The mechanisms of Ly alpha photon escape are key to understanding galaxy evolution and cosmic reionization, yet remain poorly understood. We investigate the UV-continuum sizes of 23 Ly alpha emitters (LAEs) at Cosmic Noon (1.7 < z < 3.3), extending previous size analyses to include fainter galaxies (M-UV similar or equal to -14) using gravitational lensing. Our results show that these LAEs are unusually small for their luminosity, with a mean effective radius (r(eff)) of 170 +/- 140 pc. They follow a distinct size-luminosity relation, with an intercept at M-UV = -21 approximately three times smaller than typical star-forming galaxies (SFGs) at similar redshifts. This relation, however, is consistent with that of low-redshift Green Pea galaxies, suggesting that LAEs maintain compact sizes across redshifts. We also find that Ly alpha equivalent width increases with decreasing r(eff), confirming previous findings. The small sizes of LAEs lead to high star formation surface densities (Sigma SFR =1-600 M-circle dot yr(-1) kpc(-2)), clearly separating them from typical SFGs in the Sigma SFR versus r(eff) space. Given that high Sigma SFR is linked to strong galactic outflows, our findings imply that compact morphology plays a key role in Ly alpha escape, likely facilitated by outflows that clear underdense channels in the intersetellar medium (ISM). Thus, these results demonstrate that compact size and high Sigma SFR can help identify LAEs.

The exploration of the early Universe is being transformed by the James Webb Space Telescope (JWST), which delivers unprecedented angular resolution at infrared wavelengths and opens a unique window redward of the K band (2 µm). Thanks to NIRCam, NIRISS, NIRSpec, and MIRI instruments, which provide both imaging and spectroscopy with exquisite efficiency, new classes of sources have emerged within the first years of operations. The first half-Gyr of cosmic time is now routinely probed, revealing massive blue/red galaxies and a population of Active Galactic Nuclei (AGN) appearing as “little red dots” together with a rest-frame near-infrared/optical view of sources across the reionization and post-reionization epochs. Angular resolution will remain pivotal in the 2030–2040s, when extreme adaptive optics (AO) facilities will be deployed on both (8–10) m (e.g., the VLT Multi-Conjugate Adaptive Optics (MCAO)-Assisted Visible Imager and Spectrograph, MAVIS) and on extremely large telescopes, like the 39 m ELT (e.g., Multi-conjugate adaptive Optics Relay For ELT Observation, MORFEO). Operating at the diffraction limit, these facilities will improve JWST s resolution, with ELT achieving a factor of ≃ 6 smaller Point-Spread-Function (PSF). An ELT diffraction-limited PSF (with a Full Width Half Maximum, FWHM ≈ 8–12 mas) in the near infrared will resolve spatial scales <100 pc at any redshift (z ≲ 18 ), revealing abundant star-forming clumps with sufficient sensitivity. Leveraging gravitational lensing as a cosmic telescope, even with moderate magnification factors (µ ∼ 4–8), diffraction-limited 8 m and 39 m telescopes will probe physical scales ≲ 25 pc, enabling systematic studies of star formation down to star cluster scale at cosmological distances. Such observations are poised to become routine in the 2030–2040s.

We present JWST NIRSpec IFU spectroscopic measurements of one of the faintest (M_UV>-13.6) known ionizing sources at z=5.66, dubbed Small And Lensed Source Arc, SALSA. This source is highly magnified (μ>100) by the lensing galaxy cluster Abell 2744, providing a unique opportunity to investigate the physical properties of faint sources in the Epoch of Reionization. We characterize SALSA's nebular emission using rest-frame UV and optical emission lines and investigate the relationship between its ionizing efficiency, nebular excitation, and chemical enrichment. We robustly detect Hα emission and use it to predict the Hβ flux assuming negligible dust attenuation. We also measure the [OIII]λ5007 emission and estimate oxygen abundance resulting in a high R3 index 2.82_-0.25^+0.34 and relatively low metallicity 12+log(O/H)= 7.43±0.09. SALSA presents a high ionizing production efficiency log(ξ_ion)=25.49_-0.08^+0.09 Hz erg ^-1, consistent with theoretical models from very massive stars and constant star formation rate, and a high Lyα escape fraction f_esc^Lyα=0.39±0.14. These properties place SALSA among the most extreme star-forming sources known at this epoch.

