Dense and warm neutral gas in BR1202-0725 at z = 4.7 as traced by the [O
  I] 145 um line
  We report the detection of [O I]145.5um in the BR 1202-0725 system, a compact
group at z=4.7 consisting of a quasar (QSO), a submillimeter-bright galaxy
(SMG), and three faint Lya emitters. By taking into account the previous
detections and upper limits, the [O I]/[C II] line ratios of the now five known
high-z galaxies are higher than or on the high-end of the observed values in
local galaxies ([O I]/[C II]$\gtrsim$0.13). The high [O I]/[C II] ratios and
the joint analysis with the previous detection of [N II] lines for both the QSO
and the SMG suggest the presence of warm and dense neutral gas in these highly
star-forming galaxies. This is further supported by new CO (12-11) line
detections and a comparison with cosmological simulations. There is a possible
positive correlation between the [NII]122/205 line ratio and the [O I]/[C II]
ratio when all local and high-z sources are taken into account, indicating that
the denser the ionized gas, the denser and warmer the neutral gas (or vice
versa). The detection of the [O I] line in the BR1202-0725 system with a
relatively short amount of integration with ALMA demonstrates the great
potential of this line as a dense gas tracer for high-z galaxies.
