Unresolved enrichment and metal-mixing history of diffuse gas at Cosmic Noon

Determine whether pristine gas survives during the peak of cosmic star formation, how the enrichment timescales of the circumgalactic medium and intergalactic medium compare with those of the interstellar medium, and how completely metals are mixed into diffuse gas at redshifts $1\le z\le2.2$.

Background

The paper identifies a substantial observational gap in measurements of ionized absorbers with low neutral-hydrogen column densities at Cosmic Noon, defined as $1\le z\le2.2$. Existing surveys primarily probe analogous gas at lower and higher redshifts, leaving the intermediate-redshift evolution of its metallicity distribution poorly constrained.

The unresolved issues concern three connected aspects of cosmic chemical evolution: the possible survival of pristine gas during the epoch of peak star formation and AGN activity; the relative timescales over which metals enrich the ISM, CGM, and IGM; and the degree to which metals become spatially mixed within diffuse circumgalactic and intergalactic gas. These questions motivate the BRIDGE survey and its extension to lower-column-density, IGM-like absorbers, but the quoted passage itself presents them as unresolved rather than reporting their definitive resolution.