We present a kinematic study of six IR-luminous galaxies observed with the Mid-infrared Instrument Medium-resolution Spectrometer (MIRI/MRS) on board JWST. These galaxies lie at z = 0.5-0.6, midway between the present day and the peak of cosmic star formation (SF). Our sample spans a range of SF and active galactic nucleus (AGN) contributions to the mid-IR emission. We characterize the dynamical state of these IR-luminous galaxies and assess how AGN activity influences the kinematics of the interstellar medium. Using mid-IR tomic lines, we map galaxy kinematics beyond the local Universe for the first time. The spatial resolution of MIRI/MRS (3.0 kpc for 0 .'' 46 at z similar to 0.55) allows us to resolve the internal kinematics of our targets. We compute kinematic maps in three different emission lines ([Ar II]6.99 mu m, [Ne II]12.81 mu m, and H2 0-0 S(5)6.91 mu m). Using the [Ar II]6.99 mu m kinematic maps, we derive rotation curves for these sources. All galaxies exhibit ordered rotation, with V/sigma >= 2, consistent with stable disks. Although some show minor disturbances, we find no strong evidence for recent major mergers or galaxy-wide ionized outflows. We find no correlation between V/sigma and AGN fraction, suggesting AGN activity does not significantly disrupt global kinematics or that disk disruption is not required to trigger AGN. However, galaxies with higher AGN fractions show elevated central dispersions, indicating localized turbulence, possibly due to AGN feedback, stellar feedback, accretion, or bulge structure. These IR-luminous galaxies likely represent mature, rotationally supported disks, with AGN activation occurring after disk assembly.

We present 850um imaging of the XMM-LSS field observed for 170 hours as part of the James Clerk Maxwell Telescope SCUBA-2 Large eXtragalactic Survey (S2LXS). S2LXS XMM-LSS maps an area of 9 square degrees, reaching a moderate depth of 1-sigma 4 mJy/beam. This is the largest contiguous area of extragalactic sky mapped by JCMT at 850um to date. The wide area of the S2LXS XMM-LSS survey allows us to probe the ultra-bright (S_850um > 15 mJy), yet rare submillimetre population. We present the S2LXS XMM-LSS catalogue, which comprises 40 sources detected at >5-sigma significance, with deboosted flux densities in the range of 7 mJy to 48 mJy. We robustly measure the bright-end of the 850um number counts at flux densities >7 mJy, reducing the Poisson errors compared to existing measurements. The S2LXS XMM-LSS observed number counts show the characteristic upturn at bright fluxes, expected to be motivated by local sources of submillimetre emission and high-redshift strongly lensed galaxies. We find that the observed 850um number counts are best reproduced by model predictions that include either strong lensing or source blending from a 15 arcsec beam, indicating that both may make an important contribution to the observed over-abundance of bright single dish 850um selected sources. We make the S2LXS XMM-LSS 850um map and >5-sigma catalogue presented here publicly available.

Magnetic fields are fundamental to the evolution of galaxies, playing a key role in the astrophysics of the interstellar medium and star formation. Large-scale ordered magnetic fields have been mapped in the Milky Way and nearby galaxies, but it is not known how early in the Universe such structures form. Here we report the detection of linearly polarized thermal emission from dust grains in a strongly lensed, intrinsically luminous galaxy that is forming stars at a rate more than a thousand times that of the Milky Way at redshift 2.6, within 2.5 Gyr of the Big Bang. The polarized emission arises from the alignment of dust grains with the local magnetic field. The median polarization fraction is of order one per cent, similar to nearby spiral galaxies. Our observations support the presence of a 5 kiloparsec-scale ordered magnetic field with a strength of around 500uG or lower, orientated parallel to the molecular gas disk. This confirms that such structures can be rapidly formed in galaxies, early in cosmic history.

