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The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800 Myr after the Big Bang 1 – 3 , has been a matter of debate 4 . Some models suggest that high ionizing emissivity and escape fractions ( f esc ) from quasars support their role in driving cosmic reionization 5 , 6 . Others propose that the high f esc values from bright galaxies generate sufficient ionizing radiation to drive this process 7 . Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and f esc , can effectively dominate cosmic reionization 8 , 9 . However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between M UV  ≈ −17 mag and −15 mag (down to 0.005 L ⋆ (refs.  10 , 11 )). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ ξ ion  (Hz erg −1 )] = 25.80 ± 0.14, a factor of 4 higher than commonly assumed values 12 . If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%. An analysis of eight ultra-faint galaxies during the epoch of reionization with absolute magnitudes between −17 mag and −15  mag shows that most of the photons that reionized the Universe come from dwarf galaxies.

Several theories have been proposed to describe the formation of black hole seeds in the early Universe and to explain the emergence of very massive black holes observed in the first thousand million years after the Big Bang 1 – 3 . Models consider different seeding and accretion scenarios 4 – 7 , which require the detection and characterization of black holes in the first few hundred million years after the Big Bang to be validated. Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z  = 10.6, revealing the detection of the [Ne iv ] λ 2423 and CII* λ 1335 transitions (typical of active galactic nuclei), as well as semi-forbidden nebular lines tracing gas densities higher than 10 9  cm −3 , typical of the broad line region of active galactic nuclei. These spectral features indicate that GN-z11 hosts an accreting black hole. The spectrum also reveals a deep and blueshifted CIV λ 1549 absorption trough, tracing an outflow with velocity 800−1,000 km s −1 , probably driven by the active galactic nucleus. Assuming local virial relations, we derive a black hole mass of log ( M BH / M ⊙ ) = 6.2 ± 0.3 , accreting at about five times the Eddington rate. These properties are consistent with both heavy seeds scenarios and scenarios considering intermediate and light seeds experiencing episodic super-Eddington phases. Our finding explains the high luminosity of GN-z11 and can also provide an explanation for its exceptionally high nitrogen abundance. An extensive analysis of the JWST-NIRSpec spectrum of GN-z11 shows a supermassive black hole of a few million solar masses in a galaxy 440 million years after the Big Bang.

Local and low-redshift ( z  < 3) galaxies are known to broadly follow a bimodal distribution: actively star-forming galaxies with relatively stable star-formation rates and passive systems. These two populations are connected by galaxies in relatively slow transition. By contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems 1 – 4 . These galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence—so-called mini-quenching events 5 , 6 . However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation 7 , 8 . Early quenched galaxies have been identified out to redshift z  < 5 (refs.  9 – 12 ) and these are all found to be massive ( M ⋆  > 10 10   M ⊙ ) and relatively old. Here we report a (mini-)quenched galaxy at z  = 7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue ( U – V  = 0.16 ± 0.03 mag) but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4–6 × 10 8   M ⊙ ) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching. Analysis of the JWST/NIRSpec spectrum of the recently observed Lyman-break galaxy JADES-GS+53.15508-27.80178 revealed a redshift of z  = 7.3, a Balmer break and a complete absence of nebular emission lines, indicating that quenching occurred only 700 million years after the Big Bang.

The first observations of the James Webb Space Telescope (JWST) have revolutionized our understanding of the Universe by identifying galaxies at redshift z  ≈ 13 (refs. 1 , 2 – 3 ). In addition, the discovery of many luminous galaxies at Cosmic Dawn ( z  > 10) has suggested that galaxies developed rapidly, in apparent tension with many standard models 4 , 5 , 6 , 7 – 8 . However, most of these galaxies lack spectroscopic confirmation, so their distances and properties are uncertain. Here we present JWST Advanced Deep Extragalactic Survey–Near-Infrared Spectrograph spectroscopic confirmation of two luminous galaxies at z = 14.32 − 0.20 + 0.08 and z  = 13.90 ± 0.17. The spectra reveal ultraviolet continua with prominent Lyman-α breaks but no detected emission lines. This discovery proves that luminous galaxies were already in place 300 million years after the Big Bang and are more common than what was expected before JWST. The most distant of the two galaxies is unexpectedly luminous and is spatially resolved with a radius of 260 parsecs. Considering also the very steep ultraviolet slope of the second galaxy, we conclude that both are dominated by stellar continuum emission, showing that the excess of luminous galaxies in the early Universe cannot be entirely explained by accretion onto black holes. Galaxy formation models will need to address the existence of such large and luminous galaxies so early in cosmic history. JWST–NIRSpec spectroscopic confirmation of two luminous galaxies is presented, proving that luminous galaxies were already in place 300 million years after the Big Bang and are more common than what was expected before JWST.

Using an ultra-deep, untargeted survey with the MUSE integral field spectrograph on the ESO Very Large Telescope, we obtain spectroscopic redshifts to a depth never explored before: galaxies with observed magnitudes m > 30–32. Specifically, we detect objects via Lyman- α emission at 2.9 < z < 6.7 without individual continuum counterparts in areas covered by the deepest optical/near-infrared imaging taken by the Hubble Space Telescope, the Hubble Ultra Deep Field. In total, we find more than 100 such objects in 9 square arcminutes at these redshifts, also including a number of sources that are visible only in the HST band that contains Lyman- α . Detailed HST and IRAC stacking analyses confirm the Lyman- α emission as well as the 1216 Å breaks, faint UV continua ( M UV ∼ −15), and optical emission lines: these objects are the faintest spectroscopically-confirmed galaxies at high- z . The blue UV continuum slopes and measurements/limits on the equivalent widths of Lyman- α , which in some cases exceeds 300 Å, are consistent with ages < 10 Myr, metallicities < 5% solar, and stellar masses < 10 7–8 solar masses. The nature of these types of objects is intriguing as they could be the faint star-forming sources of Reionization and could represent the initial (strong) phase of stellar mass growth in galaxies.

