--- title: SCUBA-2 Cosmology Legacy Survey (S2CLS) url: https://www.emergentmind.com/topics/scuba-2-cosmology-legacy-survey-s2cls type: topic --- # SCUBA-2 Cosmology Legacy Survey (S2CLS) Searching arXiv for the SCUBA-2 Cosmology Legacy Survey and closely related follow-up papers. The **SCUBA-2 Cosmology Legacy Survey (S2CLS)** is a blank-field submillimetre survey conducted with SCUBA-2 on the James Clerk Maxwell Telescope, designed to map large sky areas at **850 \(\mu\)m** and, in some fields, at **450 \(\mu\)m** in order to measure the number counts of submillimetre galaxies, quantify field-to-field variance, identify rare luminous sources including strongly lensed galaxies, and provide a legacy data set for multiwavelength studies of dusty star formation [1607.03904]. Survey papers describe a wide **850 \(\mu\)m** component covering about **4 square degrees** across seven extragalactic fields, while the catalogue release reports **nearly 3,000 submillimetre sources** over **\(\sim 5\)** square degrees at an average **\(1\sigma\)** depth of **1.2 mJy beam\(^{-1}\)**, approaching a measured **850 \(\mu\)m confusion limit of \(\sim 0.8\) mJy beam\(^{-1}\)** [1607.03904]. Enabled by SCUBA-2’s wide field of view, simultaneous dual-band imaging, stable calibration, and survey-grade mapping speed, S2CLS became a parent survey for interferometric follow-up, source-identification work, cosmic infrared background studies, and measurements of obscured star formation across cosmic time [1208.4622]. ## 1. Instrumental basis and survey conception S2CLS was made possible by **SCUBA-2**, a wide-field bolometric camera with a **43 square arc-minute field of view**, operating simultaneously at **850 \(\mu\)m** and **450 \(\mu\)m** using a dichroic beamsplitter [1208.4622]. The instrument comprises two focal planes, each with **5120 pixels** arranged as **four 1280-bolometer sub-arrays**, and each wavelength channel had an average yield of roughly **3400 working bolometers** on sky [1208.4622]. The detectors are **TES bolometers** with **SQUID** readout and heater-based bias compensation, allowing the arrays to remain stable as sky loading changed [1208.4622]. Survey operation depended on stable atmospheric correction, reproducible flux calibration, and scan strategies optimized for large areas. The SCUBA-2 performance paper gives empirical extinction relations, \[ \tau_{850} = 0.179 \cdot ({\mbox{PWV}} + 0.337), \qquad \tau_{450} = 1.014 \cdot ({\mbox{PWV}} + 0.142), \] and reports stable flux conversion factors over a year, with absolute deviation in performance of less than **10%** [1208.4622]. Two observing modes were central: the **“daisy”** pattern for fields smaller than the focal plane and the **“pong”** pattern for larger fields [1208.4622]. In the best weather, SCUBA-2 could map **1 square degree to 10 mJy beam\(^{-1}\) rms at 850 \(\mu\)m in about 2 hours** and **1 square degree to 60 mJy beam\(^{-1}\) rms at 450 \(\mu\)m in about 5 hours**, a mapping speed that turned degree-scale extragalactic submillimetre surveys into a practical legacy-survey mode [1208.4622]. ## 2. Survey architecture, fields, and catalogues The wide **850 \(\mu\)m** S2CLS component includes the seven canonical extragalactic fields **UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22, and GOODS-North** [1607.03904]. In the catalogue release, the survey is presented as the largest survey of its kind at **850 \(\mu\)m**, with sources spanning flux densities from **\(\sim 1\) mJy up to \(\gtrsim 10\) mJy** [1607.03904]. The same survey infrastructure also supported ultra-deep and field-specific components, such as the **EGS** deep field and the **COSMOS/CANDELS** and **UDS** deep tiers at **450 \(\mu\)m** and **850 \(\mu\)m** [1610.03551]. The **UDS** field became the best-characterized S2CLS region. In the large UDS counterpart study, the **850 \(\mu\)m** map covers about **1 deg\(^2\)** with a median rms noise of **0.89 mJy beam\(^{-1}\)** and a deepest rms of **0.82 mJy beam\(^{-1}\)** after matched