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 $\hat{I}¼m$ 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 $>3\ddot{I}ƒ$ 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 $\sim 1mJy/500km{s}^{-1}$, 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 $M_{\odot }y{r}^{-1}$. We place upper limits on the gas mass of NGP6_D1 of $M_{H2}$ $<(1.1\pm 3.5)\times 10^{11}$ $M_{\odot }$, 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.

Accepoted for publication in MNRAS