The early universe, specifically the epoch of reionization and the period shortly thereafter known as Cosmic Dawn, remains a region of intense study in astrophysics. Understanding the formation and evolution of the first galaxies during this era is crucial to our comprehension of the universe's overall development. Initial observations from the James Webb Space Telescope (JWST) have yielded a wealth of data, revealing numerous candidate galaxies at exceptionally high redshifts (z > 10 and even z > 12), hinting at a much faster and more prolific formation of luminous galaxies than many theoretical models had predicted. These observations challenge the established paradigm and necessitate further investigation to reconcile the observed data with our existing understanding of galaxy formation and evolution. The detection of these high-redshift galaxies, however, is largely based on photometric redshift estimations, which are inherently prone to uncertainties and potential contamination from foreground objects. Spectroscopic confirmation, which directly measures the redshift through the identification of spectral lines, is essential to definitively verify the distance and properties of these distant galaxies. This research directly addresses this need by focusing on spectroscopic confirmation of high-redshift galaxy candidates to provide robust evidence for the existence and characteristics of luminous galaxies in the very early universe, thereby refining and challenging the current theoretical models of galaxy formation and evolution.

Previous studies using the JWST, primarily relying on photometric redshifts, identified numerous candidate galaxies at redshifts z > 10 and even exceeding z = 12. These studies, while exciting, lacked the definitive confirmation that only spectroscopy can provide. While some spectroscopic confirmations of galaxies at z > 12 existed prior to this research, they were limited in number. The high luminosity of several of these photometrically identified galaxies, combined with their high redshift, indicated a potential discrepancy with standard galaxy formation models, which generally predict a lower density of luminous galaxies at such early times. The discrepancy highlighted the need for spectroscopic confirmation of these high redshift candidates to understand the properties of these galaxies and their implications for our understanding of galaxy formation. The existing literature provided strong motivation for this study, highlighting both the promising potential of finding high-redshift galaxies and the need for robust spectroscopic confirmation to validate their existence and properties.

This research utilized data from the JWST Advanced Deep Extragalactic Survey (JADES) campaigns. Three candidate galaxies at z > 14 were initially selected based on photometric data from JWST's NIRCam and MIRI instruments. These candidates were further analyzed using JWST's Near-Infrared Spectrograph (NIRSpec) in multi-object spectroscopic mode. NIRSpec's low-resolution prism and medium-resolution gratings were employed to probe the wavelength range 0.6–5.2 μm, with spectral resolving powers of R ≈ 100 and R ≈ 1,000, respectively. Due to the low luminosity of one candidate, the focus shifted to two brighter candidates, JADES-GS-z14-0 and JADES-GS-z14-1. The spectra were analyzed to identify the presence of Lyman-α breaks, a characteristic feature used to determine redshift. The presence of a sharp break in the flux density, indicative of a Lyman-α break, was observed in both galaxies, initially suggesting a redshift of z ≈ 14. To refine the redshift estimates and mitigate uncertainties arising from potential Lyman-α absorption by neutral hydrogen, the rest-frame UV continuum emission was parameterized using a power-law model, accounting for physical processes that influence the Lyman-break profile. This detailed spectral fitting led to more precise redshift determinations: z = 14.32⁺⁰·⁰⁸ for JADES-GS-z14-0 and z = 13.90 ± 0.17 for JADES-GS-z14-1. Further analysis included the examination of the spectral energy distributions (SEDs) within a Bayesian framework to infer physical properties such as stellar mass, star formation rate, and dust attenuation. NIRCam imaging was used to assess the size and morphology of these galaxies, providing further insights into their nature and formation mechanisms. A nearby lower redshift interloper near JADES-GS-z14-0 was also spectroscopically analyzed and confirmed (z=3.475), further supporting the high redshift of JADES-GS-z14-0 and allowing for correction of any gravitational lensing effects.

