The lowest metallicity stars are also the oldest ones and they carry the imprint of the first supernovae. Since the very first stars are likely short-lived and inaccessible to us, the lowest metallicity stars are also those that can inform us most on the first generation of stars, how they enriched their environment, and produced the first structures that, through hierarchical formation, built up the primordial constituents of the galaxies we observe today. These stars are exceedingly rare and few of them are known today. In order to efficiently improve on this situation, we have put in place the Pristine international collaboration. Focussed around a wide narrow-band photometric survey conducted at the Canada-France-Hawaii Telescope and a large dedicated spectroscopic campaign, Pristine is many times more efficient than previous attempts at finding the precious low-metallicity stars. From the detailed study of these stars, we will: hunt for the most extreme low-metallicity stars; place constraints on star formation during the earliest epoch of the universe; reliably unveil the properties of the faintest known dwarf galaxies that orbit the Milky Way and are promising cosmological probes; decompose the Milky Way into its main components to place our Galaxy in the global context of galaxy formation and evolution; deconstruct the stellar halo of the Milky Way into its constituent substructures, which will then be used to constrain the mass and shape of the Milky's Way potential. In order to reliably achieve these significant goals, we request funding to support Pristine in France and ensure the project is staffed adequately to yield high-impact scientific returns in the field of very low metallicity stars. Our team is built around experts of the field in Nice (OCA/Lagrange), Paris (GEPI), and Strasbourg (Observatoire astronomique de Strasbourg) and represents a large part of the full Pristine collaboration. To complement this team, we ask for funding for a PhD student and two postdoctoral researchers who will be spread over the three French nodes of our project, along with funds to support the team and secure the visibility and active partnership of the French team members within the full international Pristine collaboration. We wish to emphasize that the effort already invested by the current team members into preparing the survey (successful telescope time proposals, data acquisition, reduction, and calibration, start of the spectroscopic follow-up campaign) means that the Pristine project is a low risk but high return project if it were to be supported by the ANR. It builds on large facilities and surveys with a significant French involvement (CFHT, Gaia, WEAVE) and it promises numerous high impact papers to be published in the high-visibility fields of Galactic archaeology and near-field cosmology in which France is a world leader.

The study of galaxy formation and evolution is one of the major topics of current extragalactic astrophysics. In the last few years, amazing progress has been made regarding the observational study of galaxy properties up to the highest redshift. JWST is making an impressive leap forward in the exploration of the hot, ionized gas of high-redshift galaxies. However, their neutral gas remains elusive despite it being one of the primary components of such high-redshift galaxies and of their circumgalactic medium. This neutral gas can be detected through its absorption of the light of a bright background source. Long Gamma-ray bursts (LGRBs) are a powerful tool to unveil the properties of the neutral interstellar medium (ISM) of their host galaxies, thanks to the detection of gas absorption lines in the optical/near-infrared spectra of their bright afterglows, independently of the brightness of the host galaxies. Even in the epoch of the ELT and JWST, LGRBs are a unique tool to probe in detail the properties of the neutral gas (such as hydrogen and metal content, kinematics) and dust of faint galaxies, that dominate in number the population of high-redshift galaxies. Furthermore, LGRBs are transients and, once their afterglows disappear, it is possible to detect the emission of the host galaxy's ionized gas as well. To date, the limit on the use of LGRBs for such studies is the poor number of optical/near-infrared spectroscopic observations of LGRB afterglows and their host galaxies at z>2-3. Within this project, we will significantly increase this statistic thanks to the Sino-French space mission SVOM for GRB detection, and to its ground-based follow-up. We will perform and use spectroscopic observations of the afterglows of LGRBs detected by SVOM to explore the properties of faint high-redshift star-forming galaxies with the goal of answering the following questions: - What are the properties of the neutral ISM gas? How do these properties evolve? - What are the properties of dust? How do they evolve? - How is star-formation connected to such properties? - How do faint star-forming galaxies at high redshift impact the reionization? Our project is focused on three Tasks: A) the study of the abundances, metallicity and kinematics of the neutral ISM gas; B) the study of dust in high redshift galaxies; C) the combination of the properties of the cold/warm absorbing gas with those of the emitting ionized gas of LGRB host galaxies. We will exploit our observational results also in synergy with cosmological simulations of galaxies. Our project will generate, as a by-product, the redshift determination of SVOM-detected GRBs. This is a fundamental information including for the study of the GRB phenomenon itself. Our results are crucial for the scientific exploitation and impact of the SVOM mission which represents an important effort of the French astrophysical community.

