4MOST - The new wide-field, high-multiplex spectroscopic survey facility for ESO's VISTA telescope

4MOST is a wide-field, high-multiplex spectroscopic survey facility under development for the VISTA telescope of the European Southern Observatory (ESO), the only facility of its kind planned for the southern sky. Its main science drivers are in the fields of galactic archeology, high-energy physics, galaxy evolution and cosmology. 4MOST will in particular provide the spectroscopic complements to the large area surveys coming from space missions like Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like VISTA, VST, DECam, LSST and SKA. Surveys are planned to determine the chemo-dynamical structure and evolution of the Milky Way and to characterize Dark Energy as function of time through multiple methods (e.g., BAO, RSD, lensing) and tracers (galaxies, AGN, Ly-α Forest, galaxy clusters, SNe 1a).The 4MOST baseline concept features a 2.7 degree diameter field-of-view with about 2400 fibres in the focal surface that are configured by a fibre positioner based on the tilting spine principle. The fibres feed two types of spectrographs; ∼1600 fibres go to two spectrographs with resolution R>5000 (∼390-930 nm) and ∼800 fibres to a spectrograph with R>18,000 (∼392-437 nm, 515-572 nm and 605-675 nm). Both types of spectrographs are fixed-configuration, three-channel spectrographs.4MOST will have an unique operations concept in which 5 year public surveys from both the consortium and the ESO community will be combined and observed in parallel during each exposure, resulting in more than 25 million spectra of targets spread over a large fraction of the southern sky. The 4MOST Facility Simulator (4FS) was developed to demonstrate the feasibility of this observing concept. 4MOST has been accepted for implementation by ESO and has moved into its construction phase. Operations are expected to start by the early 2021.