The mechanisms of Ly$\alpha$ photon escape are key to understanding galaxy evolution and cosmic reionization, yet remain poorly understood. We investigate the UV-continuum sizes of 23 Ly$\alpha$ emitters (LAEs) at Cosmic Noon ($1.7 < z < 3.3$), extending previous size analyses to include fainter galaxies ($M_{\rm UV} \simeq -14$) using gravitational lensing. Our results show that these LAEs are unusually small for their luminosity, with a mean effective radius ($r_{\rm eff}$) of $170 \pm 140$ pc. They follow a distinct size-luminosity relation, with an intercept at $M_{\rm UV} = -21$ approximately three times smaller than typical star-forming galaxies (SFGs) at similar redshifts. This relation, however, is consistent with that of low-redshift Green Pea galaxies, suggesting that LAEs maintain compact sizes across redshifts. We also find that Ly$\alpha$ equivalent width (EW(Ly$\alpha$)) increases with decreasing $r_{\rm eff}$, confirming previous findings. The small sizes of LAEs lead to high star formation surface densities ($\Sigma$SFR $= 1-600 M_{\sun} \ \rm{yr}^{-1} \ \rm{kpc^{-2}}$), clearly separating them from typical SFGs in the $\Sigma$SFR vs. $r_{\rm eff}$ space. Given that high $\Sigma$SFR is linked to strong galactic outflows, our findings imply that compact morphology plays a key role in Ly$\alpha$ escape, likely facilitated by outflows that clear under-dense channels in the ISM. Thus, these results demonstrate that compact size and high $\Sigma$SFR can help identify Ly$\alpha$-emitters.

Galaxies at redshift z∼2-6 exhibit prominent star-forming regions (clumps). Characterisation of these galactic structures requires both high sensitivity and spatial resolution of ∼100 pc or less. Currently, these spatial scales are achievable only with the aid of strong gravitational lensing. However, lensing introduces model-dependent uncertainties and limits the sample to galaxies behind massive clusters. The upcoming 40m-class telescopes, such as the ESO Extremely Large Telescope (ELT), will enable routine studies of clumps in ubiquitous, non-lensed z≥2 galaxies. We assess the capability of MICADO, the first-light imager and spectrograph of the ELT, to characterise clumps in non-lensed z=2 galaxies. Specifically, we focus on two representative observing scenarios, providing the basis for an early scientific exploitation of the instrument. We modelled clumps in an idealised z=2 star-forming galaxy and produced mock MICADO observations in both broad- and narrow-band imaging. The latter leverages the strong emission lines typical of clumps to improve detection and characterisation. Reaching UV rest-frame magnitudes of clumps as faint as M_UV∼ -15 in optimal observing conditions, MICADO will be able to efficiently characterise clumps in non-lensed z=2 galaxies down to R_e∼ 20 pc. Moreover, the full MICADO ∼1 arcmin^2 field of view enables efficient surveys of multiple targets. This study demonstrates MICADO's potential to fill a crucial observational gap, shedding new light on clump formation and evolution at cosmic noon. Accurate PSF reconstruction will be crucial to fully harness this capability as it significantly impacts clump detection and characterisation, especially at the smallest sizes (<40 pc).

We report the discovery of a doubly imaged little red dot (LRD) candidate behind the galaxy cluster Abell 383, which we dub A383-LRD1. Initially classified as a dropout galaxy in HST imaging with several ground-based emission line detections placing it at z(spec) = 6.027, new JWST/NIRCam observations taken as part of the cycle 4 VENUS survey now reveal that the source consists of two underlying components. These consist of a red point-source with a V-shaped SED consistent with LRD selection criteria and a nearby (similar to 380 pc) compact blue companion, which was the main contributor to the previous rest-frame UV detections. Based on lensing symmetry and its SED, the LRD appears to lie at a similar redshift as well. The magnification of the two images of A383-LRD1 is mu(A) = 16.2 +/- 1.2 and mu(B) = 9.0 +/- 0.6, respectively, and the predicted time delay between them is Delta t(grav) = 5.20 +/- 0.14 yr (similar to 0.7 yr in the rest-frame). After correcting for the lensing magnification, we derived an absolute magnitude of M-UV,M- LRD = -16.8 +/- 0.3 for the LRD and M-UV,M- BC = -18.2 +/- 0.2 for the blue companion. We performed SED fits to both components, revealing the LRD to show the best fit with a black hole star (BH*) model and a substantial host galaxy, along with the blue companion with an extremely young, emission-line dominated star-forming nebula. If it is verified spectroscopically in the future, A383-LRD1 would represent the second known multiply imaged LRD detected to date, following A2744-QSO1. To our knowledge, it would also stand as the first LRD system with a confirmed detection of [C II]lambda 158 mu m emission from ALMA observations. Thanks to the lensing magnification, this system opens a unique door to studying the relation between a LRD, its host galaxy, and its environment. It represents a prime candidate for deep JWST spectroscopy and high-resolution ALMA follow-up observations.

