The Rapid Decline in Metallicity of Damped Lyα Systems at z ~ 5

Rafelski, M.; Neeleman, M.; Fumagalli, M.; Wolfe, A.M.; Prochaska, J.X.

Abstract

We present evidence that the cosmological mean metallicity of neutral atomic hydrogen gas shows a sudden decrease at z > 4.7 down to $\langle {Z}\rangle =-2.03^{+0.09}_{-0.11}$, which is 6σ deviant from that predicted by a linear fit to the data at lower redshifts. This measurement is made possible by the chemical abundance measurements of eight new damped Lyα (DLA) systems at z > 4.7 observed with the Echellette Spectrograph and Imager on the Keck II Telescope, doubling the number of measurements at z > 4.7 to 16. Possible explanations for this sudden decrease in metallicity include a change in the physical processes that enrich the neutral gas within disks, or an increase of the covering factor of neutral gas outside disks due to a lower ultraviolet radiation field and higher density at high redshift. The later possibility would result in a new population of presumably lower metallicity DLAs, with an increased contribution to the DLA population at higher redshifts resulting in a reduced mean metallicity. Furthermore, we provide evidence of a possible decrease at z > 4.7 in the comoving metal mass density of DLAs, ρmetals(z)DLA, which is flat out to z ~ 4.3. Such a decrease is expected, as otherwise most of the metals from star-forming galaxies would reside in DLAs by z ~ 6. While the metallicity is decreasing at high redshift, the contribution of DLAs to the total metal budget of the universe increases with redshift, with DLAs at z ~ 4.3 accounting for ~20% as many metals as produced by Lyman break galaxies.

Citation

Rafelski, M., Neeleman, M., Fumagalli, M., Wolfe, A., & Prochaska, J. (2014). The Rapid Decline in Metallicity of Damped Lyα Systems at z ~ 5. Astrophysical Journal Letters, 782(2), Article L29. https://doi.org/10.1088/2041-8205/782/2/l29

• Journal Article Type: Article
• Acceptance Date: Jan 15, 2014
• Online Publication Date: Feb 5, 2014
• Publication Date: Feb 20, 2014
• Deposit Date: Apr 6, 2014
• Publicly Available Date: Jul 3, 2014
• Journal: Astrophysical Journal Letters
• Print ISSN: 2041-8205
• Electronic ISSN: 2041-8213
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 782
• Issue: 2
• Article Number: L29
• DOI: https://doi.org/10.1088/2041-8205/782/2/l29
• Keywords: Galaxies: abundances, Galaxies: evolution, Galaxies: high-redshift, Galaxies: ISM, Quasars: absorption lines.
• Public URL: https://durham-repository.worktribe.com/output/1434855