First Detection of the [O i] 63 μm Emission from a Redshift 6 Dusty Galaxy

We report a ground-based detection of the [O i] 63 μm line in a z = 6.027 gravitationally lensed dusty star-forming galaxy (DSFG) G09.83808 using the Atacama Pathfinder EXperiment SEPIA 660 receiver, the first unambiguous detection of the [O i]₆₃ line beyond redshift 3, and the first obtained from the ground. The [O i]₆₃ line is robustly detected at 22 ± 5 Jy km s⁻¹, corresponding to an intrinsic (de-lensed) luminosity of (5.4 ± 1.3) × 10⁹ L _⊙. With the [O i]₆₃/[C ii] luminosity ratio of 4, the [O i]₆₃ line is the main coolant of the neutral gas in this galaxy, in agreement with model predictions. The high [O i]₆₃ luminosity compensates for the pronounced [C ii] deficit ([C ii]/FIR ≃ 4 × 10⁻⁴). Using photon-dominated region models, we derive a source-averaged gas density n = 10^4.0 cm⁻³, and FUV field strength G = 10⁴ G ₀, comparable to the z = 2–4 DSFG population. If G09.83808 represents a typical high-redshift DSFG, the [O i]₆₃ line from z = 6 non-lensed DSFGs should be routinely detectable in the Atacama Large Millimeter/submillimeter Array Band 9 observations with ∼15 minutes on-source, opening a new window to study the properties of the earliest DSFGs.