A SCUBA-2 selected Herschel-SPIRE dropout and the nature of this population

Abstract

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 +/- 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 Redshift Search Receiver and IRAM 30 m EMIR spectra of NGP6_D1 across 32 GHz of bandwidth despite reaching detection limits of similar to 1mJy/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 similar to 10(8)-10(9) M-circle dot and a star-formation rate of similar to 500 M-circle dot yr(-1). We place upper limits on the gas mass of NGP6_D1 of M-H2 <(1.1 +/- 3.5) x 10(11) M-circle dot, consistent with a gas-to-dust ratio of similar to 100-1000. We discuss the nature of NGP6_D1 in the context of the broader sub-mm population, and find that comparable SPIRE dropouts account for similar to 20 per cent of all SCUBA-2 detected sources, but with a similar flux density distribution to the general population.