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Probing the dark ages with metal absorption lines

Oh, S. Peng (2002) Probing the dark ages with metal absorption lines. Monthly Notices of the Royal Astronomical Society, 336 (3). pp. 1021-1029. ISSN 0035-8711. doi:10.1046/j.1365-8711.2002.05859.x.

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Recent observations of high-redshift quasars at z∼ 6 have finally revealed complete Gunn–Peterson absorption. However, this at best constrains the volume-weighted and mass-weighted neutral fractions to be x^V_(HI) ≥ 10^(−3) and x^M_(HI) ≥ 10^(−2) respectively; stronger constraints are not possible because of the high optical depth for hydrogen Lyman transitions. Here I suggest certain metal lines as tracers of the hydrogen neutral fraction. These lines should cause unsaturated absorption when the intergalactic medium is almost fully neutral, if it is polluted to metallicities Z∼ 10^(−3.5) – 10^(−2.5) Z_⊙. Such a minimal level of metal pollution is inevitable in the middle to late stages of reionization unless quasars rather than stars are the dominant source of ionizing photons. The O I line at 1302 Å is particularly promising: the O I and H ionization potentials are almost identical, and O I should be in very tight charge exchange equilibrium with H. The Si II 1260 Å transition might also be observable. At high redshift, overdense regions are the first to be polluted to high metallicity but the last to remain permanently ionized, as a result of the short recombination times. Such regions should produce a fluctuating O I and Si II forest, which, if observed, would indicate large quantities of neutral hydrogen. The O I forest may already be detectable in the Sloan Digital Sky Survey z= 6.28 quasar. If seen in future high-redshift quasars, the O I and Si II forests will probe the topology of reionization and metal pollution in the early Universe. If, in addition, the H I optical depth can be measured from the damping wing of a high-redshift γ-ray burst, they will yield a very robust measure of the metallicity of the high-redshift Universe.

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Additional Information:© 2002 RAS. Accepted 2002 July 5. Received 2002 June 20; in original form 2002 February 8. I am grateful to Michael Strauss for detailed comments, and Charles Steidel for an informative conversation on some of the observational issues. I also thank Xiaohui Fan, Avi Loeb, Evan Scannapieco and David Spergel for stimulating conversations, and Marc Kamionkowski for encouragement and advice. I acknowledge NSF grant AST-0096023 for support.
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Subject Keywords:galaxies: high-redshift – intergalactic medium – quasars: absorption lines – cosmology: theory – early Universe
Issue or Number:3
Record Number:CaltechAUTHORS:20180413-161551950
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Official Citation:S. Peng Oh; Probing the dark ages with metal absorption lines, Monthly Notices of the Royal Astronomical Society, Volume 336, Issue 3, 1 November 2002, Pages 1021–1029,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85861
Deposited By: George Porter
Deposited On:16 Apr 2018 15:14
Last Modified:15 Nov 2021 20:32

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