An ALMA survey of the S2CLS UDS field: Optically invisible submillimetre
  galaxies
  We analyse a robust sample of 30 near-infrared-faint (K>25.3, 5 sigma)
submillimetre galaxies selected across a 0.96 deg^2 field, to investigate their
properties and the cause of their lack of detectable optical/near-infrared
emission. Our analysis exploits precise identifications based on ALMA 870um
continuum imaging, combined with the very deep near-infrared imaging from the
UKIDSS-UDS survey. We estimate that K>25.3 submillimetre galaxies represent
15+/-2 per cent of the total population brighter than S870=3.6mJy, with an
expected surface density of ~450/deg^2 above S870>1mJy. As such they pose a
source of contamination in surveys for both high-redshift "quiescent" galaxies
and very-high-redshift Lyman-break galaxies. We show that these K-faint
submillimetre galaxies are simply the tail of the broader submillimetre
population, with comparable dust and stellar masses to K<25.3 mag submillimetre
galaxies, but lying at significantly higher redshifts (z=3.44+/-0.06 versus
z=2.36+/-0.11) and having higher dust attenuation (Av=5.2+/-0.3 versus
Av=2.9+/-0.1). We investigate the origin of the strong dust attenuation and
find indications that these K-faint galaxies have smaller dust continuum sizes
than the K<25.3 galaxies, as measured by ALMA, which suggests their high
attenuation is related to their compact sizes. We find a correlation of dust
attenuation with star-formation rate surface density (Sigma_SFR), with the
K-faint submillimetre galaxies representing the higher-Sigma_SFR and highest-Av
galaxies. The concentrated, intense star-formation activity in these systems is
likely to be associated with the formation of spheroids in compact galaxies at
high redshifts, but as a result of their high obscuration these are completely
missed in UV, optical and even near-infrared surveys.