We present our search for little red dots (LRDs) in the "J1030 field", a region of the sky around the z∼ 6.3 quasar SDSS J1030+0524, observed by the JWST EIGER program. Over 154 point-like sources selected in a JWST-based photometric catalog, we find five broad line emitters (with FWHM ≳ 1000 km s^-1) that are red (F200W - F356W > 0) and are undetected in the X-rays. We use these sources to derive the bolometric luminosity function (LF) of LRDs at z = 2.4 and z = 4.5. At z = 2.4, the space density of LRDs is only a factor of ∼ 2 lower than that of all pre-JWST active galactic nuclei (AGNs) with bolometric luminosity L_ bol≳ 3 × 10^44 erg s^-1. At z = 4.5, our estimate is consistent with those derived for LRDs based on larger areas of the sky. A similar behaviour is observed in the black hole mass function. More importantly, we study the number density of LRDs from cosmic dawn to cosmic noon. We find that there is no significant evolution in the abundance of LRDs with L_ bol≳ 3 × 10^44 erg s^-1 at z > 2. We speculate that the drop at z < 4 seen by other studies is due to their sampling of only the bright-end of the LRDs LF. At cosmic noon, the abundance of LRDs is n = 3.4^+5.6_-2.4× 10^-5 Mpc^-3, which is a factor of ∼ 350 larger than recent model predictions and is comparable with that of X-ray selected AGNs with similar bolometric luminosity. Our result may imply that, if LRDs are the early, rapid stages of supermassive black hole growth, as suggested by some models, then the formation of black hole seeds can be efficient down to epochs as recent as cosmic noon. Alternatively, LRDs may simply be a high-accretion phase in already mature black holes.

We present the first characterization of the host galaxy of a recently discovered type IIP SN at z=5.13 (SN Eos). SN Eos and its host galaxy are gravitationally lensed and multiply imaged. The total magnification μ∼53 enables spatially resolving the system, allowing us to localize the core-collapse supernova (CCSN) position and to characterize its local environment within an early galaxy. Our observation reveals that the host is an ultra-faint (M_ UV=-14.4±0.3 mag) Lyman-α emitter with a very high equivalent width. The host galaxy also shows very weak [O iii]4959,5007 lines despite an Hα line detection ([O iii]5007/Hβ<0.7 with case B recombination). Assuming that the weak [O iii] is due to low gas-phase metallicity given the low-metallicity of SN Eos itself, SN Eos plausibly marks the formation and explosion of a metal-poor star in an extremely metal-poor environment (<1 % Z_⊙), facilitating the initial stages of the chemical enrichment of the host. Finding the CCSN in such an ultra-faint galaxy at z=5.13 also indicates that the SN rate could be considerably higher in high-z, metal-poor environments, potentially implying e.g., a Z-dependent IMF, Z-dependent massive star explodability, or runaway stellar collisions in dense star clusters. Without lensing, only SN Eos would be detectable and the host would be below the detection limit in any NIRCam surveys ever performed. The Eos host galaxy can thus be representative of the origin of hostless supernovae frequently found in JWST blank field surveys.

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 272351 and 445525 (using ATMO2020 and Sonora Elf Owl models) and distances of ∼ 150650. 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 year later, we also detect their proper motions of (49 ± 8) and (24 ± 3), further indicating that at least some F277W and F356W dropouts are sub-stellar 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.

