Abstract

MOSAIC is the future multi-object fibre spectrograph for the ESO 39m Extremely Large Telescope (ELT). It is a first generation instrument, with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. While ELT first light instruments will focus on exploiting the central part of the ELT focal plane, MOSAIC is designed to cover the largest possible area (~40 arcmin²) thanks to an original stepped and tiled focal plane design which allows parallel observations in different modes. MOSAIC is a versatile multi-object spectrograph working in in both the Visible and NIR domains, offering both high multiplex and multi-IFU capabilities, optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from our Galaxy to the epoch of the reionization. In this respect, it will be a privileged survey machine for the followup of sources being discovered by current and future major facilities, such as JWST , Euclid, ROMAN or SKA . I will review the main expected scientific contributions of MOSAIC , focusing on the study of the first galaxies formed in the Universe.