LOFAR (the European LOw Frequency ARray) is the first of the new-generation radiotelescopes of the 21st century, that will culminate with SKA (Square Kilometer Array) after 2020. NenuFAR is a giant extension of LOFAR, that is also a powerful standalone low-frequency (LF) radiotelescope in the range 10-85 MHz. Its construction has started in 2014 in Nançay, supported by CNRS/INSU, Observatoire de Paris, Université d’Orléans/OSUC, and the FLOW consortium. NenuFAR is a compact antenna array (400 m in diameter) connected to the LOFAR receivers and to a local digitizer and beamformer (synthesizing narrow steerable 200 kHz beams in the sky up to a total instantaneous bandwidth of 150 MHz). We propose here to give NenuFAR the additional capability of a powerful LF imager with ~7’ resolution, by adding a realtime correlator, offline computing power, massive storage, and 6 small arrays (of 19 antennas each) within 3 km of the compact core. These extensions will considerably reduce the confusion noise limiting the imaging sensitivity of the instrument, down to about the thermal noise. The scientific topics that will greatly benefit from the use of NenuFAR in Standalone as an imager include: (a) in multi-frequency, long integration imaging: the detection of the cosmological HI signal from the “dark ages” to the reionization era through the "cosmic dawn", the systematic and sensitive search for yet unconfirmed LF radio signatures of exoplanets and star-planet plasma interactions (SPI), and the radio afterglows of high energy collapses generating gravitational waves and Gamma Ray Bursts ; (b) in snapshot imaging mode: the systematic sensitive search for fast radio transients in a broad field of view, including the prompt emission associated with gravitational waves, pulsar giant pulses, Fast Radio Bursts, and exoplanetary / stellar / SPI bursts ; Detection and study of ~10^5 galactic and extragalactic sources, with their diffuse LF emission, will also be made possible. For topics (a) & (b) the NenuFAR Radio Imager (NRI) will be in its frequency range the most powerful existing instrument before SKA (more powerful than LOFAR and the LWA), combining a high instantaneous sensitivity, very large total bandwidth, good angular resolution, and full polarization measurement capability. The proposed imaging capability, adding to its beamformer capability and its enhancement of LOFAR as a “Super Station” (LSS) will make NenuFAR a truly unique LF radio facility of the 21st century. In the SKA era, it will remain complementary to SKA in terms of spectral range and hemisphere. France has a strong expertise in development of imaging, calibration, deconvolution, and RFI mitigation algorithms for LF radioastronomy, that will be applied to and optimized for the NRI, preparing the exploitation of SKA. NenuFAR has received the official status of SKA pathfinder from the SKA office, for the experience it will bring in terms of hardware, algorithms, multimode operations (super-station / standalone beamformer / imager), and science preparation. The NRI will contribute to the organization and development of the French LF radio community – which started with the collective writing of a broad science case by 80 French and International scientists –, reinforcing its role and influence in Europe. The CNRS prospective in 2014 and the Haut Conseil TGIR in 2016 gave a high priority to the completion of NenuFAR (labelled “Infrastructure de Recherche” by the MESR), and the “Conseil Stratégique de Direction” of USN identified the NRI as the development of highest importance. The NRI project includes a Public Outreach part in the form of an Art Project integrated with the instrument and that will enhance its interest for the general public.

