High-resolution SMA imaging of bright submillimetre sources from the SCUBA-2 Cosmology Legacy Survey

Abstract

We have used the Submillimeter Array at 860$\mu$m to observe the brightest SCUBA-2 sources in 4deg${}^2$ of the Cosmology Legacy Survey. We have targeted 75 of the brightest single-dish SCUBA-2 850$\mu$m sources down to $S_{850}\approx 8$mJy, achieving an average synthesized beam of 2.4${}^{\prime \prime }$ and an average rms of ${\sigma }_{860}=1.5$mJy in our primary beam-corrected maps. We searched our maps for $4\sigma$ peaks, corresponding to $S_{860}\gtrsim 6$mJy sources, and detected 59 single galaxies and three pairs of galaxies. We include in our study 28 archival observations, bringing our sample size to 103 bright single-dish submillimetre sources with interferometric follow-up. We compute the cumulative and differential number counts of our sample, finding them to overlap with previous single-dish survey number counts within the uncertainties, although our cumulative number count is systematically lower than the parent SCUBA-2 cumulative number count by $24\pm 6$ per cent between 11 and 15mJy. We estimate the probability that a $\gtrsim 10$mJy single-dish submillimetre source resolves into two or more galaxies with similar flux densities, causing a significant change in the number counts, to be about 15 per cent. Assuming the remaining 85 per cent of the targets are ultra-luminous starburst galaxies between $z=2$-3, we find a likely volume density of $\gtrsim 400$M${}_{\odot }$yr${}^{-1}$ sources to be $\sim 3_{-0.6}^{+0.7}\times 10^{-7}$Mpc${}^{-3}$. We show that the descendants of these galaxies could be $\gtrsim 4\times 10^{11}$M${}_{\odot }$ local quiescent galaxies, and that about 10 per cent of their total stellar mass would have formed during these short bursts of star-formation.