2012MNRAS.427..520C

We present Herschel-PACS spectroscopy of the [OI]63 µm far-infrared cooling line from a sample of six unlensed and spectroscopically confirmed 870 µm 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 [OI]63 µm, one of the main photodissociation region (PDR) cooling lines, in SMGs. New high-resolution Atacama Large Millimetre Array (ALMA) interferometric 870 µm continuum imaging confirms that these six Herschel-targeted SMG counterparts are bona fide sources of submm emission. We detect [OI]63 µm in two SMGs with an SNR ≳ 3, tentatively detect [OI]63 µm 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 µm and our new PACS continuum measurements to constrain the far-infrared luminosity, L_FIR, in these SMGs to ≲ 30per cent. We find that SMGs do not show a deficit in their [OI]63 µm-to-far-infrared (FIR) continuum luminosity ratios (with ratios ranging from ≃ 0.5 to 1.5per cent), similar to what was seen previously for the [CII]158 µm-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 ( ∼ 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.