Periodic Reporting for period 4 - NEFERTITI (NEar FiEld cosmology: Re-Tracing Invisible TImes)
Okres sprawozdawczy: 2023-11-01 do 2024-10-31
The first stars profoundly affected the unfolding of our Universe. Their mass distribution is unknown, but it controls the injection of energy, momentum and newly created heavy elements (metals) into the gas, affecting subsequent star-formation and the build-up of the first galaxies. Despite extraordinary progress in theoretical modelling and observational techniques, very little is known about the properties of the first stars and of the first galaxies hosting them.
A direct exploration of their formation time is a tremendous challenge. This chapter of the cosmic history was written between 13 and 13.5 Gyr ago, an epoch that the James Webb Space Telescope (JWST) will explore. But even its superb sensitive eyes will be blind to faint dwarf galaxies: the nurseries of the first stars, and the building-blocks of present-day galaxies.
In the Local Group, ultra-faint dwarf galaxies (UFDs) represent the most common galaxy population. These small systems comprise stars that are over 13 Gyr old and that likely built-up the Galactic stellar halo. Such early Universe survivors can be observed individually to retrace the chemical evolution and star-formation history of the gas during those “invisible” times.
First stars of different masses produce unique nucleosynthetic products, which are dispersed into the gas and preserved in the photosphere of ancient second-generation stars. The chemical abundance pattern of second-generation stars can be measured with high-resolution spectroscopy and interpreted with models to uncover the properties of the first stars. Second-generation stars can be found in UFDs and in the Galactic halo but they are extremely rare. Yet, in our current epoch of wide and deep Local surveys the number of identified ultra-faint dwarf galaxies will constantly increase and that of Galactic halo stars will rise by orders of magnitude.
The project NEFERTITI aims at fully exploiting this unprecedented data flow to catch the local stellar fossils and figure out the properties of the first stars and galaxies. In particular, it is expected to make a major step-forward in our understanding of the properties of the first stars and galaxies by constraining the mass distribution of the first stars and uncovering the physical processes driving the build-up of the first star-forming systems, i.e. mini-haloes.
To this end, the NEFERTITI project adopts a novel approach that integrates theoretical and observational research in a unique way. This is reflected into the composition of the NEFERTITI Team, which is composed by a mixture of theoreticians and observers: the PI (Salvadori: theory), the long-term researcher (RTD-A, Skuladottir: obs), three post-docs (Koutsouridou: theory; Aguado. Luccheei: obs), and seven PhD students (Rossi, Gelli, Vanni, Rusta, Ciabattini, Querci, Mori) working on models/simulations to interpret observations and make predictions.
A direct exploration of their formation time is a tremendous challenge. This chapter of the cosmic history was written between 13 and 13.5 Gyr ago, an epoch that the James Webb Space Telescope (JWST) will explore. But even its superb sensitive eyes will be blind to faint dwarf galaxies: the nurseries of the first stars, and the building-blocks of present-day galaxies.
In the Local Group, ultra-faint dwarf galaxies (UFDs) represent the most common galaxy population. These small systems comprise stars that are over 13 Gyr old and that likely built-up the Galactic stellar halo. Such early Universe survivors can be observed individually to retrace the chemical evolution and star-formation history of the gas during those “invisible” times.
First stars of different masses produce unique nucleosynthetic products, which are dispersed into the gas and preserved in the photosphere of ancient second-generation stars. The chemical abundance pattern of second-generation stars can be measured with high-resolution spectroscopy and interpreted with models to uncover the properties of the first stars. Second-generation stars can be found in UFDs and in the Galactic halo but they are extremely rare. Yet, in our current epoch of wide and deep Local surveys the number of identified ultra-faint dwarf galaxies will constantly increase and that of Galactic halo stars will rise by orders of magnitude.
The project NEFERTITI aims at fully exploiting this unprecedented data flow to catch the local stellar fossils and figure out the properties of the first stars and galaxies. In particular, it is expected to make a major step-forward in our understanding of the properties of the first stars and galaxies by constraining the mass distribution of the first stars and uncovering the physical processes driving the build-up of the first star-forming systems, i.e. mini-haloes.
To this end, the NEFERTITI project adopts a novel approach that integrates theoretical and observational research in a unique way. This is reflected into the composition of the NEFERTITI Team, which is composed by a mixture of theoreticians and observers: the PI (Salvadori: theory), the long-term researcher (RTD-A, Skuladottir: obs), three post-docs (Koutsouridou: theory; Aguado. Luccheei: obs), and seven PhD students (Rossi, Gelli, Vanni, Rusta, Ciabattini, Querci, Mori) working on models/simulations to interpret observations and make predictions.
The existence of very massive metal-free stars, M* = (140-260) M, has never been proved. Yet, these elusive stars should exist according to state-of-the-art numerical simulation, and they are expected to be the key sources of early metal-enrichment, ionizing photons, and primordial stellar black holes. Probing the existence of these stars is thus fundamental not only for Cosmology but also for galaxy formation. A possible way to make this major step-forward is by catching the chemical signatures that these elusive stars left in their descendants, i.e. in long-lived low-mass stars formed out of their ``ashes”. Indeed, very massive first stars are predicted to end their life as energetic Pair Instability Supernovae (PISNe), which spread out in the surrounding Interstellar Medium (ISM) their peculiar chemical products.
By investigating the free parameter space of the problem, we studied how does the peculiar chemical abundance pattern of an ISM polluted by PISNe vary when subsequent generations of “normal” Pop II stars contaminate it. In fact, PopII stars are predicted to form early on after the first supernovae (SNe) explosions, and thus they can washed-out their key chemical signatures. Still, independent of the choice of the free parameters, we find that an ISM imprinted by the heavy elements from PISNe at a >50% level it is most likely deficient in Copper and Zinc. Further, these stars predominantly have [Fe/H] ~ 2 (Salvadori et al. 2019).