We report the discovery of a doubly imaged little red dot (LRD) candidate behind the galaxy cluster Abell 383, which we dub A383-LRD1. Initially classified as a dropout galaxy in HST imaging with several ground-based emission line detections placing it at zspec = 6.027, new JWST/NIRCam observations taken as part of the cycle 4 VENUS survey now reveal that the source consists of two underlying components. These consist of a red point-source with a V-shaped SED consistent with LRD selection criteria and a nearby (∼380 pc) compact blue companion, which was the main contributor to the previous rest-frame UV detections. Based on lensing symmetry and its SED, the LRD appears to lie at a similar redshift as well. The magnification of the two images of A383-LRD1 is μA = 16.2 ± 1.2 and μB = 9.0 ± 0.6, respectively, and the predicted time delay between them is Δtgrav = 5.20 ± 0.14 yr (∼0.7 yr in the rest-frame). After correcting for the lensing magnification, we derived an absolute magnitude of MUV, LRD = −16.8 ± 0.3 for the LRD and MUV, BC = −18.2 ± 0.2 for the blue companion. We performed SED fits to both components, revealing the LRD to show the best fit with a black hole star (BH*) model and a substantial host galaxy, along with the blue companion with an extremely young, emission-line dominated star-forming nebula. If it is verified spectroscopically in the future, A383-LRD1 would represent the second known multiply imaged LRD detected to date, following A2744-QSO1. To our knowledge, it would also stand as the first LRD system with a confirmed detection of [C II]λ158 μm emission from ALMA observations. Thanks to the lensing magnification, this system opens a unique door to studying the relation between a LRD, its host galaxy, and its environment. It represents a prime candidate for deep JWST spectroscopy and high-resolution ALMA follow-up observations.

We report the identification of a pair of faint little red dots (LRDs), dubbed Red Eyes, in a strongly-lensed galaxy at z∼7 behind the PLCKG004.5-10.5 cluster, identified from the JWST Treasury program VENUS. Red Eyes are spatially resolved on the image plane with distinct colors, while the critical curve lies far north of Red Eyes, clearly requiring two different LRDs rather than a single LRD. Red Eyes is an extremely close pair of LRDs separated by ∼70 pc in the source plane with a magnification of μ∼20, which consistently explains another counter-image detected to the north-west. Red Eyes is hosted in a typical star-forming galaxy with M_UV,int∼ -19, but its own UV emission is very faint (M_UV,int≳ -16). Moreover, Red Eyes does not reside at the galaxy center but lies at an offset position of approximately one effective radius R_e away from the galaxy center. If observed without lensing, Red Eyes would appear as a typical star-forming galaxy at z∼ 7 with M_UV∼ -19, showing no apparent LRD signatures in either morphology or SED. These results suggest that multiple off-center LRDs, similar to Red Eyes, may be commonly hidden in a typical high-z star-forming galaxy. In this case, various plausible scenarios may emerge, one of which is that intermediate-mass black holes (IMBHs) with M_BH∼10^4–6 M_⊙ may form in star clusters on a stellar disk and contribute to the growth of the central supermassive black hole via mergers, with some IMBHs detectable as luminous LRDs in a sufficiently active and massive phase.

The formation of the first stars and galaxies marked the onset of chemical enrichment, yet direct observations of such primordial systems remain elusive. Here we present James Webb Space Telescope spectroscopic observations of LAP1-B, an ultra-faint galaxy at redshift zspec = 6.625 ± 0.001, corresponding to a cosmic age of 800 million years after the Big Bang. This galaxy is strongly magnified by gravitational lensing. LAP1-B exhibits a gas-phase oxygen abundance of (4.2 ± 1.8) × 10-3 times the solar value, making it the most chemically primitive star-forming galaxy discovered to date. The galaxy displays an exceptionally hard ionizing radiation field, which is inconsistent with chemically enriched stellar populations or accreting black holes but matches theoretical predictions for an exceptionally metal-deficient stellar population1. It also shows an elevated carbon-to-oxygen abundance ratio for its metallicity in the interstellar medium, consistent with nucleosynthetic yields from a stellar population formed in the absence of initial metals2-4. The lack of detectable stellar continuum constrains the stellar mass to below 3,300 M⊙, and the dynamical mass, derived from emission-line kinematics, exceeds the combined stellar and gas mass, which indicates a dominant dark matter halo. Our findings establish LAP1-B as a 'fossil in the making', a direct high-redshift progenitor of the ancient ultra-faint dwarf galaxies observed in the local Universe and offers a rare window into the earliest stages of galaxy formation.

In this series of papers, we present dynamical models of cluster members in strong lensing (SL) galaxy clusters to independently probe the persistent discrepancy reported between SL models and cosmological simulations, in terms of total mass properties for the cluster subhalos. In this work, we focused our study on early-type galaxies within Abell 2744 (z=0.309) and MACS J0416.1-2403 (z=0.397). We took advantage of deep MUSE spectroscopic data, complemented with HFF photometry. We used a pipeline based on spectral fitting to perform kinematic measurements of the LOS velocity dispersion profiles of 109 cluster members. We modeled the galaxies assuming a dPIE total mass density distribution and a Jaffe stellar mass density distribution. From the models, we inferred the values of the central stellar velocity dispersion, σ_0, and the truncation radius, r_t, for the galaxies in our sample. We found that σ_0 is accurately recovered for all of the cluster members, while r_t is reliably measured for a fraction of galaxies in our sample, with sufficiently extended radial kinematic coverage. Our dynamical models predicted LOS velocity dispersion profiles that fit the measured ones better than those inferred from SL models. We then exploited the σ_0 measurements obtained from the dynamical models to calibrate the Faber-Jackson scaling relations for the cluster members in both galaxy clusters. When comparing our relations to those obtained in previous kinematics and SL works, we found systematically higher normalization and compatible slope and scatter values. We conclude that our dynamical measurements of σ_0 and r_t, along with calibrated scaling relations, are more robust than previous kinematic estimates which are biased by not taking into account the effects of the PSF, and should therefore be adopted as improved initial prescriptions in future SL models.