filtering, yielding **1088 detections at \(\ge 3.5\sigma\)** [1601.02630]. A high-fidelity “main” sample contains **716 sources with \(S/N \ge 4\sigma\)**, for which the false-detection rate is expected to be only **\(\sim 1\%\)** [1601.02630]. In the later AS2UDS follow-up paper, the parent S2CLS UDS map is described as covering **0.96 deg\(^2\)** at **\(\sigma_{850}=0.90\pm0.05\) mJy beam\(^{-1}\)**, with **716** single-dish sources selected above **\(>4\sigma\)** for ALMA observation [1805.05362]. S2CLS also included genuinely deep single-dish maps. In the **EGS** deep field, simultaneous SCUBA-2 observations reached a central instrumental depth of **\(\sigma_{450}=1.2\) mJy beam\(^{-1}\)** and **\(\sigma_{850}=0.2\) mJy beam\(^{-1}\)** over **\(\sim 70\)** arcmin\(^2\), detecting **57** sources at **450 \(\mu\)m** and **90** at **850 \(\mu\)m** with **\(S/N>3.5\)** [1610.03551]. In the **COSMOS/CANDELS** region, the first deep blank-field **450 \(\mu\)m** map reached **\(\sigma_{450}\approx1.3\) mJy beam\(^{-1}\)** over **140 arcmin\(^2\)** and yielded **60** discrete **450 \(\mu\)m** sources above **\(\sim 3.75\sigma\)** [1211.6668]. ## 3. Number counts, confusion, and the cosmic infrared background A central S2CLS deliverable was a high-precision measurement of submillimetre number counts. The wide **850 \(\mu\)m** survey reduced the Poisson errors on the differential counts to approximately **4% at \(S_{850}\sim 3\) mJy**, found that number counts on **0.5–1 degree** scales are generally within **50%** of the S2CLS mean for **\(S_{850}>3\) mJy**, and identified a bright-end up-turn in the counts indicative of local **850 \(\mu\)m** sources and strongly lensed high-redshift galaxies [1607.03904]. A marginal **\(2\sigma\)** density enhancement was noted in **GOODS-North** [1607.03904]. Deep-field analyses used explicit count parameterizations. In the EGS study, the differential counts were fitted with both a Schechter-like form, \[ \frac{dN}{dS}=\left(\frac{N_0}{S_0}\right)\left(\frac{S}{S_0}\right)^{1-\alpha}\exp\left(-\frac{S}{S_0}\right), \] and a double power law, \[ \frac{dN}{dS}=\frac{N_0}{S_0}\left[\left(\frac{S}{S_0}\right)^\alpha + \left(\frac{S}{S_0}\right)^\beta\right]^{-1}. \] For the full multi-survey fits, the best-fit double-power-law parameters at **850 \(\mu\)m** were **\(N_0=1900\pm600\)**, **\(S_0=4.3\pm0.5\) mJy**, **\(\alpha=2.0\pm0.4\)**, and **\(\beta=6.0\pm0.3\)** [1610.03551]. The same paper emphasizes that the EGS **850 \(\mu\)m** counts are the first blank-field single-dish counts to directly reach the regime overlapped by ALMA [1610.03551]. S2CLS also transformed empirical work on the **cosmic infrared background (CIB)**. In the first deep blank-field **450 \(\mu\)m** map, directly detected point sources contributed \[ I_\nu(450\ \mu{\rm m}) = (7.4 \pm 0.7)\times 10^{-2}\ {\rm MJy\ sr^{-1}}, \] equivalent to **\(16\pm7\%\)** of the COBE/FIRAS background at that wavelength [1211.6668]. Stacking on **1600** Spitzer/MIPS **24 \(\mu\)m** sources added **\(0.20 \pm 0.04\ {\rm MJy\ sr^{-1}}\)**, or **\(42\pm19\%\)** of the **450 \(\mu\)m** CIB, so that the combined accounting reached **\(58\pm20\%\)** [1211.6668]. In the EGS deep field, direct detections plus stacking of **24 \(\mu\)m**-selected galaxies recovered **\(0.26\pm0.03\ {\rm MJy\ sr^{-1}}\)** at **450 \(\mu\)m** and **\(0.07\pm0.01\ {\rm MJy\ sr^{-1}}\)** at **850 \(\mu\)m**, corresponding to roughly **\(60\pm20\%\)** and **\(50\pm20\%\)** of the total CIB, respectively [1610.03551]. The recovered CIB is wavelength dependent, with a peak around **\(z\sim1\)** at **450 \(\mu\)m** and around **\(z\sim2\)** at **850 \(\mu\)m** [1610.03551]. ## 4. Counterparts, redshifts, and population demographics A second major S2CLS contribution was the construction of large counterpart catalogues and redshift distributions for single-dish sources. In the UDS field, the counterpart study used an ALMA pilot sample of **52** ALMA-detected SMGs as a training set and introduced the **Optical-IR Triple Color (OIRTC)** method based