Dense and Warm Neutral Gas in BR 1202-0725 at z = 4.7 as Traced by the [O I] 145 μm Line

Abstract

We report the detection of [O i ] 145.5 μ m 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 Ly α 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.