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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.

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.

In this thesis we have exploited the power of gravitational lensing of massive clusters to probe galaxy evolution with the galaxy stellar mass functions (GSMF) and UV luminosity functions (UV LF) at z=6-9. Throughout the thesis the data utilized is from the Hubble Frontier Fields (HFF) program. We present new measurements of the evolution of the GSMF and UV LF for galaxies from z=6-9 within the HFF cluster MACSJ0416.1-2403 and its parallel field. To obtain these results, we have developed a novel method to subtract the massive foreground galaxies that lie close to the critical line from the MACSJ0416.1-2403 cluster, allowing for a deeper and cleaner detection of the faintest systems at z\\geq6. We derive the stellar masses of our sample by fitting synthetic stellar population models to their observed spectral energy distribution (SED) with the inclusion of nebular emission lines. This is the deepest and most distant mass function measured to date. We also investigate, for the first time, the correlation between β and SFR and find that there is a strong correlation between β and SFR, in that galaxies with low SFRs exhibit bluer slopes, and they also appear to get bluer with increasing redshift.

The Hubble Space Telescope (HST) archives constitute a rich dataset of high resolution images to mine for strong gravitational lenses. While many HST programs specifically target strong lenses, they can also be present by coincidence in other HST observations. We aim to identify non-targeted strong gravitational lenses in almost two decades of images from the ESA it Hubble Space Telescope archive (eHST), without any prior selection on the lens properties. We used crowdsourcing on the Hubble Asteroid Hunter (HAH) citizen science project to identify strong lenses, alongside asteroid trails, in publicly available large field-of-view HST images. We visually inspected 2354 objects tagged by citizen scientists as strong lenses to clean the sample and identify the genuine lenses. We report the detection of 252 strong gravitational lens candidates, which were not the primary targets of the HST observations. 198 of them are new, not previously reported by other studies, consisting of 45 A grades, 74 B grades and 79 C grades. The majority are galaxy-galaxy configurations. The newly detected lenses are, on average, 1.3 magnitudes fainter than previous HST searches. This sample of strong lenses with high resolution HST imaging is ideal to follow-up with spectroscopy, for lens modelling and scientific analyses. This paper presents an unbiased search of lenses, which enabled us to find a high variety of lens configurations, including exotic lenses. We demonstrate the power of crowdsourcing in visually identifying strong lenses and the benefits of exploring large archival datasets. This study shows the potential of using crowdsourcing in combination with artificial intelligence for the detection and validation of strong lenses in future large-scale surveys such as ESA's future mission Euclid or in JWST archival images.

We present new measurements of the UV spectral slope \\(\\beta\\) for galaxies at \\(z=6-9\\) in the Frontier Fields cluster MACSJ0416.1-2403 and its parallel field, to an unprecedented level of low stellar mass. We fit synthetic stellar population models to the observed spectral energy distribution and calculate \\(\\beta\\) by fitting a power law to the best-fit spectrum. With this method, we report the derivation of rest-frame UV colours of galaxies for the Frontier Fields program extending out to \\(z=9\\), probing magnitudes as faint as \\(M\\mathrm{_{UV}=-13.5}\\) at \\(z=6\\). We find no significant correlation between \\(\\beta\\) and rest-frame UV magnitude \\(M_{1500}\\) all redshifts, but we do find a strong correlation between \\(\\beta\\) and stellar mass with lower mass galaxies exhibiting bluer UV slopes. At \\(z=7\\) the bluest median value of our sample is redder than previously reported values in the literature, whereas at \\(z=9\\) our bluest data point has a median value of \\(\\beta=-2.63_{-0.43}^{+0.52}\\). Thus, we find no evidence for extreme stellar populations at \\(z>6\\). We also observe a strong correlation between \\(\\beta\\) and SFR, such that galaxies with low SFRs exhibit bluer slopes. Additionally, there exists a star formation main sequence up to \\(z = 9\\) with SFRs correlating with stellar mass. All of these relations show that \\(\\beta\\) values correlate with a process that drives both the overall SFR and stellar mass assembly. Furthermore, we observe no trend between \\(\\beta\\) and specific SFR, suggesting that \\(\\beta\\) is getting set by a global process driven by the scale of the galaxy.

One of the more surprising findings after the first year of JWST observations is the large number of spatially extended galaxies (ultra-red flattened objects, or UFOs) among the optically-faint galaxy population otherwise thought to be compact. Leveraging the depth and survey area of the JADES survey, we extend observations of the optically-faint galaxy population to an additional 112 objects, 56 of which are well-resolved in F444W with effective sizes, \\(R_e > 0.25''\\), more than tripling previous UFO counts. These galaxies have redshifts around \\(2 < z < 4\\), high stellar masses (\\(\\mathrm{log(M_*/M_{\\odot})} \\sim 10-11\\)), and star-formation rates around \\(\\sim 100-1000 \\mathrm{M_{\\odot}/yr}\\). Surprisingly, UFOs are red across their entire extents which spatially resolved analysis of their stellar populations shows is due to large values of dust attenuation (typically \\(A_V > 2\\) mag even at large radii). Morphologically, the majority of our UFO sample tends to have low Sérsic indices (\\(n \\sim 1\\)) suggesting these large, massive, optically faint galaxies have little contribution from a bulge in F444W. Further, a majority have axis-ratios between \\(0.2 < q < 0.4\\), which Bayesian modeling suggests that their intrinsic shapes are consistent with being a mixture of inclined disks and prolate objects with little to no contribution from spheroids. While kinematic constraints will be needed to determine the true intrinsic shapes of UFOs, it is clear that an unexpected population of large, disky or prolate objects contributes significantly to the population of optically faint galaxies.