Star formation histories of local dwarf galaxies, derived through resolved stellar populations, appear complex and varied. The general picture derived from hydrodynamical simulations is one of cold gas accretion and bursty star formation, followed by feedback from supernovae and winds that heat and eject the central gas reservoirs. This ejection halts star formation until the material cools and re-accretes, resulting in an episodic SFH, particularly at stellar masses below ~ 109 M⊙. Such feedback has often been cited as the driving force behind the observed slowly-rising rotation curves in local dwarfs, due to an under-density of dark matter compared to theoretical models, which is one of the primary challenges to LCDM cosmology. However, these events have not yet been directly observed at high-redshift. Recently, using HST imaging and grism spectroscopy, we have uncovered an abundant population of low-mass galaxies (M* < 109 M⊙) at z = 1 - 2 that are undergoing strong bursts of star formation, in agreement with the theoretical predictions. These Extreme Emission Line Galaxies, with high specific SFRs and shallow gravitational potential wells, are ideal places to test the theoretical prediction of strong feedback-driven outflows. Here we use deep MUSE spectroscopy to search these galaxies for signatures of outflowing material, namely kinematic offsets between absorption lines (in the restframe optical and UV), which trace cool gas, and the nebular emission lines, which define the systemic redshift of the galaxy. Although the EELGs are intrinsically very faint, stacked spectra reveal blueshifted velocity centroids for Fe II absorption, which is indicative of outflowing cold gas. This represents the first constraint on outflows in M* < 109 M⊙ galaxies at z = 1 - 2. These outflows should regulate the star formation histories of low-mass galaxies at early cosmic times and thus play a crucial role in galaxy growth and evolution.

The first observations of thejames Webb Space Telescope (JWST) have revolutionized our understanding of the Universe by identifying galaxies at redshift z ~ 13 (refs. 1-3). In addition, the discovery of many luminous galaxies at Cosmic Dawn (z > 10) has suggested that galaxies developed rapidly, in apparent tension with many standard models4 8. However, most of these galaxies lack spectroscopic confirmation, so their distances and properties are uncertain. Here we present JWST Advanced Deep Extragalactic Survey-Near-Infrared Spectrograph spectroscopic confirmation of two luminous galaxies at z = 14.32+0.08-0.20 and z = 13.90 ± 0.17. The spectra reveal ultraviolet continua with prominent Lyman-a breaks but no detected emission lines. This discovery proves that luminous galaxies were already in place 300 million years after the Big Bang and are more common than what was expected beforeJWST. The most distant of the two galaxies is unexpectedly luminous and is spatially resolved with a radius of260 parsecs. Considering also the very steep ultraviolet slope of the second galaxy, we conclude that both are dominated by stellar continuum emission, showing that the excess of luminous galaxies in the early Universe cannot be entirely explained by accretion onto black holes. Galaxy formation models will need to address the existence of such large and luminous galaxies so early in cosmic history.

We present the first results from CECILIA, a Cycle 1 JWST NIRSpec/MSA program that uses ultra-deep ~30 hour G235M/F170LP observations to target multiple electron temperature-sensitive auroral lines in the spectra of 33 galaxies at z~1-3. Using a subset of 23 galaxies, we construct two ~600 object-hour composite spectra, both with and without the stellar continuum, and use these to investigate the characteristic rest-optical (5700-8500 Angstrom) spectrum of star-forming galaxies at the peak epoch of cosmic star formation. Emission lines of eight different elements (H, He, N, O, Si, S, Ar, and Ni) are detected, with most of these features observed to be <3% the strength of H-alpha. We report the characteristic strength of three auroral lines ([NII]5756, [SIII]6313, and [OII]7322,7332), as well as other semi-strong and faint emission lines, including forbidden [NiII]7380,7414 and the OI 8449 recombination line, some of which have never before been observed outside of the local universe. Using these measurements, we find T_e[NII]=13630+/-2540 K, representing the first measurement of electron temperature using [NII] in the high-redshift universe. We also see evidence for broad line emission with a FWHM of ~536 km/s; the broad component of H-alpha is 6.01-28.31% the strength of the narrow component and likely arises from star-formation driven outflows. Finally, we briefly comment on the feasibility of obtaining large samples of faint emission lines using JWST in the future.

The Near-infrared Spectrograph (NIRSpec) on the James Webb Space Telescope is uniquely suited to studying galaxies in the distant Universe with its combination of multi-object capabilities and sensitivity over a large range in wavelength (0.6-5.3 microns). Here we present the NIRSpec Wide survey, part of the NIRSpec Instrument Science Team's Guaranteed Time Observations, using NIRSpec's microshutter array to obtain spectra of more than 3200 galaxies at \\(z>1\\) at both low- and high-resolution (\\(R\\approx100\\) and 2700) for a total of 105 hours. With 31 pointings covering \\(\\approx\\)320 arcmin\\(^2\\) across the five CANDELS fields with exquisite ancillary photometry from the Hubble Space Telescope, the NIRSpec Wide survey represents a fast and efficient way of using JWST to probe galaxies in the early Universe. Pointing centers are determined to maximize the observability of the rarest, high-value sources. Subsequently, the microshutter configurations are optimized to observe the maximum number of \"census\" galaxies with a selection function based primarily on HST/F160W magnitude, photometric/slitless grism redshift, and predicted \\ha\\ flux tracing the bulk of the galaxy population at cosmic noon (\\(z_{\\rm med}=2.0\\)). We present details on the survey strategy, the target selection, an outline of the motivating science cases, and discuss upcoming public data releases to the community.