SCUBA-2 Cosmology Legacy Survey (S2CLS)

• SCUBA-2 Cosmology Legacy Survey (S2CLS) is a blank-field submillimetre survey using dual-band imaging at 850 μm and 450 μm to map wide extragalactic areas and study dusty star formation.
• The survey leverages SCUBA-2’s wide field-of-view, TES bolometers, and stable calibration to deliver high-precision number counts, flux measurements, and assessments of source multiplicity.
• S2CLS provides a comprehensive legacy data set that supports multiwavelength follow-ups, enabling detailed investigations of the cosmic infrared background and galaxy demographics from z≈1 to z≈6.

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 (Geach et al., 2016). 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$^{-1}$ (Geach et al., 2016). 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 (Dempsey et al., 2012).

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$0m using a dichroic beamsplitter (Dempsey et al., 2012). 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 (Dempsey et al., 2012). The detectors are TES bolometers with SQUID readout and heater-based bias compensation, allowing the arrays to remain stable as sky loading changed (Dempsey et al., 2012).

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,

$\mu$1

and reports stable flux conversion factors over a year, with absolute deviation in performance of less than 10% (Dempsey et al., 2012). Two observing modes were central: the “daisy” pattern for fields smaller than the focal plane and the “pong” pattern for larger fields (Dempsey et al., 2012). In the best weather, SCUBA-2 could map 1 square degree to 10 mJy beam$\mu$2 rms at 850 $\mu$3m in about 2 hours and 1 square degree to 60 mJy beam$\mu$4 rms at 450 $\mu$5m in about 5 hours, a mapping speed that turned degree-scale extragalactic submillimetre surveys into a practical legacy-survey mode (Dempsey et al., 2012).

2. Survey architecture, fields, and catalogues

The wide 850 $\mu$6m S2CLS component includes the seven canonical extragalactic fields UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22, and GOODS-North (Geach et al., 2016). In the catalogue release, the survey is presented as the largest survey of its kind at 850 $\mu$7m, with sources spanning flux densities from $\mu$8 mJy up to $\mu$9 mJy (Geach et al., 2016). 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$0m and 850 $\mu$1m (Zavala et al., 2016).

The UDS field became the best-characterized S2CLS region. In the large UDS counterpart study, the 850 $\mu$2m map covers about 1 deg$\mu$3 with a median rms noise of 0.89 mJy beam$\mu$4 and a deepest rms of 0.82 mJy beam$\mu$5 after matched filtering, yielding 1088 detections at $\mu$6 (Chen et al., 2016). A high-fidelity “main” sample contains 716 sources with $\mu$7, for which the false-detection rate is expected to be only $\mu$8 (Chen et al., 2016). In the later AS2UDS follow-up paper, the parent S2CLS UDS map is described as covering 0.96 deg$\mu$9 at $\sim 5$0 mJy beam$\sim 5$1, with 716 single-dish sources selected above $\sim 5$2 for ALMA observation (Stach et al., 2018).

S2CLS also included genuinely deep single-dish maps. In the EGS deep field, simultaneous SCUBA-2 observations reached a central instrumental depth of $\sim 5$3 mJy beam$\sim 5$4 and $\sim 5$5 mJy beam$\sim 5$6 over $\sim 5$7 arcmin$\sim 5$8, detecting 57 sources at 450 $\sim 5$9m and 90 at 850 $1\sigma$0m with $1\sigma$1 (Zavala et al., 2016). In the COSMOS/CANDELS region, the first deep blank-field 450 $1\sigma$2m map reached $1\sigma$3 mJy beam$1\sigma$4 over 140 arcmin$1\sigma$5 and yielded 60 discrete 450 $1\sigma$6m sources above $1\sigma$7 (Geach et al., 2012).

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 $1\sigma$8m survey reduced the Poisson errors on the differential counts to approximately 4% at $1\sigma$9 mJy, found that number counts on 0.5–1 degree scales are generally within 50% of the S2CLS mean for $^{-1}$0 mJy, and identified a bright-end up-turn in the counts indicative of local 850 $^{-1}$1m sources and strongly lensed high-redshift galaxies (Geach et al., 2016). A marginal $^{-1}$2 density enhancement was noted in GOODS-North (Geach et al., 2016).

