Effect of Ly$\alpha$ Radiative Transfer on Intensity Mapping Power Spectra

Abstract

Clustering of Ly$\alpha$-emitting galaxies (LAEs) and Ly$\alpha$ line intensity mapping (LIM) are useful probes of cosmology. However, Ly$\alpha$ radiative transfer (RT) effects, such as absorption, line shift, and line broadening, and their dependence on the large-scale density and velocity fields can modify the measured LAE clustering and LIM statistics. We explore the effects of RT on the Ly$\alpha$ LIM power spectrum in two ways: using an analytic description based on linear approximations, and using lognormal mocks. The qualitative effects of intergalactic Ly$\alpha$ absorption on the LIM auto- and cross-power spectrum include a scale-dependent, reduced effective bias, reduced mean intensity, and modified redshift-space distortions. The linear absorption model does not describe the results of the lognormal simulations well. The random line shift suppresses the redshift-space power spectrum similar to the Fingers-of-God effect. In cross-correlation of LAEs or Ly$\alpha$ intensity with a non-Ly$\alpha$ tracer, the Ly$\alpha$ line shift leads to a phase shift of the complex power spectrum, i.e. a cosine damping of the real part. We study the impact of Ly$\alpha$ RT effects on the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) LAE and LIM power spectra using lognormal mocks. We find that even small amounts of IGM absorption will significantly change the measured LAE auto-power spectrum. The LAE-intensity cross-power spectrum stays within the measurement uncertainty. Therefore, HETDEX will be able to constrain Ly$\alpha$ RT effects.

13 pages, 6 figures, comments are welcome