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

D. Sobral, S. Santos, J. Matthee, A. S. P. Afonso, B. R. L. Ribeiro, J. Calhau, A. A. Khostovan

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 Lyα emitters (LAEs) from z∼2 to 6 in 16 redshift slices (SC4K). We present new Lyα luminosity functions (LFs) covering a comoving volume of ∼108 Mpc3. SC4K extensively complements ultradeep surveys, jointly covering over 4 dex in Lyα luminosity and revealing a global (2.5<z<6) synergy LF with α=−1.93+0.12−0.12, log10Φ LLyα=−3.45+0.22−0.29 Mpc−3, and log10 LLyα=42.93+0.15−0.11 erg s−1. The Schechter component of the Lyα LF reveals a factor ∼5 rise in LLyα and a ∼7 × decline in ΦLyα 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 Lyα 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 Lyα 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 Lyα/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 Lyα 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 Lyα photons to escape.

Keywords
galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: luminosity function, mass function; galaxies: statistics

Monthly Notices of the Royal Astronomical Society
Volume 476, Issue 4, Page 28
2018 February

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia