Journal Article

Supersolar metal abundances in two galaxies at <i>z</i> ∼ 3.57 revealed by the GRB 090323 afterglow spectrum*

S. Savaglio, A. Rau, J. Greiner, T. Krühler, S. McBreen, D. H. Hartmann, A. C. Updike, R. Filgas, S. Klose, P. Afonso, C. Clemens, A. Küpcü Yoldaş, F. Olivares E., V. Sudilovsky and G. Szokoly

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 1, pages 627-636
Published in print February 2012 | ISSN: 0035-8711
Published online January 2012 | e-ISSN: 1365-2966 | DOI:
Supersolar metal abundances in two galaxies at z ∼ 3.57 revealed by the GRB 090323 afterglow spectrum*

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We report on the surprisingly high metallicity measured in two absorption systems at high redshift, detected in the Very Large Telescope spectrum of the afterglow of the gamma-ray burst (GRB) GRB 090323. The two systems, at redshift z= 3.5673 and 3.5774 (separation Δv≈ 660 km s−1), are dominated by the neutral gas in the interstellar medium of the parent galaxies. From the singly ionized zinc and sulphur, we estimate oversolar metallicities of [Zn/H] =+0.29 ± 0.10 and [S/H] = +0.67 ± 0.34, in the blue and red absorber, respectively. These are the highest metallicities ever measured in galaxies at z > 3. We propose that the two systems trace two galaxies in the process of merging, whose star formation and metallicity are heightened by the interaction. This enhanced star formation might also have triggered the birth of the GRB progenitor. As typically seen in star-forming galaxies, the fine-structure absorption Si ii* is detected, both in z= 3.5774 ± 0.0005 and 3.5673 ± 0.0003. From the rest-frame ultraviolet emission in the GRB location, we derive a relatively high, not corrected for dust extinction, star formation rate ≈6 M yr−1. These properties suggest a possible connection between some high-redshift GRB host galaxies and high-z massive submillimetre galaxies, which are characterized by disturbed morphologies and high metallicities. Our result provides additional evidence that the dispersion in the chemical enrichment of the Universe at high redshift is substantial, with the existence of very metal-rich galaxies less than two billion years after the big bang.

Keywords: gamma-ray burst: individual: GRB 090323; galaxies: ISM; cosmology: observations

Journal Article.  6862 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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