Probing the epoch of reionization with JWST spectroscopy

The first three years of James Webb Space Telescope (JWST) spectroscopy have revealed a substantial population of galaxies at high-redshift, previously inaccessible to both ground-based and spaced-based telescopes. For the first time, JWST enables observations deep into the cosmic dawn, revealing the earliest sources responsible for reionizing the intergalactic medium. Among the most striking discoveries are two unresolved frontier problems: the unexpectedly high abundance of bright galaxies at z > 9, and the early emergence of supermassive black holes (SMBHs) already in place by these epochs, showing accelerated evolution relative to local scaling relations. JWST has not only delivered the most distant spectroscopically confirmed star-forming galaxies and active galactic nuclei (AGN), but has also provided statistically significant samples of galaxies throughout the Epoch of Reionization (EoR), enabling a time-resolved characterization of this critical phase in cosmic history. In this context of exciting advancements in the galaxy evolution field, this thesis presents observational constraints on early galaxy evolution and reionization derived from JWST NIRSpec spectroscopy. We first discuss the spectroscopic characterization of eight galaxies at z > 9.5 in the Abell-2744 field, confirming the photometrically inferred overdensity originally identified by the GLASS team. To date, this field remains the most overdense region at z = 10, with a number density of spectroscopically confirmed galaxies approximately three times higher than the average JWST photometric UV luminosity function derivations. Within this sample, we identify and characterize two extreme UV line emitters: GHZ9 at z = 10.145, the most distant X-ray detected galaxy and the second most distant AGN known, and GHZ2 at z = 12.34, the most distant AGN candidate identified to date. Both galaxies exhibit nitrogen-enriched emission line spectra, suggesting chemically evolved environments in the early Universe. While these sources represent the most remote analogs of the first galaxies that likely started the cosmic reionization process, in the second part of this thesis, we analyze a large statistical sample of JWST-observed Lya-break galaxies in the EGS and UDS fields to study the evolution of Lya visibility across 4 < z < 11. This analysis allows us to place new constraints on the volume-averaged neutral hydrogen fraction in the two fields during and after reionization. Finally, we caution against direct comparisons between ground-based and JWST-based Lya samples due to the impact of Lya slit losses in the NIRSpec multi object spectroscopy (MOS) configuration. We quantify this observational bias using both empirical measurements and radiation-hydrodynamic simulations, demonstrating that it can severely suppress Lya fluxes, with implications for current and future reionization studies using JWST MOS spectroscopy.