An outstanding issue in current extra-galactic studies is a better physical understanding of the interplay between dark, stellar and gaseous material including how primordial and metal-enriched gas flows in and out of galaxies on various scales. Such questions can only be addressed with a large multi-object spectroscopic survey at redshifts 1 < z < 5, corresponding to a formative period in cosmic history. The unique combination of large field of view, primary diameter, high-multiplex MOS and monolitic IFS makes WST the ideal facility to address this issue.

WST is uniquely positioned to probe the evolution of galaxies from the era of cosmic reionisation at z~7 through the peak of star-formation activity to z~1, where current observations suggest the distinction between disc and spheroidal galaxies was established. In addition to much larger and deeper surveys of colour-selected Lyman break galaxies over redshifts 2 < z < 7, WST can complement these with “blind” IFS-selected sample of Lyman alpha emitters representative of lower mass star-forming systems. The combination will provide a unique view of the assembly history and spatial distributions spanning a wide range of mass and star formation rates. The connection of galaxies to the cosmic web can be mapped with an IGM tomographic survey based on the detection of neutral hydrogen (HI) Lyman alpha absorption seen in the spectra of background 2.5 < z < 4 galaxies. By densely sampling the galaxy population, the dark matter halos within which galaxies evolve can be mapped out to redshifts z > 2. Via this key survey, the distribution of gas and galaxies can be probed both on the largest scales using WST’s high multiplex advantage, whereas on smaller scales relevant to the circumgalactic medium (CGM) or even at IGM scales, the detailed physical properties of individual galaxies and their immediate environment can be probed by the panoramic IFS to investigate the underlying physical processes governing their assembly history.