Slicing COSMOS with SC4K: the evolution of typical Ly α emitters and the Ly α escape fraction from z ∼ 2 to 6

We present and explore deep narrow- and medium-band data obtained with the Subaru and the Isaac Newton telescopes in the ~2 deg² COSMOS field. We use these data as an extremely wide, low-resolution (R~20-80) IFU survey to slice through the COSMOS field and obtain a large sample of ~4000 Lyman-α (Lya) emitters from z~2 to z~6 in 16 redshift slices (SC4K). We present new Lya luminosity functions (LFs) covering a co-moving volume of ~10⁸Mpc³. SC4K extensively complements ultra-deep surveys, jointly covering over 4 dex in Lya luminosity and revealing a global (2. 53. 5, likely linked with the evolution of the AGN population. The Lya luminosity density rises by a factor ~2 from z~2 to z~3 but is then found to be roughly constant (~1. 110^40 erg s^-1 Mpc^-3) to z~6, despite the ~0. 7 dex drop in UV luminosity density. The Lya/UV luminosity density ratio rises from 41% to 306% from z~2. 2 to z~6. Our results imply a rise of a factor of ~2 in the global ionisation efficiency (ξ ₈₎₍) and a factor ~41 in the Lya escape fraction from z~2 to z~6, hinting for evolution in both the typical burstiness/stellar populations and even more so in the typical ISM conditions allowing Lyα photons to escape.