Recent high-resolution interferometric images of submillimetre galaxies (SMGs) reveal fascinatingly complex morphologies. This raises a number of questions: how does the relative orientation of a galaxy affect its observed submillimetre emission, and does this result in an `orientation bias' in the selection and analysis of such galaxies in flux-limited cosmological surveys? We investigated these questions using the \textsc{Simba} cosmological simulation paired with the dust radiative transfer code \textsc{Powderday}. We selected eight simulated SMGs ($S_{850}\gtrsim2$ mJy) at $z = 2$, and measured the variance of their `observed' emission over 50 random orientations. Each galaxy exhibits significant scatter in its emission close to the peak of the thermal dust emission, with variation in flux density of up to a factor of 2.7. This results in an appreciable dispersion in the inferred dust temperatures and infrared luminosities ($16^{\mathrm{th}}-84^{\mathrm{th}}$ percentile ranges of 5\,K and 0.1\,dex, respectively) and therefore a fundamental uncertainty in derived parameters such as dust mass and star formation rate ($\sim$30% for the latter using simple calibrations). Using a Monte Carlo simulation we also assessed the impact of orientation on flux-limited surveys, finding a bias in the selection of SMGs towards those with face--on orientations, as well as those at lower redshifts. We predict that the orientation bias will affect flux-limited single-dish surveys, most significantly at THz frequencies, and this bias should be taken into account when placing the results of targeted follow--up studies in a statistical context.

Aims. We investigate the radio properties of a sample of 850 μm-selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS) using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array. This sample consists of 53 sources, 41 of which are detected at >5σ at 150 MHz.Methods. Combining these data with additional observations at 324 MHz, 610 MHz, and 1.4 GHz from the Giant Metrewave Radio Telescope and the Jansky Very Large Array, we find a variety of radio spectral shapes and luminosities (L1.4 GHz ranging from ~4 × 1023−1 × 1025) within our sample despite their similarly bright submillimetre flux densities (>4 mJy). We characterise their spectral shapes in terms of multi-band radio spectral indices. Finding strong spectral flattening at low frequencies in ~20% of sources, we investigate the differences between sources with extremely flat low-frequency spectra and those with ‘normal’ radio spectral indices (α > −0.25).Results. As there are no other statistically significant differences between the two subgroups of our sample as split by the radio spectral index, we suggest that any differences are undetectable in galaxy-averaged properties that we can observe with our unresolved images, and likely relate to galaxy properties that we cannot resolve, on scales ≲1 kpc. We attribute the observed spectral flattening in the radio to free–free absorption, proposing that those sources with significant low-frequency spectral flattening have a clumpy distribution of star-forming gas. We estimate an average spatial extent of absorbing material of at most several hundred parsecs to produce the levels of absorption observed in the radio spectra. This estimate is consistent with the highest-resolution observations of submillimetre galaxies in the literature, which find examples of non-uniform dust distributions on scales of ~100 pc, with evidence for clumps and knots in the interstellar medium. Additionally, we find two bright (>6 mJy) S2CLS sources undetected at all other wavelengths. We speculate that these objects may be very high redshift sources, likely residing at z > 4.

It is widely believed that ultraluminous infrared (IR) gala xies and active galactic nuclei (AGN) activity are triggered by galaxy interactions and mer ging, with the peak of activity occurring at z ∼ 2, where submillimetre galaxies are thousands of times more numerous than local ULIRGs. In this evolutionary picture, submillimetre galaxies (SMG s) would host an AGN, which would eventually grow a black hole (BH) strong enough to blow off al l of the gas and dust leaving an optically luminous QSO. To probe this evolutionary sequenc e we have focussed on the ’missing link’ sources, which demonstrate both strong starburst (SB ) and AGN signatures, in order to determine if the SB is the main power source even in SMGs when w e have evidence that an AGN is present from their IRAC colours. The best way to determine if a dominant AGN is present is to look for their signatures in the mid-infrared with the Spitzer IRS, since often even deep X-ray observations miss identifying the presence of AGN in heavil y dust-obscured SMGs. We present the results of our audit of the energy balance between star-f ormation and AGN within this special sub-population of SMGs – where the BH has grown appreciably t o begin heating the dust emission.

