First stars and dwarf galaxies

The first stars played a crucial role in the evolution of the primordial Universe since they represented the first sources of ionising photons, dust, and chemical elements heavier than helium, i.e. metals. The amount of ionizing photons, dust, and different chemical species produced by the first stars strongly depends on their mass and on the explosion energy of the very first supernovae (SNe). Thus, understanding the Initial Mass Function (IMF) of the first stars and their Energy Distribution Function (EDF) is a fundamental problem in Cosmology. Yet, these functions are very difficult to infer both from an ob- servational and a theoretical prospective, and they are still unknown. First stars, also know as Population III (Pop III) stars, are predicted to form at z ∼ 15 − 30, in so-called minihaloes with masses Mh ≈ 10^6 M⊙ and virial temperature Tvir ≤ 10^4K. Since they form out of gas with primordial composition, they are expected to be completely metal-free. These metal-free stars should produce key emission lines that might allow us to easily identify their host galaxies. However, the detection of individual Pop III stars in the high-redshift Universe poses a significant challenge, even with the employ of the newly launched James Webb Space Telescope. Thus, our current knowledge of Pop III stars arises from theoretical studies and indirect observations. Different theoretical studies and cosmological simulations agree that Pop III stars are likely more massive than present-day stars, with mass likely extending up to ∼ 1000M⊙. On the other hand, 3D simulations that study the cooling of primordial gas reveal that the proto-stellar gas clouds can experience strong fragmentation and Pop III stars could have masses lower than 1 M⊙. Indeed, if Pop III stars with masses below 0.8 M⊙ were able to form, they should be alive today, and they should dwell in the oldest and most metal-poor systems of the Local Group. Despite long searches, zero-metallicity stars have never been found among the ancient stars in our Milky Way, supporting the idea that first stars are more massive than present-day stars. If so, most of Pop III stars would evolve and explode as SNe, rapidly disappearing from the Universe in few Myrs. However, the chemical signatures of these pristine SNe could be retained in the photo- spheres of low-mass (Pop II) stars that formed from the ashes of massive Pop III stars. These low-mass long-lived stars imprinted by Pop III SNe, the so-called first stars “descendants”, can survive until today and they can be individually observed in the oldest and metal-poor environments of our Galaxy and its dwarf galaxies satellites. This is indeed the key idea behind “Stellar Archaeology”: to indirectly study the first stars by exploiting the stellar chemical abundances measured in the Local Group. Among all environments hosting ancient stars, Ultra-Faint Dwarf (UFD) galaxies (Lbol < 10^5L⊙) are the best places to look for Pop III star descendants. UFD galaxies are the most common dwarf galaxies in the Local Group, representing more than 50% of the total number of dwarf satellites. They are the oldest, most dark matter-dominated, most metal-poor, least luminous, and least chemically evolved stellar systems known. Most UFD galaxies formed more than 75% of their stars in the first Gyr of evolution and hence have old stellar populations, > 10 Gyr. Furthermore they contain the high- est fraction of extremely metal-poor stars, [Fe/H] < −3. Finally, these low-mass dwarf galaxies are predicted to be the building blocks of the Galactic stellar halo, and the first star-forming systems that hosted Pop III stars. The main goal of this Thesis is to unveil the properties of the first stars by exploit- ing the observed chemical properties of stars in Ultra-Faint Dwarf galaxies. To achieve this goal, we develop a novel theoretical model that follows the formation and chemical evolution of Boötes I, the best studied UFD galaxy. For the first time, we present a theoretical model that accounts for the incomplete sampling of the IMF for both Pop III stars and subsequent normal Pop II, and that is able to follow the chemical enrichment star-by-star and from different sources: SNe and AGB1 stars both from Pop III and Pop II stars. First, we investigate the frequency of long-living first star relics in UFDs to limit the minimum mass of the first stars. By comparing our model results with the current number of observed stars in Boötes I, and other UFDs, we can put limits on the shape and on the low-mass end of the Pop III IMF. Then, by studying the chemical signatures left by the Pop III SNe we determine the key chemical features to uniquely identify true first star descendants and we provide predictions to discover them. Finally, by exploring other ancient and metal-poor environments in our Milky Way, such as the Galactic halo and the bulge, we show how to further derive constraints on the masses and the nature of first SNe.