Abstract

We present Herschel-PACS spectroscopy of four main-sequence star-forming galaxies at z similar to 1.5. We detect [OI]63 mu m line emission in BzK-21000 at z = 1.5213, and measure a line luminosity, L-[OI]63 mu m = (3.9 +/- 0.7) x 10(9) L-circle dot. Our PDR modelling of the interstellar medium in BzK-21000 suggests a UV radiation field strength, G similar to 320G(0), and gas density, n similar to 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 [OI]63 mu m line luminosity, L-[OI]63 mu m = (1.1 +/- 0.2) x 10(9) L-circle dot. We find that the implied luminosity ratio, L-[OI]63 mu m/L-IR, 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 similar to 6. The high [OI]63 mu m line luminosities observed in BzK-21000 and the z similar to 1-3 dusty and sub-mm luminous star-forming galaxies may be associated with extended reservoirs of low density, cool neutral gas.