Detection of the [O i] 63 µm emission line from the z = 6.04 quasar J2054–0005.

This article reports the highest-redshift detection of the [O i] 63 μm fine-structure line from the luminous quasar SDSS J2054−0005 at redshift z = 6.04, based on Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 9 observations. The [O i] 63 μm line luminosity is measured as (4.5 ± 1.5) × 10⁹ L_⊙, with a luminosity ratio to far-infrared (FIR) emission consistent with values observed in the local Universe, and a line luminosity ratio to [C ii] 158 μm of 1.3 ± 0.5. Photodissociation region (PDR) modeling, accounting for contributions from active galactic nucleus (AGN) activity and ionized gas, indicates a gas density of log(n_H/cm³) = 3.7 ± 0.3 and an incident far-ultraviolet radiation field of log(G/G₀) = 3.0 ± 0.1, comparable to those found in nearby star-forming galaxies and lower-redshift sub-millimeter galaxies. The study also suggests possible self-absorption in the [O i] 63 μm line, though deeper observations are needed for confirmation, demonstrating ALMA's capability to probe dense, warm interstellar medium conditions in high-redshift quasar host galaxies.