Origin of the low ionization state in the protocluster core

Determine whether high gas-phase metallicity is the primary cause of the low O32 values observed in the core galaxies of A2744-z7p9OD.

Background

The core galaxies of A2744-z7p9OD exhibit unusually low O32 values, indicating low ionization states relative to typical galaxies at comparable redshifts. The paper evaluates several possible drivers, including stellar mass, specific star-formation rate, star-formation-rate surface density, gas-phase metallicity, electron density, and neutral-gas reservoirs.

Gas-phase metallicity cannot be robustly determined because R23 is double-valued with respect to metallicity, depends on the adopted calibration, and is often constrained only by lower limits when Hβ is undetected. Consequently, although higher metallicity could lower the ionization parameter and contribute to the observed low O32 values, the paper leaves its role unresolved.