2020ApJ...889L..11R

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) x 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 x 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 G₀, 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.