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Register a new userThe SCUBA-2 Cosmology Legacy Survey: the nature of bright submm galaxies from 2 deg2 of 850-um imaging
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Micha{\\l} Jerzy Micha{\\l}owski
Publication date
2016
fields
Physics
and research's language is
English
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We present physical properties [redshifts (z), star-formation rates (SFRs) and stellar masses (Mstar)] of bright (S850>4mJy) submm galaxies in the ~2deg2 COSMOS and UDS fields selected with SCUBA-2/JCMT. We complete the galaxy identification process for all (~2000) S/N>3.5 850-um sources, but focus our scientific analysis on a high-quality sub-sample of 651 S/N>4 sources with complete multi-wavelength coverage including 1.1-mm imaging. We check the reliability of our identifications, and the robustness of the SCUBA-2 fluxes by revisiting the recent ALMA follow-up of 29 sources in our sample. Considering >4mJy ALMA sources, our identification method has a completeness of ~86 per cent with a reliability of ~92 per cent, and only ~15-20 per cent of sources are significantly affected by multiplicity (when a secondary component contributes >1/3 of the primary source flux). The impact of source blending on the 850-um source counts as determined with SCUBA-2 is modest; scaling the single-dish fluxes by ~0.9 reproduces the ALMA source counts. For our final SCUBA-2 sample we find median z=2.40+0.10-0.04, SFR=287+-6Moyr-1, and log(Mstar/Mo)=11.12+-0.02 (the latter for 349/651 sources with optical identifications). These properties clearly locate bright submm galaxies on the high-mass end of the main sequence of star-forming galaxies out to z~6, suggesting that major mergers are not a dominant driver of the high-redshift submm-selected population. Their number densities are also consistent with the evolving galaxy stellar mass function. Hence, the submm galaxy population is as expected, albeit reproducing the evolution of the main sequence of star-forming galaxies remains a challenge for theoretical models/simulations.
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We have used the Submillimeter Array at 860$,mu$m to observe the brightest SCUBA-2 sources in 4$,$deg$^{2}$ of the Cosmology Legacy Survey. We have targeted 75 of the brightest single-dish SCUBA-2 850$,mu$m sources down to $S_{850},{approx},8,$mJy, achieving an average synthesized beam of 2.4$^{primeprime}$ and an average rms of $sigma_{860},{=},1.5,$mJy in our primary beam-corrected maps. We searched our maps for $4sigma$ peaks, corresponding to $S_{860},{gtrsim},6,$mJy sources, and detected 59 single galaxies and three pairs of galaxies. We include in our study 28 archival observations, bringing our sample size to 103 bright single-dish submillimetre sources with interferometric follow-up. We compute the cumulative and differential number counts of our sample, finding them to overlap with previous single-dish survey number counts within the uncertainties, although our cumulative number count is systematically lower than the parent SCUBA-2 cumulative number count by $24,{pm},6$ per cent between 11 and 15$,$mJy. We estimate the probability that a ${gtrsim},10,$mJy single-dish submillimetre source resolves into two or more galaxies with similar flux densities, causing a significant change in the number counts, to be about 15 per cent. Assuming the remaining 85 per cent of the targets are ultra-luminous starburst galaxies between $z,{=},2$-3, we find a likely volume density of ${gtrsim},400,$M$_{odot},$yr$^{-1}$ sources to be ${sim},3^{+0.7}_{-0.6},{times},10^{-7},$Mpc$^{-3}$. We show that the descendants of these galaxies could be ${gtrsim},4,{times},10^{11},$M$_{odot}$ local quiescent galaxies, and that about 10 per cent of their total stellar mass would have formed during these short bursts of star-formation.
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 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.
We present deep observations at 450 um and 850 um in the Extended Groth Strip field taken with the SCUBA-2 camera mounted on the James Clerk Maxwell Telescope as part of the deep SCUBA-2 Cosmology Legacy Survey (S2CLS), achieving a central instrumental depth of $sigma_{450}=1.2$ mJy/beam and $sigma_{850}=0.2$ mJy/beam. We detect 57 sources at 450 um and 90 at 850 um with S/N > 3.5 over ~70 sq. arcmin. From these detections we derive the number counts at flux densities $S_{450}>4.0$ mJy and $S_{850}>0.9$ mJy, which represent the deepest number counts at these wavelengths derived using directly extracted sources from only blank-field observations with a single-dish telescope. Our measurements smoothly connect the gap between previous shallower blank-field single-dish observations and deep interferometric ALMA results. We estimate the contribution of our SCUBA-2 detected galaxies to the cosmic infrared background (CIB), as well as the contribution of 24 um-selected galaxies through a stacking technique, which add a total of $0.26pm0.03$ and $0.07pm0.01$ MJy/sr, at 450 um and 850 um, respectively. These surface brightnesses correspond to $60pm20$ and $50pm20$ per cent of the total CIB measurements, where the errors are dominated by those of the total CIB. Using the photometric redshifts of the 24 um-selected sample and the redshift distributions of the submillimetre galaxies, we find that the redshift distribution of the recovered CIB is different at each wavelength, with a peak at $zsim1$ for 450 um and at $zsim2$ for 850um, consistent with previous observations and theoretical models.
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