Abstract
We have used the Submillimeter Array at 860$\,\mu$m to observe the brightest SCUBA-2 sources in 4$\,$deg$^{2}$ 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 15$\,$mJy. 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 ${\sim}\,3^{+0.7}_{-0.6}\,{\times}\,10^{-7}\,$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.
Highlights
The emergence of submillimetre astronomy has led to the discovery of a cosmologically important population of submm galaxies (SMGs), which appear to be among the earliest and most actively star-forming galaxies in the Universe, often reaching luminosities of a few times 1013 L and star-formation rates (SFRs) greater than a few hundred M yr−1 (e.g., Blain et al 2002; Magnelli et al 2012; Swinbank et al 2014; MacKenzie et al 2017; Michalowski et al 2017) and above (e.g. HFLS3, see Riechers et al 2013) around redshifts 2–3 (e.g., Chapman et al 2005; Simpson et al 2014, 2017)
A similar analysis performed for the Atacama Large Millimeter/submillimeter Array (ALMA) sources observed by Simpson et al (2015) resulted in completeness levels of 50 per cent for S850 > 8 mJy, 56 per cent for S850 > 9 mJy, and 73 per cent for S850 > 10 mJy, which shows that our observations offer a significant improvement in this field owing to the fact that our targets were selected from later versions of the Cosmology Legacy Survey (CLS) maps
In the UDS field, we found that none of our 23 observed Submillimeter Common User Bolometer Array (SCUBA)-2 sources break up into two components, while the ALMA follow-up results of Simpson et al (2015) reported 18 single dish sources breaking up into multiple components in 30 observations, a fraction of 0.60 ± 0.14, where the uncertainty is calculated as the square root of the number of multiples divided by the sample size
Summary
The emergence of submillimetre (submm) astronomy has led to the discovery of a cosmologically important population of submm galaxies (SMGs), which appear to be among the earliest and most actively star-forming galaxies in the Universe, often reaching luminosities of a few times 1013 L and star-formation rates (SFRs) greater than a few hundred M yr−1 (e.g., Blain et al 2002; Magnelli et al 2012; Swinbank et al 2014; MacKenzie et al 2017; Michalowski et al 2017) and above (e.g. HFLS3, see Riechers et al 2013) around redshifts 2–3 (e.g., Chapman et al 2005; Simpson et al 2014, 2017). More accurately pinpointing the submm emission directly – the only way to be fully sure that the associated positions and optical/IR counterparts are bone fide – was not possible until the leap in continuum sensitivity provided by new submm interferometers and wide-bandwidth correlators, such as those available at the Plateau de Bure Interferometer (PdBI; Guilloteau et al 1992), the Submillimeter Array (SMA; Ho et al 2004) and, most recently, the Atacama Large Millimeter/submillimeter Array (ALMA; Wootten & Thompson 2009) These have greatly aided the localisation of counterparts and the further characterization of SMGs. These have greatly aided the localisation of counterparts and the further characterization of SMGs These facilities were able to confirm that many SMGs exhibit multiplicity (e.g., Iono et al 2006; Younger et al 2007, 2009; Wang et al 2011; Smolcic et al 2012b; Hodge et al 2013; Simpson et al 2015; Miettinen et al 2015, Stach et al in prep.), where one bright single-dish submm source resolves into two or three individual SMGs. Large single-dish submm surveys (e.g., Scott et al 2002; Greve et al 2004; Wang et al 2004; Coppin et al 2006; Bertoldi et al 2007; Weiß et al 2009; Oliver et al 2010; Valiante et al 2016; Geach et al 2017), followed up by interferometers, have been important for addressing the issue of multiplicity as they provide substantial catalogues of bright single-dish sources across continuous patches of sky that interferometers can follow-up.
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