Deep-field analyses used explicit count parameterizations. In the EGS study, the differential counts were fitted with both a Schechter-like form,

$^{-1}$3

and a double power law,

$^{-1}$4

For the full multi-survey fits, the best-fit double-power-law parameters at 850 $^{-1}$5m were $^{-1}$6, $^{-1}$7 mJy, $^{-1}$8, and $^{-1}$9 (Zavala et al., 2016). The same paper emphasizes that the EGS 850 $\mu$0m counts are the first blank-field single-dish counts to directly reach the regime overlapped by ALMA (Zavala et al., 2016).

S2CLS also transformed empirical work on the cosmic infrared background (CIB). In the first deep blank-field 450 $\mu$1m map, directly detected point sources contributed

$\mu$2

equivalent to $\mu$3 of the COBE/FIRAS background at that wavelength (Geach et al., 2012). Stacking on 1600 Spitzer/MIPS 24 $\mu$4m sources added $\mu$5, or $\mu$6 of the 450 $\mu$7m CIB, so that the combined accounting reached $\mu$8 (Geach et al., 2012). In the EGS deep field, direct detections plus stacking of 24 $\mu$9m-selected galaxies recovered $\sim 0.8$0 at 450 $\sim 0.8$1m and $\sim 0.8$2 at 850 $\sim 0.8$3m, corresponding to roughly $\sim 0.8$4 and $\sim 0.8$5 of the total CIB, respectively (Zavala et al., 2016). The recovered CIB is wavelength dependent, with a peak around $\sim 0.8$6 at 450 $\sim 0.8$7m and around $\sim 0.8$8 at 850 $\sim 0.8$9m (Zavala et al., 2016).

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 $^{-1}$0, $^{-1}$1, and $^{-1}$2 colors (Chen et al., 2016). Combined with radio identifications, this yielded counterpart candidates for 80% of the Class = 1 $^{-1}$3 SCUBA-2 sample, with expected accuracy $^{-1}$4 and completeness $^{-1}$5 (Chen et al., 2016). The photometric redshift distribution of the identified SMGs is well described by a lognormal model with median $^{-1}$6, and after accounting for sources without radio and/or OIRTC counterparts the median becomes $^{-1}$7 for SMGs with $^{-1}$8 mJy (Chen et al., 2016).

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$^{-1}$9 COSMOS+UDS sample of bright $\mu$0 mJy sources, a high-quality subsample of 651 sources with complete long-wavelength coverage including 1.1-mm imaging has median $\mu$1, $\mu$2, and $\mu$3 for the sources with optical identifications (Michałowski et al., 2016). 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 $\mu$4 (Michałowski et al., 2016).

The deepest 850 $\mu$5m S2CLS data pushed this argument to lower star-formation rates. In the $\mu$6 arcmin$\mu$7 ultra-deep COSMOS/UltraVISTA/CANDELS field, SCUBA-2 reached $\mu$8 mJy, uncovered 106 sources at $\mu$9, and delivered a full-sample mean redshift of $\mu$00 (Koprowski et al., 2015). For galaxies with $\mu$01 and $\mu$02, the specific star-formation rate distribution is described by a Gaussian peaking at $\mu$03 at $\mu$04, 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 $\mu$05 and $\mu$06–3 (Koprowski et al., 2015).

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 $\mu$07 significance (Simpson et al., 2015). Of the 28 SCUBA-2-detected sources in that sample, 17 fragment into more than one SMG, corresponding to a multiplicity fraction of $\mu$08 (Simpson et al., 2015). The brightest SMG contributes on average $\mu$09 of the single-dish flux density, while multiplicity reduces the intrinsic cumulative counts by 20% at $\mu$10 mJy and by 60% at $\mu$11 mJy (Simpson et al., 2015). The same paper reports that the number density of secondary SMGs brighter than 2 mJy around the brightest sources is $\mu$12 times higher than blank-field expectations, arguing that a significant fraction of multiple systems are physically associated rather than pure line-of-sight projections (Simpson et al., 2015).

A larger but shallower SMA campaign emphasized a narrower definition of multiplicity. High-resolution 860 $\mu$13m imaging of the brightest S2CLS sources over 4 deg$\mu$14 targeted 75 bright SCUBA-2 sources and, with archival additions, assembled 103 bright single-dish submillimetre sources with interferometric follow-up (Hill et al., 2017). The cumulative number count is systematically lower than the parent SCUBA-2 cumulative number count by $\mu$15 between 11 and 15 mJy, but the paper estimates that the probability that a $\mu$16 mJy single-dish source resolves into two or more galaxies with similar flux densities is about 15 per cent (Hill et al., 2017).

