A needle in a haystack? Catching Population 3. stars during the epoch of reionization

The advent of JWST has revolutionised our understanding of the early Universe, raising unexpected and intriguing challenges for our current theoretical models. Among the most striking discoveries is the apparent overabundance of UV-bright and red, massive galaxies at high redshifts, suggesting an earlier assembly of large structures than previously anticipated; another challenge lies in the interpretation of the ionisation levels of extreme-emission-line galaxies. Substantial uncertainties still surround the physics governing the formation and evolution of the first galaxies, which may account for these apparent conundrums. One possible explanation involves a significant contribution from high-mass stars, whose formation is thought to be favored in low-metallicity environments; in fact, as we move to higher redshifts, the average metallicity of the Universe decreases, reflecting the fact that only H, He, and traces of Li were produced during the Big Bang Nucleosynthesis: heavier elements are produced later as a byproduct of stellar evolution. However, even the oldest stars that we observe already exhibit traces of metals in their atmospheres, pointing to the existence of a hidden generation of stars that must have formed from metal-free gas at earlier epochs. These first stars, known as Population III (Pop III) stars, might be the missing piece in this cosmic puzzle. Pop III stars are expected to start forming from the gravitational collapse of pristine clouds in mini-haloes (with masses ∼ 105 − 106 M⊙) at redshifts z ∼ 20 − 30 (the so-called “Cosmic Dawn”). Since cooling in these environments is predominantly sustained by H2 molecules – which are much less efficient cooling agents than metals –, Pop III stars are thought to form with a much lower star formation efficiency compared to present-day stars, and with a top-heavy mass spectrum that might extend to ∼ 100s or even ∼ 1000s M⊙. These massive stars produce copious amounts of ionising radiation and heavy elements, initiating two major processes that profoundly affect the subsequent evolution of the Universe: cosmic metal enrichment, and cosmic reionisation (causing H in the inter-galactic medium to be nearly completely ionised by redshift z ∼ 6). While a direct observation of the first stars at Cosmic Dawn is probably out of reach, a complementary probe may be encoded in the 21-cm signature of neutral H. Another tool right in our galactic backyard is cosmic archaeology: it consists in studying the atmospheres of extremely old, metal-poor stars and matching their abundance patterns to supernova explosions, to infer the properties of their Pop III progenitors. However, a third, intermediate approach may also be feasible, i.e. looking for Pop III star formation in and around massive galaxies during the Epoch of Reionization (EoR, z ∼ 6 − 10). In fact, despite the many uncertainties surrounding the physics of very-metalpoor star formation and feedback, simulations consistently predict a late Pop III star formation even into the EoR, as the in-homogeneous nature of metal enrichment allows the persistence of pristine star-forming gas pockets at late times. Most of these studies focused on relatively small simulated volumes (at best ∼ 10h −1 cMpc per side), while the primary goal of this thesis is to investigate this phenomenon in a full cosmological context. To achieve this, I employ a suite of eight cosmological simulations with the hydrodynamical code dustyGadget, which is the largest available set of simulations (∼ 50h −1 cMpc per side) including a model for Pop III star formation and feedback. I will first provide an introduction to the theory of galaxy formation and evolution within the ΛCDM cosmological model, and discuss the new observational window into the high-z Universe opened by JWST (Chapter 1). This will be followed by a comprehensive review of the theory of Pop III star formation, the transition to second-generation stars, and the observational diagnostics at both low and high redshifts (Chapter 2). I will then describe the numerical methods used to model galaxy formation in a cosmological context, with a special emphasis on the dustyGadget simulation suite (Chapter 3). Finally, I will present the results of the simulations, focusing on the statistics of Pop III star formation down to z ∼ 6 (Chapter 4), and on the detectability of Pop III stars during the EoR, either through pair-instability supernovae (Chapter 5) or through the HeIIλ1640 recombination line (Chapter 6).