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Register a new userX-UDS: The Chandra Legacy Survey of the UKIDSS Ultra Deep Survey Field
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We present the X-UDS survey, a set of wide and deep Chandra observations of the Subaru-XMM Deep/UKIDSS Ultra Deep Survey (SXDS/UDS) field. The survey consists of 25 observations that cover a total area of 0.33 deg$^{2}$. The observations are combined to provide a nominal depth of ~600 ksec in the central 100 arcmin$^{2}$ region of the field that has been imaged with Hubble/WFC3 by the CANDELS survey and $sim$200 ksec in the remainder of the field. In this paper, we outline the surveys scientific goals, describe our observing strategy, and detail our data reduction and point source detection algorithms. Our analysis has resulted in a total of 868 band-merged point sources detected with a false-positive Poisson probability of $<1times10^{-4}$. In addition, we present the results of an X-ray spectral analysis and provide best-fitting neutral hydrogen column densities, $N_{rm H}$, as well as a sample of 51 Compton-thick active galactic nucleus candidates. Using this sample, we find the intrinsic Compton-thick fraction to be 30-35% over a wide range in redshift ($z=0.1-3$), suggesting the obscured fraction does not evolve very strongly with epoch. However, if we assume that the Compton-thick fraction is dependent on luminosity, as is seen for Compton-thin sources, then our results are consistent with a rise in the obscured fraction out to $zsim3$. Finally, an examination of the host morphologies of our Compton-thick candidates shows a high fraction of morphological disturbances, in agreement with our previous results. All data products described in this paper are made available via a public website.
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We present the catalogue and properties of sources in AS2UDS, an 870-$mu$m continuum survey with the Atacama Large Millimetre/sub-millimetre Array (ALMA) of 716 single-dish sub-millimetre sources detected in the UKIDSS/UDS field by the SCUBA-2 Cosmology Legacy Survey. In our sensitive ALMA follow-up observations we detect 708 sub-millimetre galaxies (SMGs) at $>$,4.3$sigma$ significance across the $sim$,1-degree diameter field. We combine our precise ALMA positions with the extensive multi-wavelength coverage in the UDS field to fit the spectral energy distributions of our SMGs to derive a median redshift of $z_{rm phot}=$,2.61$pm$0.09. This large sample reveals a statistically significant trend of increasing sub-millimetre flux with redshift suggestive of galaxy downsizing. 101 ALMA maps do not show a $>$,4.3$sigma$ SMG, but we demonstrate from stacking {it Herschel} SPIRE observations at these positions, that the vast majority of these blank maps correspond to real single-dish sub-millimetre sources. We further show that these blank maps contain an excess of galaxies at $z_{rm phot}=$,1.5--4 compared to random fields, similar to the redshift range of the ALMA-detected SMGs. In addition, we combine X-ray and mid-infrared active galaxy nuclei activity (AGN) indicators to yield a likely range for the AGN fraction of 8--28,% in our sample. Finally, we compare the redshifts of this population of high-redshift, strongly star-forming galaxies with the inferred formation redshifts of massive, passive galaxies being found out to $zsim$,2, finding reasonable agreement -- in support of an evolutionary connection between these two classes of massive galaxy.
We report the first results of AS2UDS: an 870 $mu$m continuum survey with the Atacama Large Millimeter/Submillimeter Array (ALMA) of a total area of $sim$ 50 arcmin$^2$ comprising a complete sample of 716 submillimeter sources drawn from the SCUBA-2 Cosmology Legacy Survey (S2CLS) map of the UKIDSS/UDS field. The S2CLS parent sample covers a 0.96 degree$^2$ field at $sigma_{850}=0.90pm0.05$ mJy beam$^{-1}$. Our deep, high-resolution ALMA observations with $sigma_{rm 870}sim$ 0.25 mJy and a 0.15--0.30 FWHM synthesized beam, provide precise locations for 695 submillimetre galaxies (SMGs) responsible for the submillimeter emission corresponding to 606 sources in the low resolution, single-dish map. We measure the number counts of SMGs brighter than $S_{rm 870}geq$ 4 mJy, free from the effects of blending and show that the normalisation of the counts falls by 28 $pm$ 2% in comparison to the SCUBA-2 parent sample, but that the shape remains unchanged. We determine that 44$^{+16}_{-14}$% of the brighter single-dish sources with $S_{850}geq$ 9 mJy consist of a blend of two or more ALMA-detectable SMGs brighter than $S_{870}sim$ 1 mJy (corresponding to a galaxy with a total-infrared luminosity of $L_{IR}geq$ 10$^{12}$ L$_odot$), in comparison to 28 $pm$ 2% for the single-dish sources at $S_{rm 850}geq$ 5 mJy. Using the 46 single-dish submillimeter sources that contain two or more ALMA-detected SMGs with photometric redshifts, we show that there is a significant statistical excess of pairs of SMGs with similar redshifts ($<$1% probability of occurring by chance), suggesting that at least 30% of these blends arise from physically associated pairs of SMGs.