Observing supernovae (SNe) in the early Universe (z > 3) provides a window into how both galaxies and individual stars have evolved over cosmic time, yet a detailed study of high-redshift stars and SNe has remained difficult due to their extreme distances and cosmological redshifting. To overcome the former, searches for gravitationally lensed sources allow for the discovery of magnified SNe that appear as multiple images - further providing the opportunity for efficient follow-up. Here we present the discovery of "SN Eos": a strongly lensed, multiply-imaged, SN II at a spectroscopic redshift of z = 5.133 +/- 0.001. SN Eos exploded in a Lyman-α emitting galaxy when the Universe was only 1 billion years old, shortly after it reionized and became transparent to ultraviolet radiation. A year prior to our discovery in JWST data, archival HST imaging of SN Eos reveals rest-frame far ultraviolet ( 1,300Å) emission, indicative of shock breakout or interaction with circumstellar material in the first few (rest-frame) days after explosion. The JWST spectroscopy of SN Eos, now the farthest spectroscopically confirmed SN ever discovered, shows that SN Eos's progenitor star likely formed in a metal-poor environment (<= 0.1 Z_⊙), providing the first direct evidence of massive star formation in the metal-poor, early Universe. SN Eos would not have been detectable without the extreme lensing magnification of the system, highlighting the potential of such discoveries to eventually place constraints on the faint end of the cosmic star-formation rate density in the very early Universe.

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally lensed galaxy (mu = 17 - 21) at a redshift of 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is a typical compact and UV-faint (M-UV = -17.8) Lyman-alpha emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical H alpha, H beta, and [OIII]lambda lambda 4959, 5007 lines are spatially resolved with the high-spectral-resolution grating (G395H, R 2700), with large equivalent widths, EW(H beta+O III) greater than or similar to 1000 & Aring;, and elevated ionizing photon production efficiencies, log(xi(ion)/erg(-1)Hz) = 25.2-25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of R-eff = 3 - 8 pc in size and M similar to 2 & sdot; 105( - 5 )& sdot; 10(6) M-circle dot in mass. These clusters are characterized by steep UV slopes, beta UV less than or similar to -2.5, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-alpha emission peaking at similar to 100 km s(-1) from the systemic redshift. While the entire system is characterized by low metallicity, similar to 0.1 Z(circle dot), the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with Z less than or similar to 2% Z(circle dot), which is barely detected in NIRCam imaging; this region is displaced by > 200 pc from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

We present new JWST NIRSpec integral field unit (IFU) G395H/F290LP observations of a merging galaxy system at z = 7.88, part of A2744-z7p9OD, the most distant protocluster to date. The IFU cube reveals [O iii ] λ 5007 emissions in two previously known galaxies, ZD3 and ZD6, and a newly identified galaxy, ZD12, at z spec = 7.8762. One of the detected [O iii ]-emitting regions has a detection of the auroral [O iii ] λ 4363 line, allowing us to derive a direct metallicity of log ( O / H ) + 12 = 7.4 ± 0.2, while metallicities in other regions are measured using strong-line calibration methods. We find large deviations within the measured metallicity ( Δ log ( O / H ) ∼ 1 ), which suggests a fast chemical enrichment from intense star formation and merger-driven growth, as expected in early galaxies. Our analysis shows that metal-poor regions could easily be outshone by more enriched regions, posing a challenge for spectroscopic analysis based on integrated light (i.e., NIRSpec/MSA) against identifying metal-free star formation in the early Universe. NIRCam imaging reveals seven UV-bright clumps in ZD12, in a stellar mass range of log M * / M ⊙ ∼ 7.6 –8.9. Four of them are unresolved (≲100 pc) and intensely star-forming (>30 M ⊙ yr −1 kpc −2 ), likely contributing to the scatter in metallicity by producing an ideal environment for rapid chemical cycles. Lastly, we revisit the nature of the host protocluster by including new member galaxies identified here and in the literature, and obtain a local overdensity factor δ = 4 4 − 31 + 89 , a total halo mass M h = 5 . 8 − 0.3 + 0.2 × 1 0 11 M ⊙ , and a formal velocity dispersion 1100 ± 500 km s −1 .

