Dusty star-forming galaxies ( DSFGs ) detected at z > 4 provide important examples of the first generations of massive galaxies .
However , few examples with spectroscopic confirmation are currently known , with Hershel struggling to detect significant numbers of z > 6 DSFGs .
NGP6_D1 is a bright 850 \mu m source ( 12.3 \pm 2.5 mJy ) with no counterparts at shorter wavelengths ( a SPIRE dropout ) .
Interferometric observations confirm it is a single source , with no evidence for any optical or NIR emission , or nearby likely foreground lensing sources .
No > 3 \sigma detected lines are seen in both LMT RSR and IRAM 30m EMIR spectra of NGP6_D1 across 32 GHz of bandwidth despite reaching detection limits of \sim 1 mJy / 500 km~ { } s ^ { -1 } , so the redshift remains unknown .
Template fitting suggests that NGP6_D1 is most likely between z = 5.8 and 8.3 .
SED analysis finds that NGP6_D1 is a ULIRG , with a dust mass \sim 10 ^ { 8 } - 10 ^ { 9 } M _ { \odot } and a SFR of \sim 500 M _ { \odot } ~ { } yr ^ { -1 } .
We place upper limits on the gas mass of NGP6_D1 of M _ { H 2 } < ( 1.1 ~ { } \pm~ { } 3.5 ) \times 10 ^ { 11 } M _ { \odot } , consistent with a gas-to-dust ratio of \sim 100 - 1000 .
We discuss the nature of NGP6_D1 in the context of the broader submm population , and find that comparable SPIRE dropouts account for \sim 20 % of all SCUBA-2 detected sources , but with a similar flux density distribution to the general population .