We present new empirical constraints on the evolution of rho(H2), the cosmological mass density of molecular hydrogen, back to z approximate to 2.5. We employ a statistical approach measuring the average observed 850 mu m flux density of near-infrared selected galaxies as a function of redshift. The redshift range considered corresponds to a span where the 850 mu m band probes the Rayleigh-Jeans tail of thermal dust emission in the rest frame, and can therefore be used as an estimate of the mass of the interstellar medium. Our sample comprises of approximate to 150,000 galaxies in the UK InfraRed Telescope Infrared Deep Sky Survey Ultra-Deep Survey field with near-infrared magnitudes K-AB <= 25 mag and photometric redshifts with corresponding probability distribution functions derived from deep 12-band photometry. With a sample approximately 2 orders of magnitude larger than in previous works we significantly reduce statistical uncertainties on rho(H2) to z approximate to 2.5. Our measurements are in broad agreement with recent direct estimates from blank field molecular gas surveys, finding that the epoch of molecular gas coincides with the peak epoch of star formation with rho(H2) approximate to 2 x 10(7) M-circle dot Mpc(-7) at z approximate to 2. We demonstrate that rho(H2) can be broadly modeled by inverting the star formation rate (SFR) density with a fixed or weakly evolving star formation efficiency. This "constant efficiency" model shows a similar evolution to our statistically derived rho(H2), indicating that the dominant factor driving the peak star formation history at z approximate to 2 is a larger supply of molecular gas in galaxies rather than a significant evolution of the SFR efficiency within individual galaxies.

Fluorine is one of the most interesting elements for nuclear and stellar astrophysics1,2. Fluorine abundance was first measured for stars other than the Sun in 19921, then for a handful of metal-poor stars3, which are likely to have formed in the early Universe. The main production sites of fluorine are under debate and include asymptotic giant branch stars, the ν-process in core-collapse supernovae, and Wolf–Rayet stars4–10. Due to the difference in the mass and lifetime of progenitor stars, high-redshift observations of fluorine can help constrain the mechanism of fluorine production in massive galaxies. Here, we report the detection of HF (signal-to-noise ratio of 8) in absorption in a gravitationally lensed dusty star-forming galaxy at redshift z = 4.4 with NHF/ $${N}_{{{{{\rm{H}}}}}_{2}}$$ as high as ~2 × 10−9, indicating a very quick ramp-up of the chemical enrichment in this high-z galaxy. At z = 4.4, asymptotic giant branch stars of a few solar masses are very unlikely to dominate the enrichment. Instead, we show that Wolf–Rayet stars are required to produce the observed fluorine abundance at this time, with other production mechanisms becoming important at later times. These observations therefore provide an insight into the underlying processes driving the ramp-up phase of chemical enrichment alongside rapid stellar mass assembly in a young massive galaxy. The detection of the HF molecule in a lensed galaxy at z = 4.4 suggests a rapid chemical enrichment. Wolf–Rayet stars are the most likely providers of the fluorine.

We present an analysis of the spatial clustering of a large sample of high-resolution, interferometically identified, submillimetre galaxies (SMGs). We measure the projected cross-correlation function of ∼350 SMGs in the UKIDSS Ultra Deep-Survey Field across a redshift range of z = 1.5–3 utilizing a method that incorporates the uncertainties in the redshift measurements for both the SMGs and cross-correlated galaxies through sampling their full probability distribution functions. By measuring the absolute linear bias of the SMGs, we derive halo masses of $\log _{10}(M_{\rm halo}[{h^{-1}\, \rm M_{\odot }}])$ ∼ 12.8 with no evidence of evolution in the halo masses with redshift, contrary to some previous work. From considering models of halo mass growth rates, we predict that the SMGs will reside in haloes of mass $\log _{10}(M_{\rm halo}[{h^{-1}\, \rm M_{\odot }}])$ ∼ 13.2 at z = 0, consistent with the expectation that the majority of z = 1.5–3 SMGs will evolve into present-day spheroidal galaxies. Finally, comparing to models of stellar-to-halo mass ratios, we show that SMGs may correspond to systems that are maximally efficient at converting their gas reservoirs into stars. We compare them to a simple model for gas cooling in haloes that suggests that the unique properties of the SMG population, including their high levels of star formation and their redshift distribution, are a result of the SMGs being the most massive galaxies that are still able to accrete cool gas from their surrounding intragalactic medium.

