxies, is reshaping our understanding of galaxy formation, and raise questions about how rapidly galaxies assembled their mass in the early Universe. In parallel with follow-up studies, significant progress can be achieved by characterizing the brightest and most massive galaxies observed when the universe was 200 − 400 million years older, likely the most immediate descendants of the tantalizing objects observed at z > 10. In this talk, we will present the first results from a NIRSpec IFU R100 program targeting 12 bright, especially high-mass galaxies at just approximately 750Myr of cosmic time (6.5 < z < 8.5). Remarkably, this sample also benefits from [CII] and dust continuum estimates from the recent ALMA large program REBELS, making this a unique sample in the panorama of high-redshift studies. Leveraging the JWST’s high spatial and spectral resolution across a broad wavelength range, from the rest-frame UV to rest-frame optical, we can reconstruct spatially-resolved maps of their stellar and gas content. These maps enable us to interpret them in terms of in-situ mass assembly, merger-driven star formation, and star-formation history. We will also provide a brief outlook on their formation histories as inferred from gas-phase metallicities, but will leave most of the discussion to a companion, more in-depth presentation. These results will ultimately allow us to assess the star formation efficiency in massive galaxies at early epochs.

In this contribution, I will review a few of the key characteristics of the stellar populations and interstellar medium (ISM) of high-redshift galaxies as revealed by James Webb Space Telescope (JWST) spectroscopy. Specifically, I will discuss recent evidence for nitrogen enhancement and proposed mechanisms to explain it, the existence of galaxies with very blue UV continuum slopes, and the ionization state of emission-line galaxies. I will then focus on some recent work to understand the connection between ionization parameter, gas density, and metallicity, showing that the gas density is an important factor in modulating the ionization parameter across a large range of redshift.

Directly observing “Pop III” or zero-metallicity galaxies has long been a goal in our quest to find the first galaxies that formed in the Universe. These objects, however, have remained elusive even with 2.5 years of JWST observations. We present spectroscopy on the lowest metallicity systems yet confirmed with JWST, including the most extreme Lyman-alpha emitters at MUV = −15 (0.01L*) detected with MUSE at z =3 −7 as well as a photometrically-selected sample at z =4 −9 from the JADES survey. While these objects show significantly lower metallicities than more UV-luminous objects at the same redshifts, we find a consistent metallicity floor at ≍ 2% Zȯ. This indicates rapid chemical enrichment must be taking place in systems with light-weighted ages less than 5 Myr. However, many typical selections involving photometric redshifts and/or UV continuum slopes are biased against identification of even lower metallicities. Therefore, it is currently not possible to establish if this “floor” is real or an observational artifact. Spectroscopy of larger samples selected in less restrictive ways will be required to fully determine the answer.

To understand the formation and evolution of the Universe, it is crucial to understand how and when the first stars formed. The latest observational data reveal unprecedented information about the chemical enrichment of the early Universe, which seems to behave differently from the local Universe. The first stars, being very massive, enrich their metal-poor environment in an uncertain way. In order to predict the abundances of the first galaxies, we include nucleosynthesis yields from Population III stars up to 300M⊙, including faint supernovae, Wolf-Rayet (WR) stars and Pair-Instability Supernovae (PISN) into our state-of-the-art hydrodynamical cosmological simulations. Our code (based on Gadget-3) also includes the latest nucleosynthesis yields from population II stars (from Kobayashi et al. 2020) for all stellar mass ranges. We predict the chemical abundance evolution of galaxies for different elements from the early Universe to the local Universe. We first test the modelling of stellar feedback by comparing the observed evolution of mass–metallicity relations (MZR) and metallicity gradients of the interstellar medium. We then compare our model including Population III stars with observational data from the James Web Space Telescope (JWST). For elemental abundances, we find that the N/O abundance gives a systematically higher value, comparable to the observational data of very high-redshift galaxies such as GN-z11.

Recent ALMA studies have detected rest-frame far-infrared emission lines in star-forming galaxies at redshift (z) ∼ 6 – 9. To investigate detailed properties of high-z ALMA [O iii] 88 μm emitters, we performed JWST observations with NIRCam and NIRSpec IFU (GO-1840). In the NIRCam observations, six broad- and/or medium-band filters were used to trace rest-frame UV to optical stellar continuum emission, while NIRSpec IFU observations provide information on rest-frame optical emission lines. The powerful combination of JWST and ALMA in this project has revealed a matured stellar population in the core region of the most distant protocluster, A274z7p9OD, at z = 7.9. We also present our results of gas-phase metallicity from a direct-temperature method based on a joint analysis of the optical and far-infrared [O iii] lines, which will form the basis for similar studies combining JWST and ALMA data.

