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 similar to 2 deg(2) COSMOS field. We use these data as an extremely wide, low-resolution (R similar to 20-80) Integral Field Unit survey to slice through the COSMOS field and obtain a large sample of similar to 4000 Ly alpha emitters (LAEs) from z similar to 2 to 6 in 16 redshift slices (SC4K). We present new Ly alpha luminosity functions (LFs) covering a comoving volume of similar to 10(8) Mpc(3). SC4K extensively complements ultradeep surveys, jointly covering over 4 dex in Ly a luminosity and revealing a global (2.5 < z < 6) synergy LF with alpha = -1.93(-0.12)(+0.12), log(10) Phi(*)(Ly alpha) = -3.45(-0.29)(+0.22) Mpc(-3), and log(10) L-Ly alpha(*) = 42.93(-0.11)(+0.15) erg s(-1). The Schechter component of the Ly alpha LF reveals a factor similar to 5 rise in L-Ly alpha(*) and a similar to 7 x decline in Phi(*)(Ly alpha) from z similar to 2 to 6. The data reveal an extra power-law (or Schechter) component above L-Ly alpha approximate to 10(43.3) erg s(-1) at z similar to 2.2-3.5 and we show that it is partially driven by X-ray and radio active galactic nucleus (AGN), as their Ly alpha LF resembles the excess. The power-law component vanishes and/ or is below our detection limits above z > 3.5, likely linked with the evolution of the AGN population. The Ly alpha luminosity density rises by a factor similar to 2 from z similar to 2 to 3 but is then found to be roughly constant (1.1(-0.2)(+0.2) x 10(40) erg s(-1) Mpc(-3)) to z similar to 6, despite the similar to 0.7 dex drop in ultraviolet (UV) luminosity density. The Ly alpha/UV luminosity density ratio rises from 4 +/- 1 per cent to 30 +/- 6 per cent from z similar to 2.2 to 6. Our results imply a rise of a factor of approximate to 2 in the global ionization efficiency (xi(ion)) and a factor approximate to 4 +/- 1 in the Ly alpha escape fraction from z similar to 2 to 6, hinting for evolution in both the typical burstiness/ stellar populations and even more so in the typical interstellar medium conditions allowing Ly alpha photons to escape.