Volume 556, August 2013
|Number of page(s)||12|
|Published online||19 July 2013|
Max-Planck-Institut für extraterrestrische Physik,
2 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
3 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
4 The University of Leicester, Department of Physics and Astronomy, University Road, Leicester, LE1 7RH, UK
5 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
6 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horská 3a/22, 12800 Prague, Czech Republic
7 Università degli studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
8 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
9 Department of Astronomy & Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
10 Department of Physics, University of Warwick, Coventry CV4 7AL, UK
11 Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
12 Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
Accepted: 30 May 2013
Galaxies selected through long γ-ray bursts (GRBs) could be of fundamental importance when mapping the star formation history out to the highest redshifts. Before using them as efficient tools in the early Universe, however, the environmental factors that govern the formation of GRBs need to be understood. Metallicity is theoretically thought to be a fundamental driver in GRB explosions and energetics, but it is still, even after more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias, which prevented high-mass and thus likely high-metallicity GRB hosts from being detected in the first place, and a lack of efficient instrumentation, which limited spectroscopic studies, including metallicity measurements, to the low-redshift end of the GRB host population. The subject of this work is the very energetic GRB 110918A (Eγ,iso = 1.9 × 1054 erg), for which we measure a redshift of z = 0.984. GRB 110918A gave rise to a luminous afterglow with an intrinsic spectral slope of β = 0.70, which probed a sight-line with little extinction (AGRBV = 0.16 mag) and soft X-ray absorption (NH,X = (1.6 ± 0.5) × 1021 cm-2) typical of the established distributions of afterglow properties. However, photometric and spectroscopic follow-up observations of the galaxy hosting GRB 110918A, including optical/near-infrared photometry with the Gamma-Ray burst Optical Near-infrared Detector and spectroscopy with the Very Large Telescope/X-shooter, reveal an all but average GRB host in comparison to the z ~ 1 galaxies selected through similar afterglows to date. It has a large spatial extent with a half-light radius of R1/2 ~ 10 kpc, the highest stellar mass for z < 1.9 (log (M∗/M⊙) = 10.68 ± 0.16), and an Hα-based star formation rate of SFRHα = 41+28-16M⊙ yr-1. We measure a gas-phase extinction of AgasV ~ 1.8 mag through the Balmer decrement and one of the largest host-integrated metallicities ever of around solar using the well-constrained ratios of [N ii]/Hα and [N ii]/[O ii] (12 + log (O/H) = 8.93 ± 0.13 and 8.85+0.14-0.18, respectively). This presents one of the very few robust metallicity measurements of GRB hosts at z ~ 1, and establishes thatGRB hosts at z ~ 1 can also be very metal rich. It conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation between metallicity and energy release in GRBs.
Key words: gamma-ray burst: general / gamma-ray burst: individual: 110918A / ISM: general / galaxies: abundances / galaxies: photometry / galaxies: star formation
Based on observations made with telescopes at the European Southern Observatory at La Silla/Paranal, Chile under program 090.A-0760(A).
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2013
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