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

Abstract

We present and explore deep narrow- and medium-band data obtained with the Subaru and the Isaac Newton telescopes in the ~2 deg${}^2$ 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-$\alpha$ (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 ~10${}^8$Mpc${}^3$. SC4K extensively complements ultra-deep surveys, jointly covering over 4 dex in Lya luminosity and revealing a global (2.5<z<6) synergy LF with $\alpha =-1.93\pm 0.12$, ${\log }_{10}{\Phi }^{\ast }=-3.45_{-0.29}^{+0.22}$ Mpc${}^{-3}$ and ${\log }_{10}{L}^{\ast }=42.93_{-0.11}^{+0.15}$ erg/s. The Schechter component of the Lya LF reveals a factor ~5 rise in $L^{\ast }$ and a ~7x decline in ${\Phi }^{\ast }$ from z~2 to z~6. The data reveal an extra power-law (or Schechter) component above L~10${}^{43.3}$ erg/s at z~2.2-3.5 and we show that it is partially driven by X-ray and radio AGN, as their Lya 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 Lya luminosity density rises by a factor ~2 from z~2 to z~3 but is then found to be roughly constant (~$1.1\times 10^{40}$ 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 $4\pm 1$% to $30\pm 6$% from z~2.2 to z~6. Our results imply a rise of a factor of ~2 in the global ionisation efficiency (${\xi }_{\mathrm{ion}}$) and a factor ~$4\pm 1$ 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 Ly$\alpha$ photons to escape.

Accepted for publication in MNRAS. 24 pages + Appendices with SC4K catalogue and LFs. SC4K catalogue of LAEs, SC4K LFs and the S-SC4K compilation are available (https://goo.gl/q9yfKo) prior to publication

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