First joint absorption and T_e-based metallicity measured in a GRB host galaxy at z = 4.28 using JWST/NIRSpec

Abstract

We present the first gamma-ray burst (GRB) host galaxy with a measured absorption line and electron temperature (T_e) basedmetallicity, using the temperature sensitive [O III]λ4363 auroral line detected in the JWST/NIRSpec spectrum of the host ofGRB 050505 at redshift z = 4.28. We find that the metallicity of the cold interstellar gas, derived from the absorption lines inthe GRB afterglow, of 12 + log(O/H) ∼ 7.7 is in reasonable agreement with the temperature-based emission line metallicityin the warm gas of the GRB host galaxy, which has values of 12 + log(O/H) = 7.80±0.19 and 7.96±0.21 for two commonindicators. When using strong emission line diagnostics appropriate for high-z galaxies and sensitive to ionization parameter, wefind good agreement between the strong emission line metallicity and the other two methods. Our results imply that, for the hostof GRB050505, mixing between the warm and the cold interstellar medium along the line of sight to the GRB is efficient, andthat GRB afterglow absorption lines can be a reliable tracer of the metallicity of the galaxy. If confirmed with a large sample,this suggest that metallicities determined via GRB afterglow spectroscopy can be used to trace cosmic chemical evolution to theearliest cosmic epochs and in galaxies far too faint for emission line spectroscopy, even for JWST.

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ORCID for Phil Wiseman: orcid.org/0000-0002-3073-1512

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