Abstract The Herschel-SPIRE Dark Field (SDF), originally observed for calibration of the SPIRE instrument, is the deepest far-infrared (FIR) field to date (equivalent exposure time of ∼700 hr deg−1) at wavelengths 250–500 μm, probing below the SPIRE confusion limit. We present SCUBA-2 850 μm 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 ∼ 4. Photo-z estimates from the magphys, bagpipes and mmpz tools are consistent within the uncertainties for 3 sources (z ∼ 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. SED fitting yields average properties for SPIRE-detected sources of log10 (LFIR / L⊙) = 13. 6 ± 0. 1, log10 (Mdust / M⊙) = 8. 1 ± 0. 1, and SFRobsc = 7000 ± 1900 M⊙ yr−1. The stacked dropouts yield log10 (LFIR / L) = 12. 8^+0. 5-₀. ₇ 12. 5^+0. 4-₀. ₆, log10 (Mdust / M) = 6. 52^+0. 33-₀. ₄₁ 6. 71^+0. 19-₀. ₂₃, and SFR₎₁ₒ₂ = 700^+1500-₅₆₀ 350^+420-₂₅₀\, M⊙ yr−1 at a derived z = 6. 7^+2. 4-₃. ₀ at a fixed z = 4. 0^+0. 1-₀. ₆ contributing ∼15 ∼2 per cent of the cosmic SFRD at that epoch. We find an excess at z ∼ 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.
Parmar et al. (Wed,) studied this question.