ABSTRACT 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.

ABSTRACT We analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-millimetre galaxies (SMGs). This survey, AS2UDS, identified 707 SMGs across the ∼1 deg2 field, including ∼17 per cent, which are undetected at K ≳ 25.7 mag. We interpret their ultraviolet-to-radio data using magphys and determine a median redshift of z = 2.61 ± 0.08 (1σ range of z = 1.8–3.4) with just ∼6 per cent at z > 4. Our survey provides a sample of massive dusty galaxies at z ≳ 1, with median dust and stellar masses of Md = (6.8 ± 0.3) × 108 M⊙ (thus, gas masses of ∼1011 M⊙) and M* = (1.26 ± 0.05) × 1011 M⊙. We find no evolution in dust temperature at a constant far-infrared luminosity across z ∼ 1.5–4. The gas mass function of our sample increases to z ∼ 2–3 and then declines at z > 3. The space density and masses of SMGs suggest that almost all galaxies with M* ≳ 3 × 1011 M⊙ have passed through an SMG-like phase. The redshift distribution is well fit by a model combining evolution of the gas fraction in haloes with the growth of halo mass past a critical threshold of Mh ∼ 6 × 1012 M⊙, thus SMGs may represent the highly efficient collapse of gas-rich massive haloes. We show that SMGs are broadly consistent with simple homologous systems in the far-infrared, 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 present an overview and description of the eMERLIN Galaxy Evolution survey (eMERGE) Data Release 1 (DR1), a large program of high-resolution 1.5 GHz radio observations of the GOODS-N field comprising $\sim140$ hours of observations with eMERLIN and $\sim40$ hours with the Very Large Array (VLA). We combine the long baselines of eMERLIN (providing high angular resolution) with the relatively closely-packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5 GHz radio survey with the sensitivity ($\sim 1.5\mu$Jy beam$^{-1}$), angular resolution ($0.2"$--$0.7"$) and field-of-view ($\sim15' \times 15'$) to detect and spatially resolve star-forming galaxies and AGN at $z\gtrsim 1$. The goal of eMERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1-mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over a $\sim20$ year baseline and the development of new point spread function/primary beam models to seamlessly merge eMERLIN and VLA data in the $uv$ plane. We present early science results, including measurements of the luminosities and/or linear sizes of $\sim500$ galaxes selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of $r_{\rm eMERGE}/r_{\rm HST}\sim1.02\pm0.03$, suggesting that in most high-redshift galaxies, the $\sim$GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers which will explore the $\sim$kpc-scale radio properties of star-forming galaxies and AGN in the GOODS-N field observed by eMERGE DR1.

ABSTRACT We analyse 870 $\mu$m Atacama Large Millimetre Array (ALMA) dust continuum detections of 41 canonically selected $z$ ≃ 3 Lyman-break galaxies (LBGs), as well as 209 ALMA-undetected LBGs, in follow-up of SCUBA-2 mapping of the UKIDSS Ultra Deep Survey (UDS) field. We find that our ALMA-bright LBGs lie significantly off the local IRX-beta relation and have relatively bluer rest-frame UV slopes (as parametrized by β), given their high values of the ‘infrared excess’ (IRX ≡ LIR/LUV), relative to the average ‘local’ IRX-β relation. We attribute this finding in part to the young ages of the underlying stellar populations but we find that the main reason behind the unusually blue UV slopes are the relatively shallow slopes of the corresponding dust attenuation curves. We show that, when stellar masses, M*, are being established via SED fitting, it is absolutely crucial to allow the attenuation curves to vary (rather than fixing it on Calzetti-like law), where we find that the inappropriate curves may underestimate the resulting stellar masses by a factor of ≃2–3× on average. In addition, we find these LBGs to have relatively high specific star-formation rates (sSFRs), dominated by the dust component, as quantified via the fraction of obscured star formation $(f_{\rm obs}\equiv {\rm SFR_{\rm IR}/{\rm SFR}_{\rm UV+IR}})$. We conclude that the ALMA-bright LBGs are, by selection, massive galaxies undergoing a burst of a star formation (large sSFRs, driven, for example, by secular or merger processes), with a likely geometrical disconnection of the dust and stars, responsible for producing shallow dust attenuation curves.

