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

AbstraktAbstract

The subject of this work is the very energetic GRB 110918A (E-gamma,E-iso = 1.9 x 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 beta = 0.70, which probed a sight-line with little extinction (A(V)(GRB) = 0.16 mag) and soft X-ray absorption (N-H,N-X = (1.6 +/- 0.5) x 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 similar to 1 galaxies selected through similar afterglows to date. It has a large spatial extent with a half-light radius of R-1/2 similar to 10 kpc, the highest stellar mass for z < 1.9 (log(M-*/M-circle dot) = 10.68 +/- 0.16), and an H alpha-based star formation rate of SFRH alpha = 41(-16)(+28) M-circle dot yr(-1). We measure a gas-phase extinction of A(V)(gas) similar to 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]/H alpha and [N II]/[O II] (12 + log (O/H) = 8.93 +/- 0.13 and 8.85(-0.18)(+0.14), respectively). This presents one of the very few robust metallicity measurements of GRB hosts at z similar to 1, and establishes that GRB hosts at z similar to 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.

The subject of this work is the very energetic GRB 110918A (E-gamma,E-iso = 1.9 x 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 beta = 0.70, which probed a sight-line with little extinction (A(V)(GRB) = 0.16 mag) and soft X-ray absorption (N-H,N-X = (1.6 +/- 0.5) x 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 similar to 1 galaxies selected through similar afterglows to date. It has a large spatial extent with a half-light radius of R-1/2 similar to 10 kpc, the highest stellar mass for z < 1.9 (log(M-*/M-circle dot) = 10.68 +/- 0.16), and an H alpha-based star formation rate of SFRH alpha = 41(-16)(+28) M-circle dot yr(-1). We measure a gas-phase extinction of A(V)(gas) similar to 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]/H alpha and [N II]/[O II] (12 + log (O/H) = 8.93 +/- 0.13 and 8.85(-0.18)(+0.14), respectively). This presents one of the very few robust metallicity measurements of GRB hosts at z similar to 1, and establishes that GRB hosts at z similar to 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.