Herschel-PACS observations of [O I]63 um towards submillimetre galaxies at z~1

We present Herschel-PACS spectroscopy of the [O I]63 um far-infrared cooling line from a sample of six unlensed and spectroscopically confirmed 870 um selected submillimetre (submm) galaxies (SMGs) at 1.1 <z <1.6 from the LABOCA Extended Chandra Deep Field South (ECDFS) Submm Survey (LESS). This is the first survey of [O I]63 um, one of the main photodissociation region (PDR) cooling lines, in SMGs. New high-resolution Atacama Large Millimetre Array (ALMA) interferometric 870 mu m continuum imaging confirms that these six Herschel-targeted SMG counterparts are bona fide sources of submm emission. We detect [O I]63 um in two SMGs with an SNR greater than or similar to 3, tentatively detect [O I]63 um in one SMG and constrain the line flux for the non-detections. We also exploit the combination of submm continuum photometry from 250 to 870 um and our new PACS continuum measurements to constrain the far-infrared luminosity, LFIR, in these SMGs to less than or similar to 30 per cent. We find that SMGs do not show a deficit in their [O I]63 um-to-far-infrared (FIR) continuum luminosity ratios (with ratios ranging from similar or equal to 0.5 to 1.5 per cent), similar to what was seen previously for the [C II]158 um-to-FIR ratios in SMGs. These observed ratios are about an order of magnitude higher than what is seen typically for local ultraluminous infrared galaxies (ULIRGs), which adds to the growing body of evidence that SMGs are not simply scaled up versions of local ULIRGs. Rather, the PDR line-to-L-FIR ratios suggest that the star formation modes of SMGs are likely more akin to that of local normal (lower-luminosity) star-forming galaxies, with the bulk of the star formation occurring in extended galaxy-scale (similar to kpc) regions. These observations represent the first step towards a census of the major PDR cooling lines in typical SMGs that will be attainable with ALMA, enabling detailed modelling to probe the global properties of the star formation and the evolutionary status of SMGs.