By further developing this model, we investigate the imprint of PopIII SNe with lower masses (10-100) Msun, which can explode with a variety of energy (Skuladottir, SS et al. 2021/23). With this simple and general tool, we interpret the properties of halo stars, showing that those with [C/Fe] > +2.5 are truly second-generation objects, solely imprinted by low-energy PopIII SNe. These results were confirmed by our chemical-evolution model for ultra-faint dwarf galaxies (Rossi, SS et al. 2021/23) and for the Milky Way assembly (Koutsouridou, SS et al. 2023). Still, the degeneracy between the mass distribution of PopIII stars and the energy distribution of the first SNe make C-enhanced stars not useful (Koutsouridou, SS et al. 2023). To overcome this issue and break the degeneracy, we need to find the descendants of very massive first stars exploding as PISNe (Koutsouridou, SS, Skuladottir 2024). But where are them?
Our predictions show that the descendants of PISNe should be predominantly found in the bulge (Pagnini, SS et al. 2023), or in relatively massive ancient dwarf galaxy, such as Fornax, which can retain the chemical products of such energetic explosions (Rossi, SS et al. submitted). The ESO/4MOST large observational program 4DWARFS (512 000 fibre hours) was awarded in 2021 to our Team (PI: Skuladottir, 33 co-Is including SS, Gelli, Rossi) and it will allow us to search for these objects by exploiting the tools developed by the NEFERTITI project: the PISN-explorer to pin-point PISN descendants in large stellar surveys (Aguado, SS et al. 2023) and the NEFERTITI model, which can drive observations towards the best Local Group environments (Koutsouridou, SS et al. 2023). Alternatively, we can look for the chemical signatures of massive PISNe in distant (z > 3) and diffuse gas clouds, where the PISN fingerprints can be identified exploiting our novel chemical diagnostics (Vanni, SS et al. 2024). Indeed, the signature of PopIII SNe can be found in diffuse gaseous absorbers, which can preserve their signature because they do not easily form stars (Saccardi, SS et al. 2023).
By investigating the free parameter space of the problem, we studied how does the peculiar chemical abundance pattern of an ISM polluted by PISNe vary when subsequent generations of “normal” Pop II stars contaminate it. In fact, PopII stars are predicted to form early on after the first supernovae (SNe) explosions, and thus they can washed-out their key chemical signatures. Still, independent of the choice of the free parameters, we find that an ISM imprinted by the heavy elements from PISNe at a >50% level it is most likely deficient in Copper and Zinc. Further, these stars predominantly have [Fe/H] ~ 2 (Salvadori et al. 2019).
By further developing this model, we investigate the imprint of PopIII SNe with lower masses (10-100) Msun, which can explode with a variety of energy (Skuladottir, SS et al. 2021/23). With this simple and general tool, we interpret the properties of halo stars, showing that those with [C/Fe] > +2.5 are truly second-generation objects, solely imprinted by low-energy PopIII SNe. These results were confirmed by our chemical-evolution model for ultra-faint dwarf galaxies (Rossi, SS et al. 2021/23) and for the Milky Way assembly (Koutsouridou, SS et al. 2023). Still, the degeneracy between the mass distribution of PopIII stars and the energy distribution of the first SNe make C-enhanced stars not useful (Koutsouridou, SS et al. 2023). To overcome this issue and break the degeneracy, we need to find the descendants of very massive first stars exploding as PISNe (Koutsouridou, SS, Skuladottir 2024). But where are them?
Our predictions show that the descendants of PISNe should be predominantly found in the bulge (Pagnini, SS et al. 2023), or in relatively massive ancient dwarf galaxy, such as Fornax, which can retain the chemical products of such energetic explosions (Rossi, SS et al. submitted). The ESO/4MOST large observational program 4DWARFS (512 000 fibre hours) was awarded in 2021 to our Team (PI: Skuladottir, 33 co-Is including SS, Gelli, Rossi) and it will allow us to search for these objects by exploiting the tools developed by the NEFERTITI project: the PISN-explorer to pin-point PISN descendants in large stellar surveys (Aguado, SS et al. 2023) and the NEFERTITI model, which can drive observations towards the best Local Group environments (Koutsouridou, SS et al. 2023). Alternatively, we can look for the chemical signatures of massive PISNe in distant (z > 3) and diffuse gas clouds, where the PISN fingerprints can be identified exploiting our novel chemical diagnostics (Vanni, SS et al. 2024). Indeed, the signature of PopIII SNe can be found in diffuse gaseous absorbers, which can preserve their signature because they do not easily form stars (Saccardi, SS et al. 2023).
The most important progress beyond the state-of-the-art of the NEFERTITI project are summarized below: (1) for the first time we manage to limit strongly the IMF of PopIII stars, on the lower mass-end (Rossi, SS et al. 2021), on the characteristic mass (Pagnini, SS et al. 2023) and on the shape (Koutsouridou, SS, Skuladottir 2024); (2) by exploiting the synergy between theory and observations we pinpointed, for the very first time, the chemical signature of the first stars in high-z diffuse gaseous absorbers (Saccardi, SS et al. 2023) and we developed novel chemical diagnostics to uncover more of them with the upcoming surveys and facilities (Vanni, SS et al. 2024); (3) by using our NEFERTITI data-calibrated model for the Milky Way (MW) formation, we demonstrated that Firefly sparkle system, which has been recently observed by the JWST is indeed a MW-analogue at z ~ 8, thus establishing, for the very first time, a link between the near and the more distant Universe.