We presentHerschel -PACS spectroscopy of the [OI]63 μm far-infrared cooling line from a sample of six unlensed and spectroscopically-confirmed 87 0μm-selected submillimetre (submm) galaxies (SMGs) at 1.1 < z < 1.6 from the LABOCA Extended Chandra Deep Field South (ECDFS) Submm Survey (LESS). This is the first sur vey of [OI]63μm, one of the main photodissociation region (PDR) cooling lines, i n SMGs. New high-resolution ALMA interferometric 870μm continuum imaging confirms that these six Herschel -targeted SMG counterparts are bona fide sources of submm emission. We d etect [OI]63μm in two SMGs with a SNR> ∼ 3, tentatively detect [OI]63μm in one SMG, and constrain the line flux for the non-detections. We also exploit the combination of s ubmm continuum photometry from 250–870μm and our new PACS continuum measurements to constrain the fa r-infr red luminosity, LFIR, in these SMGs to. 30per cent. We find that SMGs do not show a deficit in their [OI]63μm–to–far-infrared (FIR) continuum luminosity ratios (wit h ratios ranging from ≃ 0.5–1.5 per cent), similar to what was seen previously for the [C II]158μm–to–FIR ratios in SMGs. These observed ratios are about an order of magnitud e higher than what is seen typically for local ultra luminous infrared galaxies (ULIRGs) , which adds to the growing body of evidence that SMGs are not simply ‘scaled up’ versions of l oca ULIRGs. Rather, the PDR line–to–LFIR ratios suggest that the star formation modes of SMGs are like ly more akin to that of local normal (lower-luminosity) star-forming gala xies, with the bulk of the star formation occurring in extended galaxy-scale ( ∼kpc) regions. These observations represent the first step towards a census of the major PDR cooling lines in typica l SMGs that will be attainable with ALMA, enabling detailed modelling to probe the global p ro erties of the star formation and the evolutionary status of SMGs.

Two decades of effort have been poured into both single-dish and interferometric millimeter-wave surveys of the sky to infer the volume density of dusty star-forming galaxies (DSFGs, with SFR>100M$_\odot$ yr$^{-1}$) over cosmic time. Though obscured galaxies dominate cosmic star-formation near its peak at $z\sim2$, the contribution of such heavily obscured galaxies to cosmic star-formation is unknown beyond $z\sim2.5$ in contrast to the well-studied population of Lyman-break galaxies (LBGs) studied through deep, space- and ground-based pencil beam surveys in the near-infrared. Unlocking the volume density of DSFGs beyond $z>3$, particularly within the first 1 Gyr after the Big Bang is critical to resolving key open questions about early Universe galaxy formation: (1) What is the integrated star-formation rate density of the Universe in the first few Gyr and how is it distributed among low-mass galaxies (e.g. Lyman-break galaxies) and high-mass galaxies (e.g. DSFGs and quasar host galaxies)? (2) How and where do the first massive galaxies assemble? (3) What can the most extreme DSFGs teach us about the mechanisms of dust production (e.g. supernovae, AGB stars, grain growth in the ISM) <1 Gyr after the Big Bang? We summarize the types of observations needed in the next decade to address these questions.

The Sloan Digital Sky Survey (SDSS) was revolutionary because of the extraordinary breadth and ambition of its optical imaging and spectroscopy. We argue that a 'sub-millimeter SDSS' - a sensitive large-area imaging+spectroscopic survey in the sub-mm window - will revolutionize our understanding of galaxy evolution in the early Universe. By detecting the thermal dust continuum emission and atomic and molecular line emission of galaxies out to z~10 it will be possible to measure the redshifts, star formation rates, dust and gas content of hundreds of thousands of high-z galaxies down to ~L*. Many of these galaxies will have counterparts visible in the deep optical imaging of the Large Synoptic Survey Telescope. This 3D map of galaxy evolution will span the peak epoch of galaxy formation all the way back to cosmic dawn, measuring the co-evolution of the star formation rate density and molecular gas content of galaxies, tracking the production of metals and charting the growth of large-scale structure.

