The low-extinction afterglow in the solar-metallicity host galaxy of γ -ray burst 110918A

摘要

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 (E _(γ,iso) = 1.9 × 10~(54) 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 (A~(GRB) _V = 0.16 magAVGRB=0.16 mag) and soft X-ray absorption (N_(H,X) = (1.6 ± 0.5) × 10~(21) 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 ~R _(1/2) ～10R1210 kpc, the highest stellar mass for z < 1.9 (log (M _-/M_(a??)) = 10.68 ± 0.16), and an Hα-based star formation rate of SFR_(Hα) = 41 ~(+28) _(-16)SFRHα=41-16+28 M _(a??) yr~(-1). We measure a gas-phase extinction of A~(gas) _V ～1.8 magAVgas1.8 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)8.85-0.18+0.14, 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.