Radiative Transfer Distortions of Lyman-α Emitters: a New Fingers-of-God Damping in the Clustering in Redshift Space

Abstract

Complex radiative transfer (RT) of Lyman-α photons poses a theoretical challenge to galaxy surveys that infer large-scale structure with Lyman-α emitters (LAEs). Guided by RT simulations, prior studies investigated the impact of RT on large-scale LAE clustering and claimed that RT induces a selection effect resulting in an anisotropic distortion even in real space, but an otherwise negligible effect in redshift space. However, our previous study, which relies on a full RT code run on the Illustris simulations, shows that the anisotropic selection effect was drastically reduced with higher spatial resolution. Adopting the same simulation framework, we further study the impact of RT on LAE clustering in redshift space. Since we measure an LAE's radial position through a spectral peak of Lyman-α emission, the frequency shift due to RT contaminates the redshift measurement and hence the inferred radial position in redshift space. We demonstrate that this additional RT offset suppresses LAE clustering along the line of sight, which can be interpreted as a novel Fingers-of-God effect. To assess this effect, we develop a theoretical framework modeling the impact of RT similarly to that of small-scale peculiar velocity, which is commonly studied in the context of redshift-space distortion. Although our findings encourage more careful modeling in LAE surveys, we also seek a method to mitigate the additional effect due to RT by making use of other information in a Lyman-α spectrum.