Galaxies selected through long \gamma -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 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 , that prevented high-mass and thus likely high-metallicity GRB hosts to be detected in the first place , and a lack of efficient instrumentation , that 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 _ { \gamma, \mathrm { iso } } = 1.9 \times 10 ^ { 54 } \mathrm { 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 \beta = 0.70 , which probed a sight-line with little extinction ( A ^ { \mathrm { GRB } } _ { V } = 0.16 \mathrm { mag } ) and soft X-ray absorption ( N _ { H,X } = ( 1.6 \pm 0.5 ) \times 10 ^ { 21 } cm ^ { -2 } ) typical of the established distributions of afterglow properties .
Photometric and spectroscopic follow-up observations of the galaxy hosting GRB 110918A , including optical/NIR photometry with GROND and spectroscopy with VLT/X-shooter , however , reveal an all but average GRB host in comparison to the z \sim 1 galaxies selected through similar afterglows to date .
It has a large spatial extent with a half-light radius of R _ { \frac { 1 } { 2 } } \sim 10 kpc , the highest stellar mass for z < 1.9 ( \log ( M _ { * } / M _ { \sun } ) = 10.68 \pm 0.16 ) , and an H \alpha -based star formation rate of SFR _ { H \alpha } = 41 ^ { +28 } _ { -16 } { M _ { \sun } yr ^ { -1 } } .
We measure a gas-phase extinction of A ^ { \mathrm { gas } } _ { V } \sim 1.8 \mathrm { 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 ] /H \alpha , and [ N II ] / [ O II ] ( 12 + log ( O/H ) = 8.93 \pm 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 \sim 1 , and establishes that GRB hosts at z \sim 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 .