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Register a new userSCUBA-2 Ultra Deep Imaging Eao Survey (Studies) III: Multi-wavelength properties, luminosity functions and preliminary source catalog of 450-$mu$m-selected galaxies
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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 $z gtrsim 2$.
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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.
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.
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 investigate the multi-wavelength properties of a sample of 450-mu m selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS). A total of 69 sources were identified above 4sigma in deep SCUBA-2 450-mu m observations overlapping the UDS and COSMOS fields and covering 210 sq. arcmin to a typical depth of sigma 450=1.5 mJy. Reliable cross identification are found for 58 sources (84 per cent) in Spitzer and Hubble Space Telescope WFC3/IR data. The photometric redshift distribution (dN/dz) of 450mu m-selected sources is presented, showing a broad peak in the redshift range 1<z<3, and a median of z=1.4. Combining the SCUBA-2 photometry with Herschel SPIRE data from HerMES, the submm spectral energy distribution (SED) is examined via the use of modified blackbody fits, yielding aggregate values for the IR luminosity, dust temperature and emissivity of <LIR>=10^12 +/- 0.8 L_sol, <T_D>=42 +/- 11 K and <beta_D>=1.6 +/- 0.5, respectively. The relationship between these SED parameters and the physical properties of galaxies is investigated, revealing correlations between T_D and LIR and between beta_D and both stellar mass and effective radius. The connection between star formation rate and stellar mass is explored, with 24 per cent of 450 mu m sources found to be ``star-bursts, i.e. displaying anomalously high specific SFRs. However, both the number density and observed properties of these ``star-burst galaxies are found consistent with the population of normal star-forming galaxies.
We present a multi-wavelength analysis of galaxies selected at 450 and 850um from the deepest SCUBA-2 observations in the Extended Groth Strip (EGS) field, which have an average depth of sigma_450=1.9 and sigma_850=0.46 mJy/beam over ~70 sq. arcmin. The final sample comprises 95 sources: 56 (59%) are detected at both wavelengths, 31 (33%) are detected only at 850um, and 8 (8%) are detected only at 450um. We identify counterparts for 75% of the whole sample. The redshift distributions of the 450 and 850um samples peak at different redshifts with median values of z=1.66 +/- 0.18 and z=2.30 +/- 0.20, respectively. However, the two populations have similar IR luminosities, SFRs, and stellar masses, with mean values of (1.5 +/- 0.2) x 10^12 L_sun, (150 +/- 20) M_sun/yr, and (9.0 +/- 0.6) x 10^10 M_sun, respectively. This places most of our sources (>85%) on the high-mass end of the `main-sequence of star-forming galaxies. Exploring the IR excess vs UV-slope (IRX-beta) relation we find that the most luminous galaxies are consistent with the Meurer law, while the less luminous galaxies lie below this relation. Using the results of a two-dimensional modelling of the HST H_160-band imaging, we derive a median Sersic index of n=1.4 +0.3 -0.1 and a median half-light radius of R_1/2 = 4.8 +/ 0.4 kpc. Based on a visual-like classification in the same band, we find that the dominant component for most of the galaxies at all redshifts is a disk-like structure, although there is a transition from irregular disks to disks with a spheroidal component at z~1.4, which morphologically supports the scenario of SMGs as progenitors of massive elliptical galaxies.
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