We present sensitive 850 mu m imaging of the Cosmological Evolution Survey (COSMOS) field using 640 hr of new and archival observations taken with SCUBA-2 at the East Asian Observatory's James Clerk Maxwell Telescope. The SCUBA-2 COSMOS survey (S2COSMOS) achieves a median noise level of sigma(850 mu m) = 1.2 mJy beam(-1) over an area of 1.6 sq. degree (MAIN; Hubble Space Telescope/Advanced Camera for Surveys footprint), and sigma(850 mu m) = 1.7 mJy beam(-1) over an additional 1 sq. degree of supplementary (SUPP) coverage. We present a catalog of 1020 and 127 sources detected at a significance level of >4 sigma and >4.3 sigma in the MAIN and SUPP regions, respectively, corresponding to a uniform 2% false-detection rate. We construct the single-dish S-850 mu m number counts at S-850 > 2 mJy and show that these S2COSMOS counts are in agreement with previous single-dish surveys, demonstrating that degree-scale fields are sufficient to overcome the effects of cosmic variance in the S-850 = 2-10 mJy population. To investigate the properties of the galaxies identified by S2COSMOS sources we measure the surface density of near-infrared-selected galaxies around their positions and identify an average excess of 2.0 +/- 0.2 galaxies within a 13 '' radius (similar to 100 kpc at z similar to 2). The bulk of these galaxies represent near-infrared-selected submillimeter galaxies and/or spatially correlated sources and lie at a median photometric redshift of z = 2.0 +/- 0.1. Finally, we perform a stacking analysis at submillimeter and far-infrared wavelengths of stellarmass-selected galaxies (M-* = 10(10)-10(12) M-circle dot) from z = 0-4, obtaining high-significance detections at 850 mu m in all subsets (signal-to-noise ratio, S/N = 4-30), and investigate the relation between far-infrared luminosity, stellar mass, and the peak wavelength of the dust spectral energy distribution. The publication of this survey adds a new deep, uniform submillimeter layer to the wavelength coverage of this well-studied COSMOS field.

We identify multi-wavelength counterparts to 1147 submillimeter sources from the S2COSMOS SCUBA-2 survey of the COSMOS field by employing a recently developed radio+machine-learning method trained on a large sample of Atacama Large Millimeter/submillimeter Array (ALMA)–identified submillimeter galaxies (SMGs), including 260 SMGs identified in the AS2COSMOS pilot survey. In total, we identify 1222 optical/near-infrared (NIR)/radio counterparts to the 897 S2COSMOS submillimeter sources with S850 > 1.6 mJy, yielding an overall identification rate of (78 ± 9)%. We find that (22 ± 5)% of S2COSMOS sources have multiple identified counterparts. We estimate that roughly 27% of these multiple counterparts within the same SCUBA-2 error circles very likely arise from physically associated galaxies rather than line-of-sight projections by chance. The photometric redshift of our radio+machine-learning-identified SMGs ranges from z = 0.2 to 5.7 and peaks at z = 2.3 ± 0.1. The AGN fraction of our sample is (19 ± 4)%, which is consistent with that of ALMA SMGs in the literature. Comparing with radio/NIR-detected field galaxy population in the COSMOS field, our radio+machine-learning-identified counterparts of SMGs have the highest star formation rates and stellar masses. These characteristics suggest that our identified counterparts of S2COSMOS sources are a representative sample of SMGs at z ≲ 3. We employ our machine-learning technique to the whole COSMOS field and identified 6877 potential SMGs, most of which are expected to have submillimeter emission fainter than the confusion limit of our S2COSMOS surveys ( mJy). We study the clustering properties of SMGs based on this statistically large sample, finding that they reside in high-mass dark matter halos ((1.2 ± 0.3) × 1013 h−1 ), which suggests that SMGs may be the progenitors of massive ellipticals we see in the local universe.