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 Multitiered Extragalactic Survey
(HerMES) Large Mode Survey, covering 274 deg2. We detected 87
per cent with a signal-to-noise ratio >3 at 850 μm. We introduce a
new method for incorporating the confusion noise in our spectral energy
distribution fitting by sampling correlated flux density fluctuations
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 σz/(1 + z)
≈ 0.21 to 0.15. Comparison spectroscopic redshifts also found little
bias ( =
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
includes examples of galaxies with the highest star formation rates in
the Universe (≫103 M⊙ yr-1).