We analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-millimetre galaxies (SMGs) detected in the SCUBA-2 Cosmology Legacy Survey 850-µm map of the UKIDSS/UDS field. This survey, AS2UDS, identified 707 SMGs across the ∼ 1 deg 2 field, including ∼17 per cent which are undetected in the optical/near-infrared to K 25.7 mag. We interpret the UV-to-radio data of these systems using a physically motivated model, magphys and determine a median photometric redshift of z = 2.61±0.08, with a 68 th percentile range of z = 1.8-3.4, with just ∼ 6 per cent at z > 4. The redshift distribution is well fit by a model combining evolution of the gas fraction in halos with the growth of halo mass past a critical threshold of ∼4×10 12 M , thus SMGs may represent the highly efficient collapse of gasrich massive halos. Our survey provides a sample of the most massive, dusty galaxies at z 1, with median dust and stellar masses of M d = (6.8±0.3) × 10 8 M (thus, gas masses of ∼ 10 11 M ) and M * = (1.26±0.05) × 10 11 M . These galaxies have gas fractions of f gas = 0.41±0.02 with depletion timescales of ∼ 150 Myr. The gas mass function evolution of our sample at high masses is consistent with constraints at lower masses from blind CO-surveys, with an increase to z ∼ 2-3 and then a decline at higher redshifts. The space density and masses of SMGs suggests that almost all galaxies with M * 2 × 10 11 M have passed through an SMG-like phase. We find no evolution in dust temperature at a constant far-infrared luminosity across z ∼ 1.5-4. We exploit dust continuum sizes to show that SMGs appear to behave as simple homologous systems in the far-infrared, having properties consistent with a centrally illuminated starburst. Our study provides strong support for an evolutionary link between the active, gas-rich SMG population at z > 1 and the formation of massive, bulge-dominated galaxies across the history of the Universe.

We have used ALMA and NOEMA to study the molecular gas reservoirs in 61 ALMA-identified submillimetre galaxies (SMGs) in the COSMOS, UDS, and ECDFS fields. We detect 12CO ($J_{\rm up} =$ 2–5) emission lines in 50 sources, and [C i](3P1 − 3P0) emission in eight, at $z =$ 1.2–4.8 and with a median redshift of 2.9 ± 0.2. By supplementing our data with literature sources, we construct a statistical CO spectral line energy distribution and find that the 12CO line luminosities in SMGs peak at Jup ∼ 6, consistent with similar studies. We also test the correlations of the CO, [C i], and dust as tracers of the gas mass, finding the three to correlate well, although the CO and dust mass as estimated from the 3-mm continuum are preferable. We estimate that SMGs lie mostly on or just above the star-forming main sequence, with a median gas depletion timescale, tdep = Mgas/SFR, of 210 ± 40 Myr for our sample. Additionally, tdep declines with redshift across z ∼ 1–5, while the molecular gas fraction, μgas = Mgas/M*, increases across the same redshift range. Finally, we demonstrate that the distribution of total baryonic mass and dynamical line width, Mbaryon–σ, for our SMGs is consistent with that followed by early-type galaxies in the Coma cluster, providing strong support to the suggestion that SMGs are progenitors of massive local spheroidal galaxies. On the basis of this, we suggest that the SMG populations above and below an 870-μm flux limit of S870 ∼ 5 mJy may correspond to the division between slow and fast rotators seen in local early-type galaxies.

We present an ALMA study of the ∼180 brightest sources in the SCUBA-2 850-μm map of the COSMOS field from the S2COSMOS survey, as a pilot study for AS2COSMOS – a full survey of the ∼1000 sources in this field. In this pilot study, we have obtained 870-μm continuum maps of an essentially complete sample of the brightest 182 sub-millimetre sources ($S_{850\, \mu \rm m}\gt $ 6.2 mJy) in COSMOS. Our ALMA maps detect 260 sub-millimetre galaxies (SMGs) spanning a range in flux density of $S_{870\, \mu \rm m}$ = 0.7–19.2 mJy. We detect more than one SMG counterpart in 34 ± 2 per cent of sub-millimetre sources, increasing to 53 ± 8 per cent for SCUBA-2 sources brighter than $S_{850\, \mu \rm m}\gt $ 12 mJy. We estimate that approximately one-third of these SMG–SMG pairs are physically associated (with a higher rate for the brighter secondary SMGs, $S_{870\, \mu \rm m}\gtrsim$ 3 mJy), and illustrate this with the serendipitous detection of bright [C ii] 157.74-μm line emission in two SMGs, AS2COS 0001.1 and 0001.2 at z = 4.63, associated with the highest significance single-dish source. Using our source catalogue, we construct the interferometric 870-μm number counts at $S_{870\, \mu \rm m}\gt $ 6.2 mJy. We use the extensive archival data of this field to construct the multiwavelength spectral energy distribution of each AS2COSMOS SMG, and subsequently model this emission with magphys to estimate their photometric redshifts. We find a median photometric redshift for the $S_{870\, \mu \rm m}\gt $ 6.2 mJy AS2COSMOS sample of z = 2.87 ± 0.08, and clear evidence for an increase in the median redshift with 870-μm flux density suggesting strong evolution in the bright end of the 870-μm luminosity function.

We construct a SCUBA-2 450-µm map in the COSMOS field that covers an area of 300 arcmin 2 and reaches a 1σ noise level of 0.65 mJy in the deepest region. We extract 256 sources detected at 450 µm with signal-to-noise ratio > 4.0 and analyze the physical properties of their multi-wavelength counterparts. We find that most of the sources are at z 3, with a median of z = 1.79 +0.03 −0.15 . About 35 +32−25 % of our sources are classified as starburst galaxies based on their total star-formation rates (SFRs) and stellar masses (M * ). By fitting the far-infrared spectral energy distributions, we find that our 450-µm-selected sample has a wide range of dust temperatures (20 K T d 60 K), with a median of T d = 38.3 +0.4 −0.9 K. We do not find a redshift evolution in dust temperature for sources with L IR > 10 12 L at z < 3. However, we find a moderate correlation where dust temperature increases with the deviation from the SFR-M * relation. The increase in dust temperature also correlates with optical morphology, which is consistent with merger-triggered starbursts in sub-millimeter galaxies. Our galaxies do not show the tight IRX-β UV correlation that has been observed in the local Universe. We construct the infrared luminosity functions of our 450-µm sources and measure their comoving SFR densities. The contribution of the L IR > 10 12 L population to the SFR density rises dramatically from z = 0 to 2 (∝ (1 + z) 3.9±1.1 ) and dominates the total SFR density at z 2.

We analyse a robust sample of 30 near-infrared-faint (KAB &gt; 25.3, 5σ) submillimetre galaxies 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 ALMA 870-μm continuum imaging, combined with very deep near-infrared imaging from the UKIDSS-UDS survey. We estimate that submillimetre galaxies with KAB &gt; 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 submillimetre galaxies represent the tail of the broader submillimetre population, with comparable dust and stellar masses to KAB ≤ 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 KAB ≤ 25.3 mag galaxies, as measured by ALMA, that suggests their high attenuation is related to their compact sizes. We identify a correlation of dust attenuation with star-formation rate surface density (ΣSFR), with the K-faint submillimetre galaxies representing the higher-ΣSFR 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 UV, optical and even near-infrared surveys.