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The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium

Becker, George D.; Pettini, Max; Rafelski, Marc; D’Odorico, Valentina; Boera, Elisa; Christensen, Lise; Cupani, Guido; Ellison, Sara L.; Farina, Emanuele Paolo; Fumagalli, Michele; López, Sebastian; Neeleman, Marcel; Ryan-Weber, Emma V.; Worseck, Gábor

The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium Thumbnail


Authors

George D. Becker

Max Pettini

Marc Rafelski

Valentina D’Odorico

Elisa Boera

Lise Christensen

Guido Cupani

Sara L. Ellison

Emanuele Paolo Farina

Sebastian López

Marcel Neeleman

Emma V. Ryan-Weber

Gábor Worseck



Abstract

We present a survey for metal absorption systems traced by neutral oxygen over 3.2 < z < 6.5. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199 QSOs with redshifts up to 6.6. In total, we detect 74 O i absorbers, of which 57 are separated from the background QSO by more than 5000 km s−1. We use a maximum likelihood approach to fit the distribution of O i λ1302 equivalent widths in bins of redshift and from this determine the evolution in number density of absorbers with W 1302 > 0.05 Å, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7–4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z ~ 6 than at lower redshifts. Absorption from highly ionized metals traced by C iv is also weaker in higher-redshift O i systems, supporting this picture. The evolution of O i absorbers implies that metal-enriched circumgalactic gas at z ~ 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.

Citation

Becker, G. D., Pettini, M., Rafelski, M., D’Odorico, V., Boera, E., Christensen, L., …Worseck, G. (2019). The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium. Astrophysical Journal, 883(2), Article 163. https://doi.org/10.3847/1538-4357/ab3eb5

Journal Article Type Article
Acceptance Date Aug 26, 2019
Online Publication Date Oct 1, 2019
Publication Date Oct 1, 2019
Deposit Date Oct 31, 2019
Publicly Available Date Oct 31, 2019
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 883
Issue 2
Article Number 163
DOI https://doi.org/10.3847/1538-4357/ab3eb5

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Copyright Statement
© 2019. The American Astronomical Society. All rights reserved.





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