Blind spectroscopy of massive lensing galaxy clusters with MUSE has revealed large numbers of gravitationally-lensed Lyman-α emitters exhibiting asymmetric profiles at 2.9 < z < 6.7, suggesting abundant outflows from low-mass star-forming galaxies in the early universe. Are these primaeval galaxies experiencing their first bursts of star formation, or established galaxies experiencing rejuvenation? With JWST rest-frame optical/NIR continuum imaging now available for many of these objects, we can search for older stellar populations. Here, we search for spectroscopic confirmation of outflows from these galaxies, finding a few high-signal-to-noise cases in which blueshifted interstellar absorption lines are detected. Next, we analyse the star formation histories with combined HST + JWST photometry. We find most them to be well characterised by very young, low metallicity stellar populations. However, despite the rest-frame optical/NIR coverage of JWST, we cannot place strict upper bounds on the mass in old stars (age > 100 Myr).

Lyman-α (Lyα) emitting galaxies (LAEs) have been the subject of a range of observational studies of cosmic reionization, tracing the emergence of the first ionized ‘bubbles’ and the evolution of the neutral fraction of the intergalactic medium (IGM). Here we discuss a series of works characterising the physical properties and environments of the most distant LAEs identified in the JWST Advanced Deep Extragalactic Survey (JADES). Around half of the z < 8 LAEs fall within large-scale galaxy overdensities, providing strong evidence for enhanced Lyα transmission in these environments. Despite findings of strong Lyα absorption at the redshift frontier (z > 9) exceeding that expected only by the IGM, several LAEs are discovered at z > 8, including one at redshift z = 13.0 that shows highly exceptional properties. These observations firmly establish LAEs as powerful probes of the unfolding of cosmic reionisation out to the earliest stages of galaxy evolution.

Recent studies of high-redshift galaxies using JWST, such as GN-z11 revealed highly elevated levels of nitrogen (N). This phenomenon extends to gravitationally-lensed galaxies like the Sunburst Arc at z = 2.37, as well as to globular clusters (GCs). We propose that this originates from the presence of very massive stars (VMSs) with masses ranging from 100 to 1000 M⊙. The He ii observed in the Sunburst Arc could also stem from the disproportionately large contribution of VMSs. We build an entirely new Framework for massive star evolution which is no longer set by Dutch or other mass-loss “recipes” but which take the physics of Γ or L/M-dependent winds into account. We discuss the mass-loss kink and the transition mass-loss rate between optically thin and thick winds, before we study the evaporative mass-loss history of VMSs. Our novel evolution models exhibit vertical evolution in the HR-diagram from the zero-age main sequence due to a self-regulatory effect driven by their wind-dominated nature, and we discuss what wind physics sets the stellar upper-mass limit. Our estimate for the Sunburst Arc in Vink (2023) suggests that the significant amounts of N found in star-forming galaxies likely arise from VMSs. We evaluate the strengths and weaknesses of previous hypotheses, including fast rotating massive stars and supermassive stars (SMSs), and we conclude that only our VMS model satisfies the relevant criteria. Finally, we advocate for the inclusion of VMSs in population synthesis and chemical evolution models, emphasizing the need for a self-consistent wind approach, which currently does not exist. Even minor inaccuracies in mass-loss rates dramatically impact the stellar evolution of VMS, as well as their ionizing and chemical feedback.

Understanding the epoch of reionization (EoR) is one of the frontier goals of modern astronomy. Lyα emission from the high-redshift galaxies has proved to be a useful tool for probing cosmic reionization. The launch of JWST has enabled the ultraviolet and optical spectroscopic studies of Lyα emitters (LAE) in the EoR. In this contribution, we present the results based on the stacking analysis of JWST/NIRSpec spectra of galaxies taken as part of the JWST GTO program JADES. The sample consists of > 250 galaxies at redshifts of ∼5-10. We divide this sample into six different sub-samples of galaxies based on the strengths of Lyα emission and redshifts and create composite spectra. We then estimate emission line ratios of optical and UV emission lines, and various physical quantities such as dust extinction, electron temperatures, ionization parameters, and ionizing photon production efficiencies to characterize the populations of LAEs and non-LAEs.

Recent James Webb Space Telescope (JWST) observations with superb angular resolution have revealed the existence of clumpy galaxies at high redshift through the detection of rest-frame optical emission lines. The formation of clumpy galaxies at lower redshift (z ≲ 4) has been investigated both observationally and theoretically, and violent disk instability driven by cold gas accretion is proposed as one of the main mechanisms. However, the formation of clumpy galaxies at z > 6 is not well understood theoretically.

