We have known for more than 30 years that galaxies like ours grow by a succession of accretion and merging events with other galaxies, in a process called hierarchical formation. It is one of the pillars of the standard cosmological model LambdaCDM. The Local Group is composed mainly of two giant spiral galaxies – which are the Milky Way and the Andromeda galaxy – and their cohort of dwarf galaxies and satellites. They form an island of about 2 Mpc in the cosmic web, embedded in the Virgo Supercluster. Because of its proximity, the Local Group is a privileged object of study where the dynamics and the chemistry of the objects and of stars can be studied at a level of precision that is not achievable in other galaxies. Therefore, the Local Group can be used as a benchmark to study the formation of galactic structures in a broader cosmological context.

Thanks to the European Gaia mission, a giant leap forward has been made in the global comprehension of our close environment. With its incredible astrometric precision, it has made possible the study of the global and internal dynamics of Local Group galaxies. Most of the orbits of the Milky Way dwarfs have been accurately reconstructed for the first time. Gaia data also allowed a measurement of the tangential velocity of the disk of the Andromeda galaxy with an unprecedented precision. Furthermore, the exquisite catalogues played a crucial role in the discovery of many new signatures of past Milky Way merger events, either visible as coherent spatial structures such as stellar streams, or as phase- space correlated features. For instance, the most important of these discoveries, the so called « Gaia-Enceladus Sausage », is the consequence of a major encounter between the Galaxy and an other massive galaxy 8-10 Gyr ago. In addition, Gaia data were essential in providing the tentative observational evidences showing that the infall of the LMC might generate significant gravitational perturbations in the outskirts of the Milky Way, which may invalidate the common hypothesis of a Galaxy in equilibrium.

Many French-based institutions and researchers are deeply involved in the analysis of the Gaia data and in their scientific exploitation at the Local Group scale. Therefore, we propose a half-day workshop on the theme: « The Local Group in the Gaia era: from the Galactic halo to Andromeda” to regroup together experts of the field. The aim of this workshop will be to give the possibility to present recent advances made in the understanding of the Local Group and to discuss how Gaia data, in synergy with upcoming large surveys, have enlarged our knowledge and will drastically revolutionize the field. Thus, the major part of this afternoon will be devoted to an overview of the main results obtained with Gaia DR2 and EDR3. But we will also dedicated some time at the end of the workshop to consider the possibilities that the new third Gaia data release will offer. This will be followed by discussions to enhance its exploitation and to build new fruitful collaborations.

Co-chair: Jean-Baptiste Salomon, Guillaume Thomas, Carine Babusiaux, François Hammer, Rodrigo Ibata