Long gamma ray bursts (GRBs) are the most energetic events known since the Big Bang and can be seen out to the farthest reaches of the Universe. They signal the catastrophic death of a star, and their very luminous and multi-wavelength afterglow provides an almost perfect background light source that shines through their host galaxy. From the absorption imprint left on GRB afterglow spectra from intervening hydrogen and heavy elements, GRBs offer a powerful probe to study the chemical composition of their distant, star forming host galaxies capable of measuring elemental abundances that are one hundred times smaller than that of our Milky Way.

Now a team lead by Dr Patricia Schady at the University of Bath have been able to combine, for the first time, these sensitive absorption-line measurements with emission line spectroscopy of the same sample of GRB host galaxies observed with the James Webb Space Telescope (JWST). It is only through the unprecedented, near-infrared sensitivity of JWST that such analysis has been made possible. Absorption and emission line data probe interstellar material at different ionisation states, and possibly also in spatially distinct regions of the galaxy. Specifically, whereas emission lines probe the ionised gas within star forming regions of a galaxy, absorption lines are especially suited to probe the neutral gas within the interstellar medium.

Dr Patricia Schady and her team used this combination of data to study the chemical composition of gas in star forming regions and of neutral gas within their sample of distant GRB host galaxies. Given that heavy elements are formed by stars, one may expect the gas within star forming regions to be more chemically enriched than the surrounding neutral material. The JWST data instead showed evidence for surprising chemical homogeneity between the multi-phase gas, implying that stellar outflows efficiently distribute newly enriched material throughout the galaxy.

Aside from the implication on feedback processes and the evolution of galaxies, these results pave the way for combining absorption and emission line measurements in the future to study the cosmic chemical evolution of the Universe out to more distant and less luminous galaxies than is possible using just a single technique.

The results from this research were published last month in the Monthly Notices of the Royal Astronomical Society.