2016ApJ...823...20K

We present the Lyα luminosity functions (LFs) derived by our deep Subaru narrowband survey that identifies a total of 3137 Lyα emitters (LAEs) at z=2.2 in five independent blank fields. This sample of LAEs is the largest to date and covers a very wide Lyα luminosity range of logL_Lyα=41.7–44.4 erg/s. We determine the Lyα LF at z=2.2 with unprecedented accuracy and obtain the best-fit Schechter parameters of L_Lyα^{*}={5.29}{-1.13}^{+1.67}×{10}^{42} erg/s, φ_Lyα^{*}={6.32}{-2.31}^{+3.08}×{10}^{-4} Mpc^–3, and α=-{1.75}_{-0.09}^{+0.10} , showing a steep faint-end slope. We identify a significant hump at the LF bright end ( logL_Lyα\gt 43.4 erg/s). Because all of the LAEs in the bright-end hump have a bright counterpart(s) in either the X-ray, UV, or radio data, this bright-end hump is not made by gravitational lensing magnification bias but by active galactic nuclei (AGNs). These AGNs allow us to derive the AGN UV LF at z ∼ 2 down to the faint magnitude limit of M_UV ≃ -22.5 and to constrain the faint-end slope of the AGN UV LF, α_AGN=-1.2±0.1, which is flatter than those at z > 4. Based on the Lyα and UV LFs from our and previous studies, we find an increase of Lyα escape fraction {f}_esc^Lyα from z ∼ 0 to 6 by two orders of magnitude. This large {f}_esc^Lyα increase can be explained neither by the evolution of stellar population nor by outflow alone, but by the evolution of neutral hydrogen H i density in the interstellar medium that enhances dust attenuation for Lyα by resonance scattering. Our uniform expanding shell models suggest that the typical H i column density decreases from {N}_{
HI}∼7×{10}^{19} (z ∼ 0) to ∼1 x 10¹⁸/cm2 (z ∼ 6) to explain the large {f}_esc^Lyα increase.