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

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 |$\mathrm{ \mu}$| 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 >3σ 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 |$\sim 1\, \mathrm{mJy}/500 \, \mathrm{km~s}^{-1}$|⁠ , 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 ∼10⁸–10⁹ M_⊙ and a star-formation rate of ∼500 M_⊙ yr⁻¹. We place upper limits on the gas mass of NGP6_D1 of M _{H 2} <(1.1 ± 3.5) × 10¹¹ M_⊙, consistent with a gas-to-dust ratio of ∼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 ∼20 per cent of all SCUBA-2 detected sources, but with a similar flux density distribution to the general population. [ABSTRACT FROM AUTHOR]