We present a spatially resolved study of stellar populations in 6 galaxies with stellar masses \\(M_*\\sim10^{10}M_\\odot\\) at \\(z\\sim3.7\\) using 14-filter JWST/NIRCam imaging from the JADES and JEMS surveys. The 6 galaxies are visually selected to have clumpy substructures with distinct colors over rest-frame \\(3600-4100Å\\), including a red, dominant stellar core that is close to their stellar-light centroids. With 23-filter photometry from HST to JWST, we measure the stellar-population properties of individual structural components via SED fitting using Prospector. We find that the central stellar cores are \\(\\gtrsim2\\) times more massive than the Toomre mass, indicating they may not form via single in-situ fragmentation. The stellar cores have stellar ages of \\(0.4-0.7\\) Gyr that are similar to the timescale of clump inward migration due to dynamical friction, suggesting that they likely instead formed through the coalescence of giant stellar clumps. While they have not yet quenched, the 6 galaxies are below the star-forming main sequence by \\(0.2-0.7\\) dex. Within each galaxy, we find that the specific star formation rate is lower in the central stellar core, and the stellar-mass surface density of the core is already similar to quenched galaxies of the same masses and redshifts. Meanwhile, the stellar ages of the cores are either comparable to or younger than the extended, smooth parts of the galaxies. Our findings are consistent with model predictions of the gas-rich compaction scenario for the buildup of galaxies' central regions at high redshifts. We are likely witnessing the coeval formation of dense central cores, along with the onset of galaxy-wide quenching at \\(z>3\\).

We analyse the photometric and spectroscopic properties of four galaxies in the epoch of reionisation (EoR) within the SMACS 0723 JWST Early Release Observations field. Given the known spectroscopic redshifts of these sources, we investigated the accuracy with which photometric redshifts can be derived using NIRCam photometry alone, finding that F115W imaging is essential to distinguish between z~8 galaxies with high equivalent width (EW) [O III] {\\lambda}5007 emission and z~10 Balmer break galaxies. We find that all four sources exhibit strong (\\geq 0.6 mag) F356W-F444W colours, which sit at the extreme end of theoretical predictions from numerical simulations. We find that these galaxies deviate (by roughly 0.5 dex) from the local correlation between [O III] {\\lambda}5007/H\\beta and [Ne III] {\\lambda}3869/[O II], which is consistent with the predictions from simulations of high-redshift galaxies having elevated line excitation ratios. We measure the [O III] {\\lambda}5007 rest-frame equivalent widths both directly from the spectroscopy, and indirectly as inferred from the strong F356W-F444W colours, finding large [O III] {\\lambda}5007 EWs of 225-1740 Å. The [O III] {\\lambda}5007 and H\\beta EWs are consistent with those seen in extreme, intensely star-forming dwarf galaxies in the local Universe. Our structural analysis indicates that these galaxies are resolved, exhibiting irregular shapes with bright clumps. In line with the predictions from the FLARES hydrodynamic simulations, such intense star formation and extreme nebular conditions are likely the norm, rather than the exception, in the EoR.

We study the ionising photon production efficiency at the end of the Epoch of Reionisation (\\(z \\sim 5.4 - 6.6\\)) for a sample of 30 Lyman-\\(\\alpha\\) emitters. This is a crucial quantity to infer the ionising photon budget of the Universe. These objects were selected to have reliable spectroscopic redshifts, assigned based on the profile of their Lyman-\\(\\alpha\\) emission line, detected in the MUSE deep fields. We exploit medium-band observations from the JWST extragalactic medium band survey (JEMS) to find the flux excess corresponding to the redshifted \\ha\\ emission line. We estimate the ultra-violet (UV) luminosity by fitting the full JEMS photometry, along with several HST photometric points, with \\texttt{Prospector}. We find a median UV continuum slope of \\(\\beta = -2.09^{+0.23}_{-0.21}\\) for the sample, indicating young stellar populations with little-to-no dust attenuation. Supported by this, we derive \\(\\xi_{ion,0}\\) with no dust attenuation and find a median value of log\\(\\frac{\\xi_{ion,0}}{\\text{Hz erg}^{-1}} = 25.44^{+0.21}_{-0.15}\\). If we perform dust attenuation corrections and assume a Calzetti attenuation law, our values are lowered by \\(\\sim 0.1\\) dex. Our results suggest Lyman-\\(\\alpha\\) emitters at the Epoch of Reionisation have enhanced \\(\\xi_{ion,0}\\) compared to previous estimations from literature, in particular, when compared to the non-Lyman-\\(\\alpha\\) emitting population. This initial study provides a promising outlook on the characterisation of ionising photon production in the early Universe. In the future, a more extensive study will be performed on the entire dataset provided by the JWST Advanced Deep Extragalactic Survey (JADES). Thus, for the first time, allowing us to place constraints on the wider galaxy populations driving reionisation.