Abstract
Abstract 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 ( ) values to O3Es with an empirical relation also show similar, anisotropic CCFs if we use only objects with higher 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.
Highlights
Matter in the universe is not uniformly distributed in space but forms large-scale filamentary structure or the cosmic web
We report that Lyα-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
With its density increasing with matter density, IGM Hi is a faithful tracer of the cosmic web that has recently come into use, the areas mapped by IGM Hi are still limited because the observations are very time-consuming
Summary
Matter in the universe is not uniformly distributed in space but forms large-scale filamentary structure or the cosmic web. With its density increasing with matter density, IGM Hi is a faithful tracer of the cosmic web that has recently come into use, the areas mapped by IGM Hi are still limited because the observations are very time-consuming. These reported discrepancies might imply that LAEs intrinsically avoid dense parts of the cosmic web for some reason (e.g., Shimakawa et al 2017; Oteo et al 2018; Shi et al 2019; Momose et al 2020)
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