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Register a new userEvolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey
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We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$mu$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$mu$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $zsim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$beta$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_ast>10^{10}M_odot$ galaxies at $0.5<z<6$. This is dominated by obscured star formation by a factor of $>10$. One third of this is accounted for by 450$mu$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $zlesssim3$, and from UV-only data at $zsim5$. The cosmic star-formation history undergoes a transition at $zsim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.
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We analyse new SCUBA-2 submillimeter and archival SPIRE far-infrared imaging of a z=1.62 cluster, Cl0218.3-0510, which lies in the UKIDSS/UDS field of the SCUBA-2 Cosmology Legacy Survey. Combining these tracers of obscured star formation activity with the extensive photometric and spectroscopic information available for this field, we identify 31 far-infrared/submillimeter-detected probable cluster members with bolometric luminosities >1e12 Lo and show that by virtue of their dust content and activity, these represent some of the reddest and brightest galaxies in this structure. We exploit Cycle-1 ALMA submillimeter continuum imaging which covers one of these sources to confirm the identification of a SCUBA-2-detected ultraluminous star-forming galaxy in this structure. Integrating the total star-formation activity in the central region of the structure, we estimate that it is an order of magnitude higher (in a mass-normalised sense) than clusters at z~0.5-1. However, we also find that the most active cluster members do not reside in the densest regions of the structure, which instead host a population of passive and massive, red galaxies. We suggest that while the passive and active populations have comparable near-infrared luminosities at z=1.6, M(H)~-23, the subsequent stronger fading of the more active galaxies means that they will evolve into passive systems at the present-day which are less luminous than the descendants of those galaxies which were already passive at z~1.6 (M(H)~-20.5 and M(H)~-21.5 respectively at z~0). We conclude that the massive galaxy population in the dense cores of present-day clusters were already in place at z=1.6 and that in Cl0218.3-0510 we are seeing continuing infall of less extreme, but still ultraluminous, star-forming galaxies onto a pre-existing structure.
We present a catalogue of nearly 3,000 submillimetre sources detected at 850um over ~5 square degrees surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850um, probing a meaningful cosmic volume at the peak of star formation activity and increasing the sample size of submillimetre galaxies selected at 850um by an order of magnitude. We describe the wide 850um survey component of S2CLS, which covers the key extragalactic survey fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1-sigma depth of S2CLS is 1.2 mJy/beam, approaching the SCUBA-2 850um confusion limit, which we determine to be ~0.8 mJy/beam. We measure the single dish 850um number counts to unprecedented accuracy, reducing the Poisson errors on the differential counts to approximately 4% at S_850~3mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5-1 degree scales are generally within 50% of the S2CLS mean for S_850>3mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2-sigma) density enhancement in the GOODS-North field. The observed number counts are in reasonable agreement with recent phenomenological and semi-analytic models. Finally, the large solid angle of S2CLS allows us to measure the bright-end counts: at S_850>10mJy there are approximately ten sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850um emission and strongly lensed high-redshift galaxies. Here we describe the data collection and reduction procedures and present calibrated maps and a catalogue of sources; these are made publicly available.
We investigate the properties of the galaxies selected from the deepest 850-micron survey undertaken to date with SCUBA-2 on the JCMT. This deep 850-micron imaging was taken in parallel with deep 450-micron imaging in the very best observing conditions as part of the SCUBA-2 Cosmology Legacy Survey. A total of 106 sources were uncovered at 850 microns from ~150, sq. arcmin in the centre of the COSMOS/UltraVISTA/CANDELS field, imaged to a typical rms depth of ~0.25 mJy. We utilise the wealth of available deep multi-frequency data to establish the complete redshift distribution for this sample, yielding <z> = 2.38 +- 0.09, a mean redshift comparable with that derived for all but the brightest previous sub-mm samples. We have also been able to establish the stellar masses of the majority of the galaxy identifications, enabling us to explore their location on the star-formation-rate:stellar-mass (SFR:M*) plane. Crucially, our new deep sample reaches flux densities equivalent to SFR ~ 100 Msun/yr, enabling us to confirm that sub-mm galaxies form the high-mass end of the `main sequence (MS) of star-forming galaxies at z > 1.5 (with a mean specific SFR of sSFR = 2.25 +- 0.19 /Gyr at z ~ 2.5). Our results are consistent with no significant flattening of the MS towards high masses at these redshifts, suggesting that reports of such flattening possibly arise from under-estimates of dust-enshrouded star-formation activity in massive star-forming galaxies. However, our findings add to the growing evidence that average sSFR rises only slowly at high redshift, resulting in log(sSFR) being an apparently simple linear function of the age of the Universe.
