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

Abstract

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) Integral Field Unit survey to slice through the COSMOS field and obtain a large sample of ~4000 Ly α emitters (LAEs) from z ~ 2 to 6 in 16 redshift slices (SC4K). We present new Ly α luminosity functions (LFs) covering a comoving volume of ~108 Mpc3. SC4K extensively complements ultradeep surveys, jointly covering over 4 dex in Ly α luminosity and revealing a global (2.5 < z < 6) synergy LF with α = -1.93-0.12+0.12, log10 ϕ Lya* = -3.45-0.29+0.22 Mpc-3, and log10 L Lya* = 42.93-0.11+0.15 erg s-1. The Schechter component of the Ly α LF reveals a factor ~5 rise in L Lya* and a ~7 × decline in ϕ Lya* from z ~ 2 to 6. The data reveal an extra power-law (or Schechter) component above LLy α ≈ 1043.3 erg s-1 at z ~ 2.2-3.5 and we show that it is partially driven by X-ray and radio active galactic nucleus (AGN), as their Ly α 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 α luminosity density rises by a factor ~2 from z ~ 2 to 3 but is then found to be roughly constant (1.1-0.2+0.2 × 1040 erg s-1 Mpc-3) to z ~ 6, despite the ~0.7 dex drop in ultraviolet (UV) luminosity density. The Ly α/UV luminosity density ratio rises from 4 ± 1 per cent to 30 ± 6 per cent from z ~ 2.2 to 6. Our results imply a rise of a factor of ≈2 in the global ionization efficiency (Ξion) and a factor ≈4 ± 1 in the Lya escape fraction from z ~ 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 α photons to escape.