2021ApJ...913...41L

We report the detection of [O I] 145.5 µm 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 Lyα 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] >= 0.13). The high [O I]/[C II] ratios and the joint analysis with previous detection of [N II] lines for both of 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 [N II] 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 BR 1202-0725 system with a relatively short amount of integration with Atacama Large Millimeter/submillimeter Array (ALMA) demonstrates the great potential of this line as a dense gas tracer for high-z galaxies.