Red, redder, reddest: SCUBA-2 imaging of colour-selected Herschel sources

High-redshift, luminous, dusty star forming galaxies (DSFGs) constrain the extremity of galaxy formation theories. The most extreme are discovered through follow-up on candidates in large area surveys. Here we present extensive 850 µm SCUBA-2 follow-up observations of 188 red DSFG candidates from the Herschel Multi-tiered Extragalactic Survey (HerMES) Large Mode Survey, covering 274 deg2. We detected 87 per cent with a signal-to-noise ratio > 3 at 850 m. We introduce a new method for incorporating the confusion noise in our spectral energy distribution fitting by sampling correlated flux density uctuations from a confusion limited map. The new 850 µm data provide a better constraint on the photometric redshifts of the candidates, with photometric redshift errors decreasing from sz=(1 + z) ˜ 0:21 to 0:15. Comparison spectroscopic redshifts also found little bias (<(z-zspec)=(1+zspec)> = 0:08). The mean photometric redshift is found to be 3.6 with a dispersion of 0:4 and we identify 21 DSFGs with a high probability of lying at z > 4. After simulating our selection effects we find number counts are consistent with phenomenological galaxy evolution models. There is a statistically significant excess of WISE-1 and SDSS sources near our red galaxies, giving a strong indication that lensing may explain some of the apparently extreme objects. Nevertheless, our sample should include examples of galaxies with the highest star formation rates in the Universe (>> 103 M?yr-1).