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An ALMA survey of the S2CLS UDS field: optically invisible submillimetre galaxies

Smail, Ian; Dudzevičiūtė, U.; Stach, S.M.; Almaini, O.; Birkin, J.E.; Chapman, S.C.; Chen, Chian-Chou; Geach, J.E.; Gullberg, B.; Hodge, J.A.; Ikarashi, S.; Ivison, R.J.; Scott, D.; Simpson, Chris; Swinbank, A.M.; Thomson, A.P.; Walter, F.; Wardlow, J.L.; van der Werf, P.

An ALMA survey of the S2CLS UDS field: optically invisible submillimetre galaxies Thumbnail


S.M. Stach

O. Almaini

S.C. Chapman

Chian-Chou Chen

J.E. Geach

B. Gullberg

J.A. Hodge

S. Ikarashi

R.J. Ivison

D. Scott

Chris Simpson

A.P. Thomson

F. Walter

J.L. Wardlow

P. van der Werf


We analyse a robust sample of 30 near-infrared-faint (KAB > 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 KAB > 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 KAB ≤ 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 (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 KAB ≤ 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 (SFR), with the K-faint SMGs representing the higher 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 galaxies are completely missed in ultraviolet, optical, and even near-infrared surveys.


Smail, I., Dudzevičiūtė, U., Stach, S., Almaini, O., Birkin, J., Chapman, S., …van der Werf, P. (2021). An ALMA survey of the S2CLS UDS field: optically invisible submillimetre galaxies. Monthly Notices of the Royal Astronomical Society, 502(3), 3426-3435.

Journal Article Type Article
Acceptance Date Jan 28, 2021
Online Publication Date Feb 3, 2021
Publication Date 2021-04
Deposit Date May 5, 2021
Publicly Available Date Sep 16, 2021
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 502
Issue 3
Pages 3426-3435


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Copyright Statement
2021 The Author(s).<br /> Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative<br /> Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium,<br /> provided the original work is properly cited

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