Deposited in DRO : 21 June 2013 Version of attached le : Other Peer-review status of attached

We presentHerschel -PACS spectroscopy of the [OI]63 μm far-infrared cooling line from a sample of six unlensed and spectroscopically-confirmed 87 0μm-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 sur vey of [OI]63μm, one of the main photodissociation region (PDR) cooling lines, i n SMGs. New high-resolution ALMA interferometric 870μm continuum imaging confirms that these six Herschel -targeted SMG counterparts are bona fide sources of submm emission. We d etect [OI]63μm in two SMGs with a SNR> ∼ 3, tentatively detect [OI]63μm in one SMG, and constrain the line flux for the non-detections. We also exploit the combination of s ubmm continuum photometry from 250–870μm and our new PACS continuum measurements to constrain the fa r-infr red luminosity, LFIR, in these SMGs to. 30per cent. We find that SMGs do not show a deficit in their [OI]63μm–to–far-infrared (FIR) continuum luminosity ratios (wit h ratios ranging from ≃ 0.5–1.5 per cent), similar to what was seen previously for the [C II]158μm–to–FIR ratios in SMGs. These observed ratios are about an order of magnitud e higher than what is seen typically for local ultra luminous infrared galaxies (ULIRGs) , which adds to the growing body of evidence that SMGs are not simply ‘scaled up’ versions of l oca ULIRGs. Rather, the PDR line–to–LFIR ratios suggest that the star formation modes of SMGs are like ly more akin to that of local normal (lower-luminosity) star-forming gala xies, with the bulk of the star formation occurring in extended galaxy-scale ( ∼kpc) regions. These observations represent the first step towards a census of the major PDR cooling lines in typica l SMGs that will be attainable with ALMA, enabling detailed modelling to probe the global p ro erties of the star formation and the evolutionary status of SMGs.