First detection of the [OI] 63-um emission from a redshift 6 dusty
  galaxy
  We report a ground-based detection of the [OI] 63-um line in a z=6.027
gravitationally lensed dusty star-forming galaxy (DSFG) G09.83808 using the
APEX SEPIA 660 receiver, the first unambiguous detection of the [OI]63 line
beyond redshift 3, and the first obtained from the ground. The [OI]63 line is
robustly detected at 22$\pm$5 Jy km s$^{-1}$, corresponding to an intrinsic
(de-lensed) luminosity of $(5.4\pm1.3)\times10^{9}$ L$_\odot$. With the
[OI]63/[CII] luminosity ratio of 4, the [OI]63 line is the main coolant of the
neutral gas in this galaxy, in agreement with model predictions. The high
[OI]63 luminosity compensates for the pronounced [CII] deficit
([CII]/FIR$\simeq4\times10^{-4}$). Using photon-dominated region models, we
derive a source-averaged gas density $n=10^{4.0}$ cm$^{-3}$, and far-UV 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 [OI]63 line from z=6 non-lensed
DSFGs should be routinely detectable in ALMA Band 9 observations with $\sim$15
min on-source, opening a new window to study the properties of the earliest
DSFGs.
