The <i>Herschel</i> -SPIRE Dark Field III: probing the deepest <i>Herschel</i> field with SCUBA-2

ABSTRACT The Herschel-SPIRE Dark Field (SDF), originally observed for calibration of the SPIRE instrument, is the deepest far-infrared field to date (equivalent exposure time of $\sim 700$ h deg$^{-1}$) at wavelengths 250–$500\,\, \mu$m, probing below the SPIRE confusion limit. We present SCUBA-2 $850\,\, \mu$m observations covering the central Deep Region, detecting 36 sources with number counts consistent with previous surveys at this wavelength. Cross-identification with multiwavelength catalogues reveals 20 SPIRE-dropouts, whose stacked SPIRE fluxes suggest a photometric redshift of $z \sim 4$. Photo-z estimates from the magphys, bagpipes, and mmpz tools are consistent within the uncertainties for 3 sources ($z \sim 1$–2), while 13 are inconsistent, and 20 lack sufficient data due to counterpart ambiguity or non-detections. We flag a total of 9 sources as potentially lensed and 15 as potential multiples. Spectral energy distribution fitting yields average properties for SPIRE-detected sources of log$_{10}(L_{\text{FIR}}$/L$_{\odot }) = 13.6\pm 0.1$, log$_{10}(M_{\text{dust}}$/M$_{\odot }) = 8.1\pm 0.1$, and SFR$_{\text{obsc}} = 7000\pm 1900\,$ M$_{\odot}$ yr$^{-1}$. The stacked dropouts yield log$_{10}(L_{\text{FIR}}$/L$_{\odot }) = 12.8^{+0.5}_{-0.7} [12.5^{+0.4}_{-0.6}]$, log$_{10}(M_{\text{dust}}$/M$_{\odot }) = 6.52^{+0.33}_{-0.41} [6.71^{+0.19}_{-0.23}]$, and SFR$_{\text{obsc}} = 700^{+1500}_{-560} [350^{+420}_{-250}]\,$ M$_{\odot}$ yr$^{-1}$ at a derived $z = 6.7^{+2.4}_{-3.0}$ (at a fixed $z = 4.0^{+0.1}_{-0.6}$) contributing $\sim 15$ ($\sim 2$) per cent of the cosmic star formation rate density (SFRD) at that epoch. We find an excess at $z\sim 1$ in the SFRD of SPIRE-detected sources, consistent with the sub-mJy bump in SPIRE number counts previously identified in this field, possibly tracing large-scale structure.