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arXiv · 2010.02250

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

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

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.

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Ian Smail, U. Dudzevičiūtė, S. M. Stach, O. Almaini, J. E. Birkin, S. C. Chapman, Chian-Chou Chen, J. E. Geach, B. Gullberg, J. A. Hodge, S. Ikarashi, R. J. Ivison, D. Scott, Chris Simpson, A. M. Swinbank, A. P. Thomson, F. Walter, J. L. Wardlow, P. van der Werf. 2020-10-05. An ALMA survey of the S2CLS UDS field: Optically invisible submillimetre galaxies. https://doi.org/10.1093/mnras/stab283

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