We present Herschel-PACS spectroscopy of the [O I]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 [O I]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 [O I]63 μm in two SMGs with an SNR ≳ 3,
tentatively detect [O I]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, LFIR,
in these SMGs to ≲30 per cent. We find that SMGs do not show a
deficit in their [O I]63 μm-to-far-infrared (FIR) continuum
luminosity ratios (with ratios ranging from ≃0.5 to 1.5 per cent),
similar to what was seen previously for the [C II]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-LFIR 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.