THE SCUBA-2 COSMOLOGY LEGACY SURVEY: ALMA RESOLVES THE BRIGHT-END OF THE SUB-MILLIMETER NUMBER COUNTS

We present high-resolution 870 μm Atacama Large Millimeter/sub-millimeter Array (ALMA) continuum maps of 30 bright sub-millimeter sources in the UKIDSS UDS field. These sources are selected from deep, 1 degree2 850 μm maps from the SCUBA-2 Cosmology Legacy Survey, and are representative of the brightest sources in the field (median ${S}_{\mathrm{SCUBA}-2}$ = 8.7 ± 0.4 mJy). We detect 52 sub-millimeter galaxies (SMGs) at >4σ significance in our 30 ALMA maps. In ${61}_{-15}^{+19}\%$ of the ALMA maps the single-dish source comprises a blend of ≥2 SMGs, where the secondary SMGs are Ultra-luminous Infrared Galaxies (ULIRGs) with ${L}_{\mathrm{IR}}$ $\gtrsim $ 1012 ${\text{}}{L}_{\odot }$. The brightest SMG contributes on average ${80}_{-2}^{+6}\%$ of the single-dish flux density, and in the ALMA maps containing ≥2 SMGs the secondary SMG contributes ${25}_{-5}^{+1}\%$ of the integrated ALMA flux. We construct source counts and show that multiplicity boosts the apparent single-dish cumulative counts by 20% at S870 > 7.5 mJy, and by 60% at S870 > 12 mJy. We combine our sample with previous ALMA studies of fainter SMGs and show that the counts are well-described by a double power law with a break at 8.5 ± 0.6 mJy. The break corresponds to a luminosity of ~6 × 1012 ${\text{}}{L}_{\odot }$ or a star formation rate (SFR) of ~103 ${\text{}}{M}_{\odot }\;{\mathrm{yr}}^{-1}$. For the typical sizes of these SMGs, which are resolved in our ALMA data with ${R}_{{\rm{e}}}$ = 1.2 ± 0.1 kpc, this yields a limiting SFR density of ~100 ${\text{}}{M}_{\odot }$ yr−1 kpc−2 Finally, the number density of S870 $\gtrsim $ 2 mJy SMGs is 80 ± 30 times higher than that derived from blank-field counts. An over-abundance of faint SMGs is inconsistent with line-of-sight projections dominating multiplicity in the brightest SMGs, and indicates that a significant proportion of these high-redshift ULIRGs are likely to be physically associated.