The SCUBA-2 Cosmology Legacy Survey: The nature of bright submm galaxies from 2 deg2 of 850-μm imaging

We present physical properties [redshifts (z), star-formation rates (SFRs) and stellar masses (Mstar)] of nearly 2000 bright (S850 > 4 mJy) submm galaxies in the ~2 deg2 COSMOS and UDS fields selected with SCUBA-2 on the JCMT, representing the largest homogeneous sample of 850-um-selected sources to date. We check the reliability of our identifications, and the robustness of the SCUBA-2 fluxes by revisiting the recent ALMA follow-up. Considering > 4 mJy ALMA sources, our identification method has a completeness of ~86 per cent with a reliability of ~92 per cent, and only ~15-20 per cent of sources are significantly affected by multiplicity (when a secondary component is brighter than a third of the primary one). The impact of source blending on the 850-um source counts as determined with SCUBA-2 is modest; scaling the single-dish fluxes by ~0.9 reproduces the ALMA source counts. We find median values of z = 2.40+0.10-0.04, SFR = 287+-6 Mo yr-1, and log(Mstar/Mo) = 11.12+-0.02. These properties clearly locate bright submm galaxies on the high-mass end of the `main sequence' of star-forming galaxies out to z~6, thus suggesting that major mergers are not a dominant driver of the high-redshift submm-selected population. Their number densities are also consistent with recent determinations of the evolving galaxy stellar mass function. Hence, the submm galaxy population is as expected, albeit reproducing the evolution of the main sequence of star-forming galaxies remains a challenge for theoretical models/simulations.