BRIGHT AND FAINT ENDS OF Lyα LUMINOSITY FUNCTIONS AT z = 2 DETERMINED BY THE SUBARU SURVEY: IMPLICATIONS FOR AGNs, MAGNIFICATION BIAS, AND ISM H I EVOLUTION

摘要

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 erg s?1. We determine the Lyα LF at z = 2.2 with unprecedented accuracy and obtain the best-fit Schechter parameters of erg s?1, Mpc?3, and , showing a steep faint-end slope. We identify a significant hump at the LF bright end ( erg s?1). 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 from z ~ 0 to 6 by two orders of magnitude. This large 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 (z ~ 0) to ~1 × 1018 cm?2 (z ~ 6) to explain the large increase.