J. M. Simpson, Ian Smail, A. M. Swinbank, S. C. Chapman, J. E. Geach, R. J. Ivison, A. P. Thomson, I. Aretxaga, A. W. Blain, W. I. Cowley, Chian Chou Chen, K. E K Coppin, J. S. Dunlop, A. C. Edge, D. Farrah, E. Ibar, A. Karim, K. K. Knudsen, R. Meijerink, M. J. MichałowskiD. Scott, M. Spaans, P. P. Van Der Werf

Research output: Contribution to journalArticlepeer-review


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 degree<sup>2</sup> 850 μm maps from the SCUBA-2 Cosmology Legacy Survey, and are representative of the brightest sources in the field (median S<inf>SCUBA-2</inf>= 8.7 ± 0.4 mJy). We detect 52 sub-millimeter galaxies (SMGs) at >4σ significance in our 30 ALMA maps. In 61<inf>-15</inf><sup>+19</sup>% 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<inf>IR</inf> ≳ 10<sup>12</sup> L<inf>⊙</inf>. The brightest SMG contributes on average 80<inf>-2</inf><sup>+6</sup>% of the single-dish flux density, and in the ALMA maps containing ≥2 SMGs the secondary SMG contributes 25<inf>-5</inf><sup>+1</sup>% of the integrated ALMA flux. We construct source counts and show that multiplicity boosts the apparent single-dish cumulative counts by 20% at S<inf>870</inf> > 7.5 mJy, and by 60% at S<inf>870</inf> > 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 × 10<sup>12</sup> L<inf>⊙</inf> or a star formation rate (SFR) of ∼10<sup>3</sup> M<inf>⊙</inf> yr<sup>-1</sup>. For the typical sizes of these SMGs, which are resolved in our ALMA data with R<inf>e</inf> = 1.2 ± 0.1 kpc, this yields a limiting SFR density of ∼100 M<inf>⊙</inf> yr<sup>-1</sup> kpc<sup>-2</sup> Finally, the number density of S<inf>870</inf> ≳ 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.

Original languageEnglish
Article number128
Number of pages13
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 10 Jul 2015


  • galaxies: abundances
  • galaxies: high-redshift
  • galaxies: star formation
  • galaxies: starburst
  • submillimeter: galaxies


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