Catch Me if You Can: Biased Distribution of Ly$\alpha$-emitting Galaxies according to the Viewing Direction

Abstract

We report that Ly$\alpha$-emitting galaxies (LAEs) may not faithfully trace the cosmic web of neutral hydrogen (HI), but their distribution is likely biased depending on the viewing direction. We calculate the cross-correlation (CCF) between galaxies and Ly$\alpha$ forest transmission fluctuations on the near and far sides of the galaxies separately, for three galaxy samples at $z\sim2$: LAEs, [OIII] emitters (O3Es), and continuum-selected galaxies. We find that only LAEs have anisotropic CCFs, with the near side one showing lower signals up to $r=3-4~h^{-1}$ comoving Mpc. This means that the average HI density on the near side of LAEs is lower than that on the far-side by a factor of $2.1$ under the Fluctuating Gunn-Peterson Approximation. Mock LAEs created by assigning Ly$\alpha$ equivalent width ($EW_\text{Ly$\alpha$}^\text{obs}$) values to O3Es with an empirical relation also show similar, anisotropic CCFs if we use only objects with higher $EW_\text{Ly$\alpha$}^\text{obs}$ than a certain threshold. These results indicate that galaxies on the far side of a dense region are more difficult to be detected ("hidden") in Ly$\alpha$ because Ly$\alpha$ emission toward us is absorbed by dense neutral hydrogen. If the same region is viewed from a different direction, a different set of LAEs will be selected as if galaxies are playing hide-and-seek using HI gas. Care is needed when using LAEs to search for overdensities.