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Register a new userSCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES): Faint-End Counts at 450 um
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The SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) is a three-year JCMT Large Program aiming at reaching the 450 $mu$m confusion limit in the COSMOS-CANDELS region, to study a representative sample of the high-redshift far-infrared galaxy population that gives rise to the bulk of the far-infrared background. We present the first-year data from STUDIES. We have reached a 450 $mu$m noise level of 0.91~mJy for point sources at the map center, covered an area of 151 arcmin$^2$, and detected 98 and 141 sources at 4.0 and 3.5 $sigma$, respectively. Our derived counts are best constrained in the 3.5-25 mJy regime using directly detected sources. Below the detection limits, our fluctuation analysis further constrains the slope of the counts down to 1 mJy. The resulting counts at 1-25 mJy are consistent with a power law having a slope of $-2.59$ ($pm0.10$ for 3.5-25 mJy, and $^{+0.4}_{-0.7}$ for 1-3.5 mJy). There is no evidence of a faint-end termination or turn-over of the counts in this flux density range. Our counts are also consistent with previous SCUBA-2 blank-field and lensing cluster surveys. The integrated surface brightness from our counts down to 1 mJy is $90.0pm17.2$ Jy deg$^{-2}$, which can account for up to $83^{+15}_{-16}%$ of the COBE 450 $mu$m background. We show that Herschel counts at 350 and 500 $mu$m are significantly higher than our 450 $mu$m counts, likely caused by its large beam and source clustering. High-angular resolution instruments like SCUBA-2 at 450 $mu$m are therefore highly beneficial for measuring the luminosity and spatial density of high-redshift dusty galaxies.
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We present structural parameters and morphological properties of faint 450-um selected submillimeter galaxies (SMGs) from the JCMT Large Program, STUDIES, in the COSMOS-CANDELS region. Their properties are compared to an 850um selected and a matched star-forming samples. We investigate stellar structures of 169 faint 450-um sources (S450=2.8-29.6mJy; S/N>4) at z<3 using HST near-infrared observations. Based on our spectral energy distribution fitting, half of such faint SMGs (LIR=10^11.65+-0.98Lsun) lie above the star-formation rate (SFR)/stellar mass plane. The size-mass relation shows that these SMGs are generally similar to less-luminous star-forming galaxies selected by NUV-r vs. r-J colors. Because of the intrinsic luminosity of the sample, their rest-frame optical emission is less extended than the 850um sources (S850>2mJy), and more extended than the star-forming galaxies in the same redshift range. For the stellar mass and SFR matched sample at z~=1 and z~=2, the size differences are marginal between faint SMGs and the matched galaxies. Moreover, faint SMGs have similar Sersic indices and projected axis ratios as star-forming galaxies with the same stellar mass and SFR. Both SMGs and the matched galaxies show high fractions (~70%) of disturbed features at z~=2, and the fractions depend on the SFRs. These suggest that their star formation activity is related to galaxy merging, and the stellar structures of SMGs are similar to those of star-forming galaxies. We show that the depths of submillimeter surveys are approaching the lower luminosity end of star-forming galaxies, allowing us to detect galaxies on the main sequence.
We analyze an extremely deep 450-$mu$m image ($1sigma=0.56$,mJy,beam$^{-1}$) of a $simeq 300$,arcmin$^{2}$ area in the CANDELS/COSMOS field as part of the SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES). We select a robust (signal-to-noise ratio $geqslant 4$) and flux-limited ($geqslant 4$,mJy) sample of 164 sub-millimeter galaxies (SMGs) at 450-$mu$m that have $K$-band counterparts in the COSMOS2015 catalog identified from radio or mid-infrared imaging. Utilizing this SMG sample and the 4705 $K$-band-selected non-SMGs that reside within the noise level $leqslant 1$,mJy,beam$^{-1}$ region of the 450-$mu$m image as a training set, we develop a machine-learning classifier using $K$-band magnitude and color-color pairs based on the thirteen-band photometry available in this field. We apply the trained machine-learning classifier to the wider COSMOS field (1.6,deg$^{2}$) using the same COSMOS2015 catalog and identify a sample of 6182 450-$mu$m SMG candidates with similar colors. The number density, radio and/or mid-infrared detection rates, redshift and stellar mass distributions, and the stacked 450-$mu$m fluxes of these SMG candidates, from the S2COSMOS observations of the wide field, agree with the measurements made in the much smaller CANDELS field, supporting the effectiveness of the classifier. Using this 450-$mu$m SMG candidate sample, we measure the two-point autocorrelation functions from $z=3$ down to $z=0.5$. We find that the 450-$mu$m SMG candidates reside in halos with masses of $simeq (2.0pm0.5) times10^{13},h^{-1},rm M_{odot}$ across this redshift range. We do not find evidence of downsizing that has been suggested by other recent observational studies.
We construct a SCUBA-2 450-$mu$m map in the COSMOS field that covers an area of 300 arcmin$^{2}$ and reaches a 1$sigma$ noise level of 0.65 mJy in the deepest region. We extract 256 sources detected at 450 $mu$m with signal-to-noise ratio $>$ 4.0 and analyze the physical properties of their multi-wavelength counterparts. We find that most of the sources are at $zlesssim3$, with a median of $z = 1.79^{+0.03}_{-0.15}$. About $35^{+32}_{-25}$% of our sources are classified as starburst galaxies based on their total star-formation rates (SFRs) and stellar masses ($M_{ast}$). By fitting the far-infrared spectral energy distributions, we find that our 450-$mu$m-selected sample has a wide range of dust temperatures (20 K $ lesssim T_{rm d} lesssim$ 60 K), with a median of ${T}_{rm d} = 38.3^{+0.4}_{-0.9}$ K. We do not find a redshift evolution in dust temperature for sources with $L_{rm IR}$ > $10^{12}$ $rm L_odot$ at $z<3$. However, we find a moderate correlation where dust temperature increases with the deviation from the SFR-$M_{ast}$ relation. The increase in dust temperature also correlates with optical morphology, which is consistent with merger-triggered starbursts in sub-millimeter galaxies. Our galaxies do not show the tight IRX-$beta_{rm UV}$ correlation that has been observed in the local Universe. We construct the infrared luminosity functions of our 450-$mu$m sources and measure their comoving SFR densities. The contribution of the $L_{rm IR}$ > $10^{12}$ $rm L_odot$ population to the SFR density rises dramatically from $z$ = 0 to 2 ($propto$ ($1+z$)$^{3.9pm1.1}$) and dominates the total SFR density at $z gtrsim 2$.
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.
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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