Radiative Transfer Modeling of Lyman Alpha Emitters. I. Statistics of Spectra and Luminosity

Abstract

We combine a cosmological reionization simulation with box size of 100Mpc/h on a side and a Monte Carlo Lyman-alpha (Lya) radiative transfer code to model Lyman Alpha Emitters (LAEs) at z~5.7. The model introduces Lya radiative transfer as the single factor for transforming the intrinsic Lya emission properties into the observed ones. Spatial diffusion of Lya photons from radiative transfer results in extended Lya emission and only the central part with high surface brightness can be observed. Because of radiative transfer, the appearance of LAEs depends on density and velocity structures in circumgalactic and intergalactic media as well as the viewing angle, which leads to a broad distribution of apparent (observed) Lya luminosity for a given intrinsic Lya luminosity. Radiative transfer also causes frequency diffusion of Lya photons. The resultant Lya line is asymmetric with a red tail. The peak of the Lya line shifts towards longer wavelength and the shift is anti-correlated with the apparent to intrinsic Lya luminosity ratio. The simple radiative transfer model provides a new framework for studying LAEs. It is able to explain an array of observed properties of z~5.7 LAEs in Ouchi et al. (2008), producing Lya spectra, morphology, and apparent Lya luminosity function (LF) similar to those seen in observation. The broad distribution of apparent Lya luminosity at fixed UV luminosity provides a natural explanation for the observed UV LF, especially the turnover towards the low luminosity end. The model also reproduces the observed distribution of Lya equivalent width (EW) and explains the deficit of UV bright, high EW sources. Because of the broad distribution of the apparent to intrinsic Lya luminosity ratio, the model predicts effective duty cycles and Lya escape fractions for LAEs.