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We report the detection of [O I]145.5um in the BR 1202-0725 system, a compactgroup at z=4.7 consisting of a quasar (QSO), a submillimeter-bright galaxy(SMG), and three faint Lya emitters. By taking into account the previousdetections and upper limits, the [O I]/[C II] line ratios of the now five knownhigh-z galaxies are higher than or on the high-end of the observed values inlocal galaxies ([O I]/[C II]$\gtrsim$0.13). The high [O I]/[C II] ratios andthe joint analysis with the previous detection of [N II] lines for both the QSOand the SMG suggest the presence of warm and dense neutral gas in these highlystar-forming galaxies. This is further supported by new CO (12-11) linedetections and a comparison with cosmological simulations. There is a possiblepositive 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 thatthe denser the ionized gas, the denser and warmer the neutral gas (or viceversa). The detection of the [O I] line in the BR1202-0725 system with arelatively short amount of integration with ALMA demonstrates the greatpotential of this line as a dense gas tracer for high-z galaxies.

Dense and warm neutral gas in BR1202-0725 at z = 4.7 as traced by the [O I] 145 um line