A large, chemically enriched, neutral gas reservoir in a galaxy at z = 6.782

The physical and chemical characterization of galaxies formed within the first billion years after the Big Bang remains one of the central goals in contemporary astrophysics. For the last two decades, optical and near-infrared spectroscopy of long gamma-ray bursts (GRBs) have been heralded as an effective diagnostic to probe the interstellar medium (ISM) of the galaxies hosting these events and their metal and dust content, reaching even the most distant redshifts. An opportunity to fulfill this expectation was provided by the recent blast triggered by the Neil Gehrels Swift Observatory of GRB 240218A at redshift z = 6.782. From the GRB explosion, we were able to study a high-redshift galaxy selected in a complementary way with respect to flux-limited surveys, not depending on galaxy luminosity and stellar mass. Furthermore, the GRB afterglow allowed us to perform a coring of the galaxy ISM and to determine its kinematic and chemical properties. We present the VLT/X-shooter spectrum of its afterglow enabling the detection and the detailed characterization of neutral-hydrogen, low-ionization, high-ionization and fine-structure absorption lines, as well as excited level transitions. From this rich variety of absorption lines associated with gas inside and around the GRB host galaxy, we determined the metallicity, kinematics, chemical abundance pattern and dust depletion. This provides the first detailed characterization of the neutral gas of a galaxy at z > 6.5. Thanks to the presence of fine-structure absorption lines, we were able to estimate the distance of the closest absorbing gas clouds as d II = 620₋₁₄₀⁺²³⁰ pc. We determine a high neutral hydrogen column density, log(N(H I)/cm⁻²) = 22.5 ± 0.3, which is the highest one at z ≳ 6 determined so far for a GRB host galaxy, as well as a surprisingly high metal column density, log(N(Zn II)/cm⁻²)≥14.3. The observed metallicity of the host galaxy system is [Zn/H] ≥ −0.8, with a dust depletion level of [Zn/Fe] > 0.4. The high hydrogen column density, metal abundances, and dust depletion in the neutral gas align with those of the ionized gas of very high-redshift galaxies unveiled by ALMA and JWST, testifying that a rapid build-up of metals and dust, and massive neutral hydrogen reservoirs seem to be common features of galaxies in the early Universe. This research highlights the remarkable potential of GRBs as tools to investigate the detailed properties of galaxies deep into the re-ionization epoch, and stresses the importance of new missions capable of enlarging the sample of very high-redshift GRBs.