Detection of the [O I] 63 μm emission line from the $z = 6.04$ quasar J2054-0005

We report the highest-redshift detection of [O I] 63 μm from a luminous quasar, J2054-0005, at $z=6.04$ based on the Atacama Large Millimeter/sub-millimeter Array Band 9 observations. The [O I] 63 μm line luminosity is (4.5±1.5) × 10⁹~L, corresponding to the [O I] 63 μm-to-far-infrared luminosity ratio of ≈ 6.7×10⁻⁴, which is consistent with the value obtained in the local universe. Remarkably, [O I] 63 μm is as bright as [C II] 158 μm, resulting in the [O I]-to-[C II] line luminosity ratio of 1.3±0.5. Based on a careful comparison of the luminosity ratios of [O I] 63 μm, [C II] 158 μm, and dust continuum emission to models of photo-dissociation regions, we find that J2054-0005 has a gas density log(nH/cm⁻³)=3.7±0.3 and an incident far-ultraviolet radiation field of log(G/G₀)= 3.0±0.1, showing that [O I] 63 μm serves as an important coolant of the dense and warm gas in J2054-0005. A close examination of the [O I] and [C II] line profiles suggests that the [O I] line may be partially self-absorbed, however deeper observations are needed to verify this conclusion. Regardless, the gas density and incident radiation field are in a broad agreement with the values obtained in nearby star-forming galaxies and objects with [O I] 63 μm observations at $z=1-3$ with the Herschel Space Observatory. These results demonstrate the power of ALMA high-frequency observations targeting [O I] 63 μm to examine the properties of photo-dissociation regions in high-redshift galaxies.