Krühler, T., Fynbo, J. P. U., Geier, S., Hjorth, J., Malesani, D., Milvang-Jensen, B., Levan, Andrew J., Sparre, M., Watson, D. J. and Zafar, T. (2012) The metal-enriched host of an energeticγ-ray burst at z ≈ 1.6. Astronomy & Astrophysics, Volume 546 . Article number A8. doi:10.1051/0004-6361/201118670 ISSN 0004-6361.

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Abstract

Context. The star-forming nature of long γ-ray burst (GRB) host galaxies provides invaluable constraints on the progenitors of GRBs and might open a short-cut to the characteristics of typical star-forming galaxies throughout the history of the Universe. Due to the absence of near-infrared (NIR) spectroscopy, however, detailed investigations, specifically a determination of the gas-phase metallicity of gamma-ray burst hosts, was largely limited to redshifts z < 1 to date. Aims. We observed the galaxy hosting GRB 080605 at z = 1.64 using optical/NIR spectroscopy and high-resolution HST/WFC3 imaging in the rest-frame wavelength range between 1150 and 8700 Å. These data allow us to study a z > 1 GRB host in unprecedented detail and investigate the relation between GRB hosts and field galaxies. Methods. We availed of VLT/X-shooter optical/NIR spectroscopy to measure the metallicity, electron density, star-formation rate (SFR), and reddening of the host of GRB 080605. Specifically, we used different strong-line diagnostics to robustly measure the gas-phase metallicity within the interstellar medium (ISM) for the first time based on [N ii] at this redshift. Results. The host of the energetic (E γ,iso ~ 2 × 10 53 erg) GRB 080605 at z ~ 1.64 is a morphologically complex, vigorously star-forming galaxy with an Hα-derived SFR of 31 -6 +12 M ȯ yr -1. Its ISM is significantly enriched with metals. Specifically, [N ii]/Hα = 0.14 ± 0.02, which yields an oxygen abundance 12 + log (O/H) between 8.3 and 8.6 depending on the adopted strong-line calibrator. This corresponds to values in the range of 0.4-0.8 Z ȯ. For its measured stellar mass M * = 8.0 -1.6 +1.3 × 10 9 M ȯ and SFR, this value is consistent with the fundamental metallicity relation defined by star-forming field galaxies. The absence of strong Lyα emission constrains the escape fraction of resonantly-scattered Lyα photons to f esc ≲ 0.08. Conclusions. Our observations provide a detailed picture of the conditions in the ISM of a highly star-forming galaxy with irregular morphology at z ~ 1.6. They include the first robust metallicity measurement based on [N ii] for a GRB host at z > 1 and directly illustrate that GRB hosts are not necessarily metal-poor, both on absolute scales as well as relative to their stellar mass and SFR. GRB hosts could thus be fair tracers of the population of ordinary star-forming galaxies at high redshift.

Item Type:	Journal Article
Divisions:	Faculty of Science, Engineering and Medicine > Science > Physics
Journal or Publication Title:	Astronomy & Astrophysics
Publisher:	EDP Sciences
ISSN:	0004-6361
Official Date:	October 2012
Dates:	Date Event October 2012 Published
Volume:	Volume 546
Page Range:	Article number A8
DOI:	10.1051/0004-6361/201118670
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	European Commission under the Marie Curie IntraEuropean Fellowship Programme in FP7, European Research Council-StG (ERC), Danish National Research Foundation
Grant number:	EGGS-278202 (ERC)

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