With stunning clarity, JWST has revealed the Universe's first billion years. The scientific community is analyzing a wealth of JWST imaging and spectroscopic data from that era, and is in the process of rewriting the astronomy textbooks. Here, 1.5 years into the JWST science mission, we provide a snapshot of the great progress made towards understanding the initial chapters of our cosmic history. We highlight discoveries and breakthroughs, topics and issues that are not yet understood, and questions that will be addressed in the coming years, as JWST continues its revolutionary observations of the Early Universe. While this compendium is written by a small number of authors, invited to ISSI Bern in March 2024 as part of the 2024 ISSI Breakthrough Workshop, we acknowledge the work of a large community that is advancing our collective understanding of the evolution of the Early Universe.

We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at z similar to 2.33 in the field around the z similar to 6.3 quasar SDSS J1030+0524. Using JWST/NIRCam images, we identified it as a bright outlier in the F200W - F356W color versus F356W magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from He I lambda 10830 and Pa gamma line, both displaying a narrow and a broad (FWHM greater than or similar to 2000 km s(-1)) component. The He I line is affected by an absorption feature, tracing dense gas with He I column density in the 2(3)S level N(He I) similar to 0.5 - 1.2 x 10(14) cm(-2), depending on the location of the absorber, which is outflowing at a speed of Delta(v) = -830(-148)(+131) km s(-1). As observed in the majority of LRDs, BiRD does not exhibit any X-ray or radio emission down to 3.7 x 10(42) erg s(-1) and 3 x 10(39) erg s(-1), respectively. The black hole mass and the bolometric luminosity, both inferred from the Pa gamma broad component, amount to M-BH similar to 10(8) M-circle dot and L-bol similar to 3 x 10(45) erg s(-1), respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, namely, GN-28074 (nicknamed Rosetta Stone) at z similar to 2.26 and RUBIES-BLAGN-1 at z similar to 3.1. The blueshifted He I absorption detected for all three sources suggests that gas outflows could be common in LRDs. We derived a first estimate of the space density of LRDs at z similar to 2 - 3 based on JWST data, as a function of the bolometric luminosity and black hole mass. The space density Phi(L) = 4.0(-2.4)(+4.0) x 10(-6) Mpc(-3) dex(-1) is only a factor of similar to 2 - 3 lower than that of UV-selected quasars with comparable bolometric luminosity and redshift, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend has also been observed with respect to black hole masses. As suggested by recent theories, if LRDs can indeed serves as probes of the very first and rapid growth of black hole seeds, our finding suggests that the formation of black hole seeds continues to be efficient at least up to cosmic noon.

One of the fundamental questions of cosmology is the origin and mechanism(s) responsible for the reionization of the Universe beyond z ∼ 6. Many studies have focused on Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) observations of local ( z ∼ 0.3) galaxies emitting ionizing radiation (Lyman continuum, or LyC) for insight. However, LyC measurements can depend on chance alignment of optically thin channels with the observer. In addition, low signal in the spectra of these faint LyC emitters inhibits constraints on gas geometry and stellar populations. To circumvent these limitations, we analyze stacks of a consolidated sample of HST/COS observations of the LyC in 89 galaxies at z ∼ 0.3. From fitting of the continuum, we obtain information about the underlying stellar populations, emergent LyC, and neutral interstellar medium geometry. We find that most LyC nondetections are not leaking appreciable LyC ( f esc LyC < 1%), but also that exceptional cases point to spatial variations in the LyC escape fraction f esc LyC . Stellar populations younger than 3 Myr lead to an increase in ionizing feedback, which in turn increases the isotropy of LyC escape. Wolf–Rayet stars and 3–6 Myr populations appear to play little role in LyC escape. Mechanical feedback from supernovae in 8–10 Myr stellar populations is important for anisotropic gas distributions needed for LyC escape. While mechanical feedback is necessary for any LyC escape, high f esc LyC (>5%) also requires a confluence of young stars and ionizing feedback. A two-stage burst of star formation is critical to producing this optimal LyC escape scenario, and should be considered fundamental to identifying LyC emitters at the Epoch of Reionization.