2020MNRAS.499.1788W

We present Herschel-PACS spectroscopy of four main-sequence star-forming galaxies at z ∼ 1.5. We detect [OI]63 µm line emission in BzK-21000 at z = 1.5213, and measure a line luminosity, L_[O I]63 µm_ = (3.9±0.7)×10⁹ L_☉. Our PDR modelling of the interstellar medium in BzK-21000 suggests a UV radiation field strength, G ∼ 320G₀, and gas density, n ∼ 1800 cm^–3, consistent with previous LVG modelling of the molecular CO line excitation. The other three targets in our sample are individually undetected in these data, and we perform a spectral stacking analysis which yields a detection of their average emission and an [O I]63 µm line luminosity, L_[O I]63 µm_ = (1.1±0.2)×10⁹ L_☉. We find that the implied luminosity ratio, L_[O I]63 µm_/LIR_, of the undetected BzK-selected star-forming galaxies broadly agrees with that of low-redshift star-forming galaxies, while BzK-21000 has a similar ratio to that of a dusty star-forming galaxy at z ∼ 6. The high [O I]63 µm line luminosities observed in BzK-21000 and the z ∼ 1-3 dusty and sub-mm luminous star-forming galaxies may be associated with extended reservoirs of low density, cool neutral gas.