on **\(z-K\), \(K-[3.6]\), and \([3.6]-[4.5]\)** colors [1601.02630]. Combined with radio identifications, this yielded counterpart candidates for **80%** of the **Class = 1 \(\ge 4\sigma\)** SCUBA-2 sample, with expected accuracy **\(82\pm20\%\)** and completeness **\(69\pm16\%\)** [1601.02630]. The photometric redshift distribution of the identified SMGs is well described by a lognormal model with median **\(z=2.3\pm0.1\)**, and after accounting for sources without radio and/or OIRTC counterparts the median becomes **\(z=2.6\pm0.1\)** for SMGs with **\(S_{850}>1\) mJy** [1601.02630]. Broader demographic analyses showed that bright S2CLS SMGs are massive systems at the high-mass end of the star-forming population. In the homogeneous **2.18 deg\(^2\)** COSMOS+UDS sample of bright **\(S_{850}>4\) mJy** sources, a high-quality subsample of **651** sources with complete long-wavelength coverage including **1.1-mm imaging** has median **\(z=2.40^{+0.10}_{-0.04}\)**, **\(\mathrm{SFR}=287\pm6\ M_\odot\,{\rm yr}^{-1}\)**, and **\(\log(M_\star/M_\odot)=11.12\pm0.02\)** for the sources with optical identifications [1610.02409]. The same study concludes that these properties locate bright submm galaxies on the high-mass end of the **main sequence** of star-forming galaxies out to **\(z\sim6\)** [1610.02409]. The deepest **850 \(\mu\)m** S2CLS data pushed this argument to lower star-formation rates. In the **\(\sim 150\)** arcmin\(^2\) ultra-deep COSMOS/UltraVISTA/CANDELS field, SCUBA-2 reached **\(\sigma_{850}\simeq0.25\) mJy**, uncovered **106** sources at **\(>5\sigma\)**, and delivered a full-sample mean redshift of **\(\langle z\rangle = 2.38\pm0.09\)** [1509.07144]. For galaxies with **\(M_\star>7\times10^{10}\ M_\odot\)** and **\(z>1.5\)**, the specific star-formation rate distribution is described by a Gaussian peaking at **\(\mathrm{sSFR}=2.25\pm0.19\ {\rm Gyr}^{-1}\)** at **\(z\simeq2.5\)**, and the paper finds no significant evidence for a flattening or turnover of the main sequence at the high-mass end out to at least **\(M_\star\sim2\times10^{11}\ M_\odot\)** and **\(z\sim2\)–3** [1509.07144]. ## 5. Interferometric resolution and the multiplicity problem S2CLS rapidly became the parent catalogue for interferometric tests of how single-dish submillimetre sources are assembled within the **14.6–14.8 arcsec** SCUBA-2 beam. The first ALMA pilot study targeted **30** bright single-dish sources from the UDS field and detected **52** SMGs at **\(>4\sigma\)** significance [1505.05152]. Of the **28** SCUBA-2-detected sources in that sample, **17** fragment into more than one SMG, corresponding to a multiplicity fraction of **\(61^{+19}_{-15}\%\)** [1505.05152]. The brightest SMG contributes on average **\(80^{+6}_{-2}\%\)** of the single-dish flux density, while multiplicity reduces the intrinsic cumulative counts by **20%** at **\(S_{870}>7.5\) mJy** and by **60%** at **\(S_{870}>12\) mJy** [1505.05152]. The same paper reports that the number density of secondary SMGs brighter than **2 mJy** around the brightest sources is **\(80\pm30\)** times higher than blank-field expectations, arguing that a significant fraction of multiple systems are physically associated rather than pure line-of-sight projections [1505.05152]. A larger but shallower SMA campaign emphasized a narrower definition of multiplicity. High-resolution **860 \(\mu\)m** imaging of the brightest S2CLS sources over **4 deg\(^2\)** targeted **75** bright SCUBA-2 sources and, with archival additions, assembled **103** bright single-dish submillimetre sources with interferometric follow-up [1710.02231]. The cumulative number count is systematically lower than the parent SCUBA-2 cumulative number count by **\(24\pm6\%\)** between **11 and 15 mJy**, but the paper estimates that the probability that a **\(\gtrsim 10\) mJy** single-dish source resolves into **two or more galaxies with similar flux densities** is about **15 per cent** [1710.02231]. The decisive survey-scale result came from **AS2UDS**, an ALMA **870 \(\mu\)m** survey