Bright and Faint Ends of Lyα Luminosity Functions at z = 2 Determined by the Subaru Survey: Implications for AGN, Magnification Bias, and ISM HI Evolution
Abstract
We present the Lya luminosity functions (LFs) derived by our deep Subaru narrowband survey that identifies a total of 3,137 Lya emitters (LAEs) at z = 2.2 in five independent blank fields. The sample of these LAEs is the largest, to date, and covers a very wide Lya luminosity range of LLyα = 41.7-44.4 erg s-1. We determine the Lya LF at z = 2.2 with unprecedented accuracies, and obtain the best-fit Schechter parameters of L*Lyα = 5.29+1.67-1.13 × 1042 erg s-1, φ*Lyα = 6.32+3.08-2.31 × 10-4 Mpc-3, and α = -1.75+0.10-0.09 showing a steep faint-end slope. We identify a significant hump at the LF bright end ( LLyα > 43.4 erg s-1). Because all of the LAEs in the bright-end hump have (a) bright counterpart(s) either in the X-ray, UV, or radio data, this bright-end hump is not made by gravitational lensing magnification bias but AGNs. These AGNs allow us to derive the AGN UV LF at z 2 down to the faint magnitude limit of MUV -22.5, and to constrain the faint-end slope of AGN UV LF, αAGN=-1.2 0.1, that is flatter than those at z > 4. Based on the Lya and UV LFs from our and previous studies, we find the increase of Lya escape fraction fLyαesc from z 0 to 6 by two orders of magnitude. This large fLyαesc increase can be explained neither by the evolution of stellar population nor outflow alone, but the evolution of neutral hydrogen HI density in inter-stellar medium that enhances dust attenuation for Lya by resonance scattering. Our uniform expanding shell models suggest that the typical HI column density decreases from NHI 7 × 1019 (z 0) to 1 × 1018 cm-2 (z 6) to explain the large fLyαesc increase.
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