In order to study the properties of (sub-)galactic clumps around the epoch of reionization (z ≳ 6), we use FirstLight, a suite of zoom-in high-resolution (∼20 pc) cosmological simulations. We devise a physical model of Hii regions and calculate spatially resolved [Oiii] line emission. We identify the giant clumps that are bright in [Oiii] 5007Å line with flux greater than ∼ 10−18 ergs−1 cm−2, to be detected by JWST. Among 62 simulated galaxies that have stellar masses of (0.5 – 6) × 1010 M⊙ at z = 5, we find clumps in 1828 snapshots in the redshift range z = 5.5 – 9.5. About one-tenth of the snapshots show the existence of clumpy systems with two or more components. Most of the clumps are formed by mergers and can be characterized by their ages; central clumps dominated by stellar populations older than 50 Myr, and off-centered clumps dominated by younger stellar populations with specific star formation rates of ∼ 50 Gyr−1. The latter type of young clumps is formed from gas debris in the tidal tails of major mergers. The merger-induced clumps are short-lived, and merge within a dynamical time of several tens million years. The number density of the clumpy systems is estimated to be ∼ 10−5 cMpc−3, which is large enough to be detected in recent JWST surveys.

Understanding the cosmic evolution of the barred galaxy population is fundamental to quantifying their impact on the internal secular evolution of galaxies and dating the formation of the first dynamically cold stellar disks in the Universe. In this context, the new imaging capability provided by JWST is revolutionizing the study of high-z disk galaxies, unveiling for the first time their complex morphologies. In this contribution, I discuss the discovery of ceers-2112, a Milky Way-like barred spiral galaxy observed in the CEERS survey. The morphological analysis of the stacked NIRCam images undoubtedly discloses the presence of a stellar bar and hints of spiral arms. The combined analysis of HST/ACS, HST/WFC3, and JWST/NIRCam photometry places the galaxy at redshift z = 3.03. Thus, this bar was already formed when the Universe was only 2 Gyr old. The star formation history of ceers-2112 reveals that this system is a Milky Way-like galaxy, with M* = 4 × 109 M⊙. Remarkably, this discovery proves that mature stellar (and not only gaseous) disks were in place as early as ∼ 12 Gyrs ago (z ∼ 5) and that baryons were dominating over dark matter in ceers-2112 at these early times.

Cosmological magneto-hydrodynamical simulations such as IllustrisTNG describe the first chapters of our cosmic history. In our study, we focus on the evolution of the physical and chemical properties of galaxies in a series of mergers. We processed the catalogue data and the merger tree files of the TNG100-1 and the TNG300-1 simulations and investigated the evolution of progenitor galaxies, their merger companions (next progenitor) and their common descendants. We calculated the galaxies’ average star formation rate (SFR), metallicity and mass at different redshifts between 0 < z < 15 and found that the SFR of the descendant galaxies is increasing through the mergers. We traced both the large-scale structure and mass build-up in events involving dwarf galaxies. We reconstructed the observed cosmic star formation rate density variation through a large time interval (12 > z > 0) and confronted the model predictions with recent JWST observations. We discuss how the dwarf galaxies influenced the mass increase of galaxies from both observational and computational approaches. Our results show that a high fraction of galaxy mergers is connected to superclusters, especially at lower redshifts.

What reionized the universe? What are the masses, star formation rates, and formation histories of galaxies in the early universe? Answering these questions requires a detailed understanding of the nebular emission properties in galaxies. However, the complex physics governing nebular emission in galaxies, particularly in the early universe, often defy simple low-dimensional models. We present Cue, a highly flexible tool for interpreting nebular emission across a wide range of abundances and ionizing conditions of galaxies at different redshifts. Unlike typical nebular models used to interpret extragalactic nebular emission, our model does not require a specific ionizing spectrum as a source, instead approximating the ionizing spectrum with a 4-part piece-wise power-law, along with freedom in C/O, N/O, O/H, electron density, and total ionizing photon budget. This flexibility allows us to either marginalize over or directly measure the incident ionizing radiation, thereby directly interrogating the source of the ionizing photons in distant galaxies via their nebular emission. We demonstrate the use of Cue on a variety of simulated and real JWST data. For example, Cue reproduces the emission lines and ionizing production efficiency of GS-NDG-9422, a peculiar z = 6 galaxy with a claimed top-heavy IMF, with an more orthodox ionizing continuum consistent with a combination of normal low-metallicity stellar populations and a low-luminosity AGN. Additionally, we show the inferences of Cue applied to JWST/NIRSpec spectroscopy of z ∼ 2 galaxies (PI: Belli, ID 1810), specifically identifying the dominant ionizing sources in normal galaxies at the cosmic noon.