We report the results of a search for serendipitous [CII] 157.74$mu$m emitters at $zsim4.4$-$4.7$ using the Atacama Large Millimeter/submillimeter Array (ALMA). The search exploits the AS2UDS continuum survey, which covers ~50 arcmin$^2$ of the sky towards 695 luminous ($S_{870}gtrsim1$mJy) submillimeter galaxies (SMGs), selected from the SCUBA-2 Cosmology Legacy Survey (S2CLS) 0.96deg$^2$ Ultra Deep Survey (UDS) field. We detect ten candidate line emitters, with an expected false detection rate of ten percent. All of these line emitters correspond to 870$mu$m continuum-detected sources in AS2UDS. The emission lines in two emitters appear to be high-J CO, but the remainder have multi-wavelength properties consistent with [CII] from $zsimeq4.5$ galaxies. Using our sample, we place a lower limit of $>5times10^{-6}$Mpc$^{-3}$ on the space density of luminous ($L_{rm IR} simeq 10^{13}$Lsun) SMGs at $z=4.40$-$4.66$, suggesting $ge7$percent of SMGs with $S_{870mu{rm m}}gtrsim1$mJy lie at $4<z<5$. From stacking the high-resolution ($sim0.15$ full-width half maximum) ALMA $870mu$m imaging, we show that the [CII] line emission is more extended than the continuum dust emission, with an average effective radius for the [CII] of $r_{rm e} = 1.7^{+0.1}_{-0.2}$kpc compared to $r_{rm e} = 1.0pm0.1$kpc for the continuum (rest-frame $160mu$m). By fitting the far-infrared photometry for these galaxies from $100$-$870mu$m, we show that SMGs at $zsim4.5$ have a median dust temperature of $T_{rm d}=55pm4$K. This is systematically warmer than $870mu$m-selected SMGs at $zsimeq2$, which typically have temperatures around $35$K. These $zsimeq4.5$ SMGs display a steeper trend in the luminosity-temperature plane than $zle2$ SMGs. We discuss the implications of this result in terms of the selection biases of high redshift starbursts in far-infrared/submillimeter surveys.
We analyse 870um Atacama Large Millimetre Array (ALMA) dust continuum detections of 41 canonically-selected z~3 Lyman-break galaxies (LBGs), as well as 209 ALMA-undetected LBGs, in follow-up of SCUBA-2 mapping of the UKIDSS Ultra Deep Survey (UDS) field. We find that our ALMA-bright LBGs lie significantly off the locally calibrated IRX-beta relation and tend to have relatively bluer rest-frame UV slopes (as parametrised by beta), given their high values of the infrared excess (IRX=L_IR/L_UV), relative to the average local IRX-beta relation. We attribute this finding in part to the young ages of the underlying stellar populations but we find that the main reason behind the unusually blue UV slopes are the relatively shallow slopes of the corresponding dust attenuation curves. We show that, when stellar masses are being established via SED fitting, it is absolutely crucial to allow the attenuation curves to vary (rather than fixing it on Calzetti-like law), where we find that the inappropriate curves may underestimate the resulting stellar masses by a factor of ~2-3x on average. In addition, we find these LBGs to have relatively high specific star-formation rates (sSFRs), dominated by the dust component, as quantified via the fraction of obscured star formation ( f_obs = SFR_IR/SFR_(UV+IR)). We conclude that the ALMA-bright LBGs are, by selection, massive galaxies undergoing a burst of a star formation (large sSFRs, driven, for example, by secular or merger processes), with a likely geometrical disconnection of the dust and stars, responsible for producing shallow dust attenuation curves.
We present multiwavelength identifications for the counterparts of 1088 submillimeter sources detected at 850$mu$m in the SCUBA-2 Cosmology Legacy Survey study of the UKIDSS-UDS field. By utilising an ALMA pilot study on a subset of our bright SCUBA-2 sample as a training set, along with the deep optical-near-infrared data available in this field, we develop a novel technique, Optical-IR Triple Color (OIRTC), using $z-K$, $K-[3.6]$, $[3.6]-[4.5]$ colors to select the candidate submillimeter galaxy (SMG) counterparts. By combining radio identification and the OIRTC technique, we find counterpart candidates for 80% of the Class = 1 $geq4,sigma$ SCUBA-2 sample, defined as those that are covered by both radio and OIR imaging and the base sample for our scientific analyses. Based on the ALMA training set, we expect the accuracy of these identifications to be $82pm20$%, with a completeness of $69pm16$%, essentially as accurate as the traditional $p$-value technique but with higher completeness. We find that the fraction of SCUBA-2 sources having candidate counterparts is lower for fainter 850$mu$m sources, and we argue that for follow-up observations sensitive to SMGs with $S_{850}gtrsim 1$ mJy across the whole ALMA beam, the fraction with multiple counterparts is likely to be $>40$% for SCUBA-2 sources at $S_{850} gtrsim 4$ mJy. We find that the photometric redshift distribution for the SMGs is well fit by a lognormal distribution, with a median redshift of $z=2.3pm0.1$. After accounting for the sources without any radio and/or OIRTC counterpart, we estimate the median redshift to be $z=2.6pm0.1$ for SMGs with $S_{850} >1$ mJy. We also use this new large sample to study the clustering of SMGs and the the far-infrared properties of the unidentified submillimeter sources by stacking their Herschel SPIRE far-infrared emission.
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