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Register a new userCharacterisation of SCUBA-2 450um and 850um-selected Galaxies in the COSMOS Field
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Caitlin M. Casey
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We present deep 450um and 850um observations of a large, uniformly covered 394arcmin^2 area in the COSMOS field obtained with the SCUBA-2 instrument on the James Clerk Maxwell Telescope (JCMT). We achieve root-mean-square noise values of 4.13mJy at 450um and 0.80mJy at 850um. The differential and cumulative number counts are presented and compared to similar previous works. Individual point sources are identified at >3.6sigma significance, a threshold corresponding to a 3-5% sample contamination rate. We identify 78 sources at 450um and 99 at 850um, with flux densities S450=13-37mJy and S850=2-16mJy. Only 62-76% of 450um sources are 850um detected and 61-81% of 850um sources are 450um detected. The positional uncertainties at 450um are small (1-2.5) and therefore allow a precise identification of multiwavelength counterparts without reliance on detection at 24um or radio wavelengths; we find that only 44% of 450um-selected galaxies and 60% of 850um-sources have 24um or radio counterparts. 450um-selected galaxies peak at <z>=1.95+-0.19 and 850um=selected galaxies peak at <z>=2.16+-0.11. The two samples occupy similar parameter space in redshift and luminosity, while their median SED peak wavelengths differ by ~10-50um (translating to deltaTdust =8-12K, where 450um-selected galaxies are warmer). The similarities of the 450um and 850um populations, yet lack of direct overlap between them, suggests that submillimeter surveys conducted at any single far-infrared wavelength will be significantly incomplete (~>30%) at censusing infrared-luminous star formation at high-z.
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The first deep blank-field 450um map (1-sigma~1.3mJy) from the SCUBA-2 Cosmology Legacy Survey (S2CLS), conducted with the James Clerk Maxwell Telescope (JCMT) is presented. Our map covers 140 arcmin^2 of the COSMOS field, in the footprint of the HST CANDELS area. Using 60 submillimetre galaxies (SMGs) detected at >3.75-sigma, we evaluate the number counts of 450um-selected galaxies with flux densities S_450>5mJy. The 8-arcsec JCMT beam and high sensitivity of SCUBA-2 now make it possible to directly resolve a larger fraction of the cosmic infrared background (CIB, peaking at ~200um) into the individual galaxies responsible for its emission than has previously been possible at this wavelength. At S_450>5mJy we resolve (7.4[+/-]0.7)x10^-2 MJy/sr of the CIB at 450um (equivalent to 16[+/-]7% of the absolute brightness measured by COBE at this wavelength) into point sources. A further ~40% of the CIB can be recovered through a statistical stack of 24um emitters in this field, indicating that the majority (~60%) of the CIB at 450um is emitted by galaxies with S_450>2mJy. The average redshift of 450um emitters identified with an optical/near-infrared counterpart is estimated to be <z>=1.3, implying that the galaxies in the sample are in the ultraluminous class (L_IR~1.1x10^12 L_sun). If the galaxies contributing to the statistical stack lie at similar redshifts, then the majority of the CIB at 450um is emitted by galaxies in the LIRG class with L_IR>3.6x10^11 L_sun.
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 present an ALMA study of the ~180 brightest sources in the SCUBA-2 map of the COSMOS field from the S2COSMOS survey, as a pilot study for AS2COSMOS - a full survey of the ~1,000 sources in this field. In this pilot we have obtained 870-um continuum maps of an essentially complete sample of the brightest 182 sub-millimetre sources (S_850um=6.2mJy) in COSMOS. Our ALMA maps detect 260 sub-millimetre galaxies (SMGs) spanning a range in flux density of S_870um=0.7-19.2mJy. 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_850um>12mJy. We estimate that approximately one-third of these SMG-SMG pairs are physically associated (with a higher rate for the brighter secondary SMGs, S_870um>3mJy), and illustrate this with the serendipitous detection of bright [CII] 157.74um line emission in two SMGs, AS2COS0001.1 & 0001.2 at z=4.63, associated with the highest significance single-dish source. Using our source catalogue we construct the interferometric 870um number counts at S_870um>6.2mJy. 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_870um>6.2mJy AS2COSMOS sample of z=2.87+/-0.08, and clear evidence for an increase in the median redshift with 870-um flux density suggesting strong evolution in the bright-end of the 870um luminosity function.
We present extensive observations of a sample of distant, submm galaxies detected in the field of the massive cluster lens, A1835, using SCUBA. Taken in conjunction with earlier observations of other submm-selected sources (Ivison et al. 1998; Smail et al. 1999; Soucail et al. 1999) we now have detailed, multi-wavelength observations of 7 examples of the submm population, having exploited the combination of achromatic amplification by cluster lenses and lavish archival datasets. These sources, all clearly at z > 1, illustrate the wide range in the radio and optical properties of distant submm-selected galaxies. We include detailed observations of the first candidate `pure starburst submm galaxy at high redshift, a z=2.56 interacting galaxy which shows no obvious sign of hosting an AGN. The remaining sources have varying degrees of inferred AGN activity (3 from 7 of the most luminous show some evidence of the presence of an AGN) although even when an AGN is obviously present it is still not apparent if reprocessed radiation from this source dominates the submm emission. In contrast with the variation in the spectral properties, we see relatively homogeneous morphologies for the population, with a large fraction of merging or interacting systems. Our study shows that virtually identical SEDs are seen for galaxies which exhibit strikingly different opt/UV spectral-line characteristics. We conclude that standard opt/UV spectral classifications are misleading when applied to distant, highly obscured galaxies and that we must seek other means of determining the various contributions to the overall energy budget of submm galaxies and hence to the far-IR extragalactic background.
We present near-infrared spectroscopic observations from VLT ISAAC of thirteen 250mu m-luminous galaxies in the CDF-S, seven of which have confirmed redshifts which average to <z > = 2.0 pm 0.4. Another two sources of the 13 have tentative z > 1 identifications. Eight of the nine redshifts were identified with H{alpha} detection in H- and K-bands, three of which are confirmed redshifts from previous spectroscopic surveys. We use their near-IR spectra to measure H{alpha} line widths and luminosities, which average to 415 pm 20 km/s and 3 times 10^35 W (implying SFR(H{alpha})~200 M_odot /yr), both similar to the H{alpha} properties of SMGs. Just like SMGs, 250 mu m-luminous galaxies have large H{alpha} to far-infrared (FIR) extinction factors such that the H{alpha} SFRs underestimate the FIR SFRs by ~8-80 times. Far-infrared photometric points from observed 24mu m through 870mu m are used to constrain the spectral energy distributions (SEDs) even though uncertainty caused by FIR confusion in the BLAST bands is significant. The population has a mean dust temperature of Td = 52 pm 6 K, emissivity {beta} = 1.73 pm 0.13, and FIR luminosity LFIR = 3 times 10^13 L_odot. Although selection at 250mu m allows for the detection of much hotter dust dominated HyLIRGs than SMG selection (at 850mu m), we do not find any >60 K hot-dust HyLIRGs. We have shown that near-infrared spectroscopy combined with good photometric redshifts is an efficient way to spectroscopically identify and characterise these rare, extreme systems, hundreds of which are being discovered by the newest generation of IR observatories including the Herschel Space Observatory.
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