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

Abstract

We report a ground-based detection of the [O i ] 63 μ 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) × 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 × 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 ∼15 minutes on-source, opening a new window to study the properties of the earliest DSFGs.