Catch Me if You Can: Biased Distribution of Lyα-emitting Galaxies according to the Viewing Direction

We report that Lyα-emitting galaxies (LAEs) may not faithfully trace the cosmic web of neutral hydrogen (H i), but their distribution is likely biased depending on the viewing direction. We calculate the cross-correlation function (CCF) between galaxies and Lyα forest transmission fluctuations on the near and far sides of the galaxies separately, for three galaxy samples at z ∼ 2: LAEs, [O iii] 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 H i 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 Lyα equivalent width (${{EW}}_{{\rm{Ly}}\alpha }^{{\rm{obs}}}$) values to O3Es with an empirical relation also show similar, anisotropic CCFs if we use only objects with higher ${{EW}}_{{\rm{Ly}}\alpha }^{{\rm{obs}}}$ 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 Lyα because Lyα 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 H i gas. Care is needed when using LAEs to search for overdensities.