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

We report a ground-based detection of the [O i] 63 mu m 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 10(9) 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 = 10(4) 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 similar to 15 minutes on-source, opening a new window to study the properties of the earliest DSFGs.