The most significant finding is the spectroscopic confirmation of two luminous galaxies at exceptionally high redshifts (z ≈ 14). These represent some of the earliest galaxies ever spectroscopically confirmed, pushing back the known limits of galaxy formation. The most distant galaxy, JADES-GS-z14-0, exhibits a surprisingly high luminosity, surpassing many previously known galaxies at higher redshifts, including GN-z11 and GHZ2, by a significant margin. JADES-GS-z14-0 is the third most UV luminous of the 700 z > 8 candidates in JADES. The galaxy’s high luminosity is particularly noteworthy given the rapid evolution of the halo mass function expected in cold dark matter cosmology. Analysis of this object suggests deviation from the predicted dimensional scaling between halo mass and luminosity, indicating unforeseen astrophysical processes are at play. Both galaxies displayed prominent Lyman-α breaks in their spectra, but notably lacked significant emission lines, suggesting that their ultraviolet light is primarily produced by stellar continuum emission rather than active galactic nuclei (AGNs). The absence of emission lines, combined with their steep ultraviolet slopes, further supports the predominance of stellar emission. Detailed SED modelling suggests that both galaxies possess relatively young stellar populations (less than 300 Myr old) and experience moderate dust attenuation. The analysis of JADES-GS-z14-0's NIRCam image revealed an extended morphology, with a deconvolved half-light radius of 260 ± 20 pc, a significantly larger size than that of many high-luminosity galaxies at similar redshifts. In contrast, JADES-GS-z14-1 shows a compact morphology. The inferred properties of both galaxies suggest high star-formation rates and high escape fractions of ionizing photons, signifying important roles in the reionization of the universe. A tentative detection of CIII] emission in JADES-GS-z14-0 (at 3.6σ significance) provides further support for the high-redshift determination. The number density of these luminous galaxies at z≈14 is more than ten times higher than pre-JWST extrapolations predicted.

The findings presented in this study significantly impact our understanding of galaxy formation in the early universe. The spectroscopic confirmation of two luminous galaxies at z ≈ 14 strongly supports the accumulating evidence that bright galaxies formed much earlier and more abundantly than previously thought. The unexpectedly high luminosity and extended morphology of JADES-GS-z14-0 challenge existing models of galaxy formation, indicating that either the star formation efficiency was higher in these early galaxies than expected or the initial mass function was top-heavy. The absence of prominent emission lines and the observed UV slopes in both galaxies strongly suggest that their luminosity is largely attributable to a young stellar population, rather than AGN activity. This discovery necessitates a re-evaluation of theoretical models, pushing for refinements to account for the observed prevalence of bright, massive galaxies at such early epochs. The observed moderate dust attenuation suggests that dust enrichment mechanisms are also different at these early times, and models must incorporate this aspect for better agreement with the observations. Future research should focus on characterizing other galaxies with similar properties and using high-resolution spectroscopic observations to understand the details of their stellar populations and chemical enrichment history. Further investigation into the escape fraction of ionizing photons will shed light on the role these galaxies played in the reionization of the universe.

This paper presents the first spectroscopic confirmation of luminous galaxies at z ≈ 14, significantly advancing our understanding of galaxy formation in the early universe. The unexpected luminosity and size of JADES-GS-z14-0 challenge existing models and highlight the need for further refinement. The data suggest that the early universe hosted a larger population of luminous galaxies than previously anticipated. Future observations with facilities like ALMA and JWST's MIRI are essential to further probe these galaxies’ properties and refine our models of early galaxy formation and evolution.

While this study provides strong spectroscopic confirmation of two high-redshift galaxies, some limitations should be acknowledged. The reliance on the Lyman-α break for redshift determination, while common, introduces some uncertainty due to potential intergalactic absorption. The tentative detection of CIII] emission in JADES-GS-z14-0 requires further confirmation. Finally, the sample size of confirmed galaxies is still limited. Further observations are needed to obtain a more statistically significant sample of high redshift galaxies to better constrain the galaxy luminosity function at this epoch.