Comparing emission- and absorption-based gas-phase metallicities in GRB host galaxies at z=2-4 using JWST

Abstract

JWST/NIRSpec is providing sensitive spectroscopic observations of distant galaxies, extending our view of cosmic chemical evolution out to the epoch of reionization and down to galaxy masses 1-2 dex lower than previously possible at z>2. These observations nevertheless remain heavily dominated by light from luminous star forming regions. An alternative and sensitive probe of the metallicity of galaxies is through absorption lines imprinted on the luminous afterglow spectra of long gamma ray burst (GRBs) from intervening material within their host galaxy. However, these two independent but complementary probes need to be cross-calibrated before they can be combined. We present the first results from a cycle-1 JWST program to investigate the relation between the metallicity of the neutral gas measured with GRB afterglow absorption lines to the emission line metallicity of the star forming regions of the GRB host measured with NIRSpec. Using an initial sample of seven GRB host galaxies at z=2-4, we find a tight relation between absorption and emission line metallicities when using the recent Laseter et al. (2023) R^ metallicity diagnostic, implying a relatively chemically-homogeneous multi-phase interstellar medium, and indicating that absorption and emission line probes can be directly combined to investigate the chemical enrichment of galaxies. However, the relation is less clear when using other diagnostics, such as R23 and R3. Ultimate confirmation of the relation between absorption and emission line metallicities will require a more direct determination of the emission line metallicity via the detection of temperature-sensitive auroral lines in our GRB host galaxy sample.

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

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