Catch Me if You Can: Biased Distribution of Ly\${\textbackslash}alpha\$-emitting Galaxies according to the Viewing Direction. Momose, R., Shimasaku, K., Nagamine, K., Shimizu, I., Kashikawa, N., Ando, M., & Kusakabe, H. arXiv e-prints, 2104:arXiv:2104.10580, April, 2021.
Paper abstract bibtex We report that Ly\${\textbackslash}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\${\textbackslash}alpha\$ forest transmission fluctuations on the near and far sides of the galaxies separately, for three galaxy samples at \$z{\textbackslash}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{\textasciitilde}h{\textasciicircum}\{-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\${\textbackslash}alpha\$ equivalent width (\$EW_{\textbackslash}text\{Ly\$\textbackslashalpha\$\}{\textasciicircum}{\textbackslash}text\{obs\}\$) values to O3Es with an empirical relation also show similar, anisotropic CCFs if we use only objects with higher \$EW_{\textbackslash}text\{Ly\$\textbackslashalpha\$\}{\textasciicircum}{\textbackslash}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\${\textbackslash}alpha\$ because Ly\${\textbackslash}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.
@article{momose_catch_2021,
title = {Catch {Me} if {You} {Can}: {Biased} {Distribution} of {Ly}\${\textbackslash}alpha\$-emitting {Galaxies} according to the {Viewing} {Direction}},
volume = {2104},
shorttitle = {Catch {Me} if {You} {Can}},
url = {http://adsabs.harvard.edu/abs/2021arXiv210410580M},
abstract = {We report that Ly\${\textbackslash}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\${\textbackslash}alpha\$ forest
transmission fluctuations on the near and far sides of the galaxies
separately, for three galaxy samples at \$z{\textbackslash}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{\textasciitilde}h{\textasciicircum}\{-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\${\textbackslash}alpha\$ equivalent width
(\$EW\_{\textbackslash}text\{Ly\${\textbackslash}alpha\$\}{\textasciicircum}{\textbackslash}text\{obs\}\$) values to O3Es with an empirical
relation also show similar, anisotropic CCFs if we use only objects with
higher \$EW\_{\textbackslash}text\{Ly\${\textbackslash}alpha\$\}{\textasciicircum}{\textbackslash}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\${\textbackslash}alpha\$ because
Ly\${\textbackslash}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.},
urldate = {2021-05-04},
journal = {arXiv e-prints},
author = {Momose, Rieko and Shimasaku, Kazuhiro and Nagamine, Kentaro and Shimizu, Ikkoh and Kashikawa, Nobunari and Ando, Makoto and Kusakabe, Haruka},
month = apr,
year = {2021},
keywords = {Astrophysics - Astrophysics of Galaxies},
pages = {arXiv:2104.10580},
}
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We calculate the cross-correlation (CCF) between galaxies and Ly\\${\\textbackslash}alpha\\$ forest transmission fluctuations on the near and far sides of the galaxies separately, for three galaxy samples at \\$z{\\textbackslash}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{\\textasciitilde}h{\\textasciicircum}\\{-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\\${\\textbackslash}alpha\\$ equivalent width (\\$EW_{\\textbackslash}text\\{Ly\\$\\textbackslashalpha\\$\\}{\\textasciicircum}{\\textbackslash}text\\{obs\\}\\$) values to O3Es with an empirical relation also show similar, anisotropic CCFs if we use only objects with higher \\$EW_{\\textbackslash}text\\{Ly\\$\\textbackslashalpha\\$\\}{\\textasciicircum}{\\textbackslash}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\\${\\textbackslash}alpha\\$ because Ly\\${\\textbackslash}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. 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We calculate the\ncross-correlation (CCF) between galaxies and Ly\\${\\textbackslash}alpha\\$ forest\ntransmission fluctuations on the near and far sides of the galaxies\nseparately, for three galaxy samples at \\$z{\\textbackslash}sim2\\$: LAEs, [OIII] emitters\n(O3Es), and continuum-selected galaxies. We find that only LAEs have\nanisotropic CCFs, with the near side one showing lower signals up to\n\\$r=3-4{\\textasciitilde}h{\\textasciicircum}\\{-1\\}\\$ comoving Mpc. This means that the average HI density on\nthe near side of LAEs is lower than that on the far-side by a factor of\n\\$2.1\\$ under the Fluctuating Gunn-Peterson Approximation. Mock LAEs\ncreated by assigning Ly\\${\\textbackslash}alpha\\$ equivalent width\n(\\$EW\\_{\\textbackslash}text\\{Ly\\${\\textbackslash}alpha\\$\\}{\\textasciicircum}{\\textbackslash}text\\{obs\\}\\$) values to O3Es with an empirical\nrelation also show similar, anisotropic CCFs if we use only objects with\nhigher \\$EW\\_{\\textbackslash}text\\{Ly\\${\\textbackslash}alpha\\$\\}{\\textasciicircum}{\\textbackslash}text\\{obs\\}\\$ than a certain threshold. These\nresults indicate that galaxies on the far side of a dense region are\nmore difficult to be detected (\"hidden\") in Ly\\${\\textbackslash}alpha\\$ because\nLy\\${\\textbackslash}alpha\\$ emission toward us is absorbed by dense neutral hydrogen. If\nthe same region is viewed from a different direction, a different set of\nLAEs will be selected as if galaxies are playing hide-and-seek using HI\ngas. 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