The Herschel -SPIRE Dark Field III: Probing the deepest Herschel field with SCUBA-2

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 SFRobsc = 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.