of all **716** UDS S2CLS sources detected above **\(>4\sigma\)** [1805.05362]. The ALMA observations, with **\(\sigma_{870}\sim0.25\) mJy beam\(^{-1}\)** and **0.15–0.30 arcsec** resolution, yielded **695** ALMA-detected SMGs across **606** SCUBA-2 pointings [1805.05362]. The SMG number counts brighter than **\(S_{870}\ge4\) mJy** have the same functional shape as the parent SCUBA-2 counts but a normalization lower by **\(28\pm2\%\)**; the fitted differential counts are described by a double power law with **\(N_0=1200^{+200}_{-300}\ {\rm deg^{-2}}\)**, **\(S_0=5.1\pm0.7\) mJy**, **\(\alpha=5.9^{+1.3}_{-0.9}\)**, and **\(\beta=0.4\pm0.1\)** [1805.05362]. Using a “multiple” definition of more than one ALMA-detected SMG with **\(S_{870}\ge1\) mJy** within the ALMA primary beam, the survey finds **\(11\pm1\%\)** multiples in the full **716-pointing** sample, **\(28\pm2\%\)** among sources with **\(S^{\rm deb}_{850}\ge5\) mJy**, and **\(44^{+16}_{-14}\%\)** among the brightest sources with **\(S^{\rm deb}_{850}\ge9\) mJy** [1805.05362]. Photometric-redshift comparisons for **46** pairs show a significant excess of close-redshift pairs, with **24/46** satisfying **\(\Delta z_{\rm phot}<0.25\)** and a **\(<1\%\)** chance probability relative to random pairing, leading to the conclusion that at least **\(\sim 30\%\)** of all multiple-SMG SCUBA-2 fields arise from physically associated pairs [1805.05362]. A common misconception is that the multiplicity literature is internally inconsistent. The published values differ because the measurements are not defined identically. The SMA study asks how often a **\(\gtrsim 10\) mJy** source resolves into near-equal components above **\(S_{860}\gtrsim6\) mJy** [1710.02231], whereas the ALMA studies count fainter secondaries down to **\(S_{870}\sim1\) mJy** within much deeper, higher-resolution maps [1505.05152; 1805.05362]. This suggests that multiplicity is highly sensitive to angular resolution, detection threshold, and the operational definition of what constitutes a blended source. ## 6. Scientific reach beyond source catalogues Because S2CLS combined large area with deep imaging and extensive ancillary coverage, it supported studies well beyond blank-field source counts. In the UDS field, stacking of **Lyman break galaxies** at **\(z\approx3,4,\) and \(5\)** yielded average **850 \(\mu\)m** flux densities of **\(0.249\pm0.029\)**, **\(0.411\pm0.064\)**, and **\(0.875\pm0.229\) mJy**, respectively [1407.6712]. Template fitting gave **\(L_{\rm IR}=3.2^{+0.8}_{-0.6}\times10^{11}\ L_\odot\)**, **\(5.5^{+0.3}_{-0.4}\times10^{11}\ L_\odot\)**, and **\(11.0^{+4.2}_{-5.9}\times10^{11}\ L_\odot\)**, corresponding to **\(\mathrm{SFR}_{\rm IR}=55^{+14}_{-10}\)**, **\(93^{+5}_{-7}\)**, and **\(186^{+71}_{-101}\ M_\odot\,{\rm yr}^{-1}\)** [1407.6712]. Corrected over **\(310^{10}\ M_\odot\)** the obscured star-formation-rate density exceeds the unobscured one by a factor of **\(>10\)** and that the cosmic star-formation history undergoes a transition at **\(z\sim3\)–4**, where predominantly unobscured growth is overtaken by obscured star formation [1607.04283]. Field-specific studies used S2CLS to probe environment and very high redshift. In the **\(z=1.62\)** cluster **Cl0218.3−0510** in UDS, S2CLS **850 \(\mu\)m** imaging, combined with SPIRE, radio, and redshift information, identified **31** far-infrared/submillimetre-detected probable cluster members with **\(L_{\rm bol}>10^{12}L_\odot\)** and a mass-normalized star-formation rate of **\(800\pm500\ {\rm yr}^{-1}(10^{-14}\ M_\odot^{-1})\)**, about an order of magnitude higher than clusters at **\(z\sim0.5\)–1** [1312.3329]. At still higher redshift, the AS2UDS ALMA cubes were used to identify candidate **\(z\sim4.5\)** **[CII]** emitters among S2CLS-selected SMGs: **10** candidate line emitters were found, the inferred space density of luminous SMGs at **\(z=4.40\)–4.66** is **\(>5\times10^{-6}\ {\rm Mpc}^{-3}\)**, and the analysis suggests that **\(\ge 7\%\)** of **\(S_{870}\gtrsim1\) mJy** SMGs lie at **\(4