2021MNRAS.502.3426S

We analyse a robust sample of 30 near-infrared-faint (K_AB > 25.3, 5σ) submillimetre galaxies (SMGs) selected from a 0.96 deg² field to investigate their properties and the cause of their faintness in optical/near-infrared wavebands. Our analysis exploits precise identifications based on Atacama Large Millimeter Array (ALMA) 870-µm continuum imaging, combined with very deep near-infrared imaging from the UKIDSS Ultra Deep Survey. We estimate that SMGs with K_AB > 25.3 mag represent 15 ± 2 per cent of the total population brighter than S₈₇₀ = 3.6 mJy, with a potential surface density of ∼450 deg^–2 above S₈₇₀ >= 1 mJy. 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 SMGs represent the tail of the broader submillimetre population, with comparable dust and stellar masses to K_AB <= 25.3 mag SMGs, but lying at significantly higher redshifts (z = 3.44 ± 0.06 versus z = 2.36 ± 0.11) and having higher dust attenuation (A_V = 5.2 ± 0.3 versus A_V = 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_AB <= 25.3 mag galaxies, as measured by ALMA, which suggests their high attenuation is related to their compact sizes. We identify a correlation of dust attenuation with star formation rate surface density (Σ_SFR), with the K-faint SMGs representing the higher Σ_SFR and highest A_V 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 galaxies are completely missed in ultraviolet, optical, and even near-infrared surveys.