I will present new results that reveal the nature and evolution of primeval galaxies, picked up in large and deep surveys due to their Lyman-alpha emission. I will show how Lyman-alpha emitters (LAEs) are an incredibly rare population of galaxies in the local Universe, but become dominant into the epoch of re-ionisation and that they have the properties and the number density necessary to dominate and drive cosmic re-ionisation.

Lyman-α (Lyα) is intrinsically the brightest line emitted from active galaxies, but its resonant nature and susceptibility to dust as a rest-frame UV photon makes Lyα very hard to interpret due to the uncertain Lyα escape fraction, until now. By conducting large blind surveys at different cosmic times and the necessary spectroscopic follow-up, we have obtained a simple empirical relation that robustly retrieves the Lyα escape fraction as a function of Lyα rest-frame EW. We show that the relation constrains a well-defined anti-correlation between ionisation efficiency and dust extinction in Lyman-alpha emitters (LAEs) which is likely an evolutionary sequence for very young metal poor sources going through a LAE phase. We also reveal how those LAEs/primeval galaxies have high escape fractions of not just Lyman-alpha photons, but also LyC, and how the production and escape seem to be linked together.