Galactic research has entered a thrilling epoch. Our knowledge of Galactic stellar populations, until few years ago mostly confined to stars at the solar vicinity, is rapidly extending to large regions of the disc and bulge of our Galaxy. Large spectroscopic surveys are acquiring an unprecedented amount of data, with radial velocities and chemical composition for hundred thousands stars, from the innermost regions to the periphery of the Milky Way disc, up to ~ 15 kpc from the Galactic center. This unique, because unprecedented, cartography of our Galaxy will acquire all its potential with the publication of the data from Gaia, the European astrometric mission, which will deliver positions and proper motions for 1 billion objects, and radial velocities for about one tenth of them. Without waiting for the final catalogue of the mission (planned for ~ 2022), with the second release of the Gaia data, scheduled by early 2017, the astrometric solution for most the sky will be made public, together with radial velocities for some ten millions stars. In less than two years from now, we will thus be able to reconstruct the orbits of several millions stars in the Galaxy, to have detailed chemical abundances for some hundred thousands and ages for several thousands. The tremendous amount of data that the mission will deliver will need efficient tools for their analysis but also sophisticated models for their interpretation. We are interested to answer to some of the simplest but still unraveled questions of Galactic studies : What are the characteristics of the different Milky Way stellar populations? How were they shaped over time? What is the evolutionary link between them? Which of them is the result of in-situ star formation or rather the deposit of structures accreted over time? The uniqueness of our approach consists in aiming to address them by using different and complementary numerical methodologies, that we have been developing in the last years and that are usually used independently one of the others: test particle methods, orbits reconstruction, chemical evolution, N-body simulations. From test particle methods, where the motion of “test particles” is integrated in a gravitational potential made of a thin and a thick stellar disc, an optional classical bulge, a rotating stellar bar, and a dark matter halo, we expect to have information about the level of complexity of the Galaxy. Can we describe it today "simply" as a disc galaxy evolved secularly in the last 8-9 Gyr under the effect of stellar asymmetries, and its main resonances? Or rather does the comparison with data available with the first Gaia releases indicate some complexity that these simple models are not able to reproduce? The reconstruction of the orbits of some ten million stars should help to understand the origin of the complexity possibly not captured by test particle methods, by quantifying the level of discontinuity in the orbital properties of the different galactic stellar populations. Chemical evolution models will reinforce our understanding, by providing: age-chemistry-kinematics relations, identifying the different chemical patterns and their possible in-situ, ex-situ origin; the star formation history of the Galaxy, and the mass of stars locked in the different stellar populations. All these ingredients will provide the basis for setting the scene for new N-body simulations, that will fully implement dissipative processes and detailed chemical enrichment. With them we aim at reconstructing possible evolutionary paths for the Milky Way in the last 8-9 Gyr, describing the chemo-dynamical links between the inner disc, the bar, and the bulge, and exploring scenarios for the accretion history of the Galaxy. Each of these methodologies, separately, allows to reconstruct some pieces of the puzzle of the chemo-dynamical processes experienced by the Milky Way. All of them, together, should allow us to build a robust and coherent picture of its evolution.

In a hierarchically-formed Universe, the Milky Way is a test-bed to study in details the mechanisms that shape galaxies. The synergy between the Gaia space satellite and the ground-based spectroscopic survey WEAVE gives access, for the first time, to more than thirty tracers of the past of our Galaxy for a million stars of the extended Solar neighbourhood, and to a dozen of tracers for another two million stars outside of it. Our project concerns the study of the Galactic disc, a structure that encodes both internal (e.g. stellar radial migration) and external (e.g. accretion events) mechanisms that come into play in the chemo-dynamical evolution of our Galaxy. We have built a versatile team with nodes in Nice, Paris and Strasbourg, including experts in Galaxy evolution, simulations and modelling. The team members are heavily involved in both WEAVE and Gaia in order to extract the maximum of information available in those combined catalogues. Over the course of the four-year MWDisc ANR project, and alongside to the accumulation of the WEAVE data (starting in Q1 2021), we aim to produce added-value catalogues for the WEAVE stellar targets (containing homogeneous stellar chemical abundances, ages, orbits and extinctions) and models associated to the diffusion of mono-age populations by time-varying perturbations and superposition of perturbations (associated to the spiral arms and the Galactic bar). These models and catalogues will allow us, in turn, to evaluate the star formation history in various regions of the disc, put constraints on the merger tree of the Milky Way (including the analysis of existing simulations of ours), to link the geometrical properties of the thin and thick disc with their chemical counterparts and finally to characterize the efficiency of radial migration throughout the disc.