We present high-resolution 870-um ALMA continuum maps of 30 bright sub-millimeter sources in the UKIDSS UDS field. These sources are selected from deep, 1-square degrees 850-um maps from the SCUBA--2 Cosmology Legacy Survey, and are representative of the brightest sources in the field (median SCUBA2 flux S_850=8.7+/-0.4 mJy). We detect 52 sub-millimeter galaxies (SMGs) at >4-sigma significance in our 30 ALMA maps. In 61+/-17% of the ALMA maps the single-dish source comprises a blend of >=2 SMGs, where the secondary SMGs are Ultra--Luminous Infrared Galaxies (ULIRGs) with L_IR>10^12 Lo. The brightest SMG contributes on average 80+/-4% of the single-dish flux density, and in the ALMA maps containing >=2 SMGs the secondary SMG contributes 25+/-3% of the integrated ALMA flux. We construct source counts and show that multiplicity boosts the apparent single-dish cumulative counts by 20% at S_870>7.5mJy, and by 60% at S_870>12mJy. We combine our sample with previous ALMA studies of fainter SMGs and show that the counts are well-described by a double power-law with a break at 8.5+/-0.6mJy. The break corresponds to a luminosity of ~6x10^12Lsol or a star-formation rate of ~1000Mo/yr. For the typical sizes of these SMGs, which are resolved in our ALMA data with r=1.2+/-0.1kpc, this yields a limiting SFR density of ~100Msol/yr/kpc2. Finally, the number density of S_870>2mJy SMGs is 80+/-30 times higher than that derived from blank-field counts. An over-abundance of faint SMGs is inconsistent with line-of-sight projections dominating multiplicity in the brightest SMGs, and indicates that a significant proportion of these high-redshift ULIRGs must be physically associated.
We present statistically significant detections at 850um of the Lyman Break Galaxy (LBG) population at z=3, 4, and 5 using data from the Submillimetre Common User Bolometer Array 2 (SCUBA-2) Cosmology Legacy Survey (S2CLS) in the United Kingdom Infrared Deep Sky Survey Ultra Deep Survey (UKIDSS-UDS) field. We employ a stacking technique to probe beneath the survey limit to measure the average 850um flux density of LBGs at z=3, 4, and 5 with typical ultraviolet luminosities of L(1700A)~10^29 erg/s/Hz. We measure 850um flux densities of (0.25 +/- 0.03, (0.41 +/- 0.06), and (0.88 +/- 0.23) mJy respectively, and find that they contribute at most 20 per cent to the cosmic far-infrared background at 850um. Fitting an appropriate range of spectral energy distributions to the z=3, 4, and 5 LBG stacked 24-850um fluxes, we derive infrared (IR) luminosities of L(8-1000um)~3.2, 5.5, and 11.0x10^11 Lsun (corresponding to star formation rates of ~50-200 Msun/yr) respectively. We find that the evolution in the IR luminosity density of LBGs is broadly consistent with model predictions for the expected contribution of luminous IR galaxy (LIRG) to ultraluminous IR galaxy (ULIRG) type systems at these epochs. We also see a strong positive correlation between stellar mass and IR luminosity. Our data are consistent with the main sequence of star formation showing little or no evolution from z=3 to 5. We have also confirmed that, for a fixed mass, the reddest LBGs (UV slope Beta -> 0) are indeed redder due to dust extinction, with SFR(IR)/SFR(UV) increasing by approximately an order of magnitude over -2<Beta<0 such that SFR(IR)/SFR(UV)~20 for the reddest LBGs. Furthermore, the most massive LBGs also tend to have higher obscured-to-unobscured ratio, hinting at a variation in the obscuration properties across the mass range.
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