An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: number counts of submillimeter galaxies

Abstract

We report the first results of AS2UDS: an 870 $\hat{I}¼$m continuum survey with the Atacama Large Millimeter/Submillimeter Array (ALMA) of a total area of $\sim$ 50 arcmin${}^2$ comprising a complete sample of 716 submillimeter sources drawn from the SCUBA-2 Cosmology Legacy Survey (S2CLS) map of the UKIDSS/UDS field. The S2CLS parent sample covers a 0.96 degree${}^2$ field at $\ddot{I}{ƒ}_{850}=0.90\pm 0.05$ mJy beam${}^{-1}$. Our deep, high-resolution ALMA observations with $\ddot{I}{ƒ}_{870}\sim$ 0.25 mJy and a 0.15"--0.30" FWHM synthesized beam, provide precise locations for 695 submillimetre galaxies (SMGs) responsible for the submillimeter emission corresponding to 606 sources in the low resolution, single-dish map. We measure the number counts of SMGs brighter than $S_{870}\ge$ 4 mJy, free from the effects of blending and show that the normalisation of the counts falls by 28 $\pm$ 2% in comparison to the SCUBA-2 parent sample, but that the shape remains unchanged. We determine that 44${}_{-14}^{+16}$% of the brighter single-dish sources with $S_{850}\ge$ 9 mJy consist of a blend of two or more ALMA-detectable SMGs brighter than $S_{870}\sim$ 1 mJy (corresponding to a galaxy with a total-infrared luminosity of $L_{IR}\ge$ 10${}^{12}$ L${}_{\odot }$), in comparison to 28 $\pm$ 2% for the single-dish sources at $S_{850}\ge$ 5 mJy. Using the 46 single-dish submillimeter sources that contain two or more ALMA-detected SMGs with photometric redshifts, we show that there is a significant statistical excess of pairs of SMGs with similar redshifts ($<$1% probability of occurring by chance), suggesting that at least 30% of these blends arise from physically associated pairs of SMGs.

11 pages, 4 figures, fixed typo (sorry Rob)