The decisive survey-scale result came from AS2UDS, an ALMA 870 $\mu$17m survey of all 716 UDS S2CLS sources detected above $\mu$18 (Stach et al., 2018). The ALMA observations, with $\mu$19 mJy beam$\mu$20 and 0.15–0.30 arcsec resolution, yielded 695 ALMA-detected SMGs across 606 SCUBA-2 pointings (Stach et al., 2018). The SMG number counts brighter than $\mu$21 mJy have the same functional shape as the parent SCUBA-2 counts but a normalization lower by $\mu$22; the fitted differential counts are described by a double power law with $\mu$23, $\mu$24 mJy, $\mu$25, and $\mu$26 (Stach et al., 2018). Using a “multiple” definition of more than one ALMA-detected SMG with $\mu$27 mJy within the ALMA primary beam, the survey finds $\mu$28 multiples in the full 716-pointing sample, $\mu$29 among sources with $\mu$30 mJy, and $\mu$31 among the brightest sources with $\mu$32 mJy (Stach et al., 2018). Photometric-redshift comparisons for 46 pairs show a significant excess of close-redshift pairs, with 24/46 satisfying $\mu$33 and a $\mu$34 chance probability relative to random pairing, leading to the conclusion that at least $\mu$35 of all multiple-SMG SCUBA-2 fields arise from physically associated pairs (Stach et al., 2018).

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 $\mu$36 mJy source resolves into near-equal components above $\mu$37 mJy (Hill et al., 2017), whereas the ALMA studies count fainter secondaries down to $\mu$38 mJy within much deeper, higher-resolution maps (Simpson et al., 2015, Stach et al., 2018). 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 $\mu$39 and $\mu$40 yielded average 850 $\mu$41m flux densities of $\mu$42, $\mu$43, and $\mu$44 mJy, respectively (Coppin et al., 2014). Template fitting gave $\mu$45, $\mu$46, and $\mu$47, corresponding to $\mu$48, $\mu$49, and $\mu$50 (Coppin et al., 2014). Corrected over $\mu$51, canonical UV-selected LBGs contribute about 14–20% of the 850 $\mu$52m background (Coppin et al., 2014).

S2CLS also became a long-wavelength anchor for studies of obscured star formation in AGN hosts and massive galaxies. Using S2CLS 850 $\mu$53m maps in five fields and ancillary 24–500 $\mu$54m data, a stacking analysis of 1957 X-ray selected AGN at $\mu$55 found average host-galaxy star-formation rates of 80–600 $\mu$56 and a flat trend of SFR with X-ray luminosity across about 3 orders of magnitude in $\mu$57 (Ramasawmy et al., 2019). A complementary analysis of the deepest 450/850 $\mu$58m S2CLS imaging in AEGIS, COSMOS, and UDS, deconfused with T-PHOT and paired with CANDELS/3D-HST priors, concluded that for galaxies with $\mu$59 the obscured star-formation-rate density exceeds the unobscured one by a factor of $\mu$60 and that the cosmic star-formation history undergoes a transition at $\mu$61–4, where predominantly unobscured growth is overtaken by obscured star formation (Bourne et al., 2016).

Field-specific studies used S2CLS to probe environment and very high redshift. In the $\mu$62 cluster Cl0218.3−0510 in UDS, S2CLS 850 $\mu$63m imaging, combined with SPIRE, radio, and redshift information, identified 31 far-infrared/submillimetre-detected probable cluster members with $\mu$64 and a mass-normalized star-formation rate of $\mu$65, about an order of magnitude higher than clusters at $\mu$66–1 (Smail et al., 2013). At still higher redshift, the AS2UDS ALMA cubes were used to identify candidate $\mu$67 [CII] emitters among S2CLS-selected SMGs: 10 candidate line emitters were found, the inferred space density of luminous SMGs at $\mu$68–4.66 is $\mu$69, and the analysis suggests that $\mu$70 of $\mu$71 mJy SMGs lie at $\mu$72 (Cooke et al., 2018). Stacked high-resolution ALMA imaging further showed that the [CII] emission is more extended than the dust continuum, with $\mu$73 kpc for [CII] and $\mu$74 kpc for the continuum (Cooke et al., 2018).

Taken together, these results established S2CLS as more than a source catalogue. It became the observational framework within which single-dish submillimetre number counts, interferometric multiplicity, counterpart identification, the redshift distribution of dusty galaxies, the resolved CIB, and the obscured component of galaxy growth could be addressed in a common survey ecosystem.