The metal-enriched host of an energetic y-ray burst at z ≈ 1.6

T. Krühler, J. P. U. Fynbo, S. Geier, J. Hjorth, D. Malesani, B. Milvang-Jensen, A. J. Levan, M. Sparre, D. J. Watson, T. Zafar

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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 [NII] at this redshift.

Results. The host of the energetic (Ey,iso ~ 2 × 1053 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 yr-1. Its ISM is significantly enriched with metals. Specifically, [NII]/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. For its measured stellar mass M* = 8.0-1.6+1.3 × 109 M 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 fesc ≲ 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 [NII] 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.
Original languageEnglish
Article numberA8
Pages (from-to)1-11
Number of pages11
JournalAstronomy and Astrophysics
Volume546
DOIs
Publication statusPublished - Oct 2012
Externally publishedYes

Keywords

  • gamma-ray burst: general
  • gamma-ray burst: individual: GRB 080605
  • ISM: abundances
  • galaxies: star formation
  • galaxies: high-redshift

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    Krühler, T., Fynbo, J. P. U., Geier, S., Hjorth, J., Malesani, D., Milvang-Jensen, B., ... Zafar, T. (2012). The metal-enriched host of an energetic y-ray burst at z ≈ 1.6. Astronomy and Astrophysics, 546, 1-11. [A8]. https://doi.org/10.1051/0004-6361/201118670