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Durham University

Department of Physics

Staff profile

Publication details for Professor Ian Smail

Simpson, J. M., Smail, I., Swinbank, A. M., Chapman, S. C., Geach, J. E., Ivison, R. J., Thomson, A. P., Aretxaga, I., Blain, A. W., Cowley, W. I., Chen, C.-C., Coppin, K. E. K., Dunlop, J. S., Edge, A. C., Farrah, D., Ibar, E., Karim, A., Knudsen, K. K., Meijerink, R., Michałowski, M. J., Scott, D., Spanns, M. & van der Werf, P. P. (2015). The SCUBA-2 Cosmology Legacy Survey: ALMA resolves the bright-end of the submillimeter number counts. The Astrophysical Journal 807(2): 128.

Author(s) from Durham


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.