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Citation for published item:
Stach, S.M. and Dudzevicit, U. and Smail, I. and Swinbank, A.M. and Geach, J.E. and Simpson, J.M. and An,
F.X. and Almaini, O. and Arumugam, V. and Blain, A.W. and Chapman, S.C. and Chen, C-C. and Conselice,
C.F. and Cooke, E.A. and Coppin, K.E.K. and da Cunha, E. and Dunlop, J.S. and Farrah, D. and Gullberg,
B. and Hodge, J.A. and Ivison, R. and Kocevski, D.D. and Micha
l
owski, M.J. and Miyaji, T. and Scott, D.
and Thomson, A.P. and Wardlow, J.L. and Weiss, A. and Werf, P. van der (2019) 'An ALMA survey of the
SCUBA-2 cosmology legacy survey UKIDSS/UDS eld : source catalogue and properties.', Monthly notices of
the Royal Astronomical Society., 487 (4). pp. 4648-4668.
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https://doi.org/10.1093/mnras/stz1536
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AS2UDS: Source catalogue and properties 1
An ALMA survey of the SCUBA-2 Cosmology Legacy
Survey UKIDSS/UDS field: Source catalogue and
properties
Stuart M. Stach
1
,
?
U. Dudzeviˇci¯ut˙e,
1
Ian Smail,
1
A. M. Swinbank,
1
J. E. Geach,
2
J. M. Simpson,
3
Fang Xia An,
4,1
Omar Almaini,
5
Vinodiran Arumugam,
6,7
A. W. Blain,
8
S. C. Chapman,
9
Chian-Chou Chen,
6
C. J. Conselice,
5
E. A. Cooke,
1
K. E. K. Coppin,
2
E. da Cunha,
10
J. S. Dunlop,
7
Duncan Farrah,
11,12
B. Gullberg,
1
J. A. Hodge,
13
R. J. Ivison,
6,7
Dale D. Kocevski,
14
M. J. Micha lowski,
15
Takamitsu Miyaji,
16
Douglas Scott,
17
A. P. Thomson,
18
J. L. Wardlow,
19
Axel Weiss,
20
P. van der Werf
13
1
Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, DH1 3LE, UK
2
Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
3
Academia Sinica Institute of Astronomy and Astrophysics, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan
4
Purple Mountain Observatory, China Academy of Sciences, 2 West Beijing Road, Nanjing 210008, China
5
School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
6
European Southern Observatory, Karl Schwarzschild Strasse 2, Garching, Germany
7
Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
8
Department of Physics and Astronomy, University of Leicester,University Road, Leicester LE1 7RH, UK
9
Department of Physics and Atmospheric Science, Dalhousie University Halifax, NS B3H 3J5, Canada
10
Research School of Astronomy and Astrophysics, The Australian National University, Canberra ACT 2611, Australia
11
Department of Physics and Astronomy, University of Hawaii, 2505 Correa Road, Honolulu, HI 96822, USA
12
Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA
13
Leiden Observatory, Leiden University, P.O. box 9513, NL-2300 RA Leiden, The Netherlands
14
Department of Physics and Astronomy, Colby College, Waterville, ME 04961, USA
15
Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. S loneczna 36, 60-286 Pozna´n, Poland
16
Instututo de Astronom´ıa sede Ensenada, Universidad Nacional Aut´onoma de M´exico, Ensenada, 22860, M´exico
17
Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
18
The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
19
Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK
20
Max-Planck-Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69 D-53121 Bonn, Germany
3 June 2019
ABSTRACT
We present the catalogue and basic properties of sources in AS2UDS, an 870-µm 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 > 4.3σ
significance across the ∼ 1-degree diameter field. We combine our precise ALMA positions with the
extensive multi-wavelength coverage in the UDS field which yields spectral energy distributions
for our SMGs and a median redshift of z
phot
= 2.61±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 > 4.3σ SMG, but we demonstrate from stacking 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
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 z ∼ 2, finding reasonable agreement – in support of
an evolutionary connection between these two classes of massive galaxy.
Key words: galaxies:starburst – galaxies:high-redshift – sub-millimetre:galaxies
?
E-mail: stuart.m.stach@durham.ac.uk
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2 Stach et al.
1 INTRODUCTION
Over twenty years ago the first, deep, sub-millimetre wave-
length surveys taken at the James Clerk Maxwell Telescope
(JCMT) uncovered a population of sub-millimetre bright
galaxies (SMGs – e.g. Smail et al. 1997; Hughes et al. 1998;
Barger et al. 1998), which were interpreted as showing some
of highest rates of star formation observed in galaxies across
the whole history of the Universe. Their sub-millimetre emis-
sion originates from the reprocessed ultra-violet starlight
that has been absorbed by dust and re-emitted in the rest-
frame far-infrared. This population of highly obscured galax-
ies are most easily selected at sub-millimetre wavelengths
and so are termed ’sub-millimetre galaxies’ (SMGs).
The selection of these star-forming galaxies at sub-
millimetre wavelengths has both advantages and disad-
vantages. A major advantage is the strongly negative K-
correction at sub-millimetre wavelengths arising from the
slope of the Rayleigh-Jeans tail of their far-infrared/sub-
millimetre spectral energy distributions (SED). As a result
of this negative K-correction, a flux limited sub-millimetre
survey provides a uniform selection in terms of far-infrared
luminosity (at a fixed dust temperature) for sources across a
redshift range of z = 1–6 (figure 4: Blain et al. 2002). Thus
sub-millimetre observations are a very effective means to sur-
vey for the most strongly star-forming galaxies in the high
redshift Universe. However, a major disadvantage of current
single dish observatories operating at sub-millimetre wave-
lengths is their modest angular resolution, 15–30
00
FWHM,
which is too coarse to allow the counterpart to the sub-
millimetre emission to be easily identified at shorter wave-
lengths, as several candidate galaxies can be encompassed by
the single-dish beam. Hence early attempts to pinpoint the
location of SMGs to sub-arcsecond resolutions exploited the
FIR–radio correlation (e.g. Ivison et al. 1998; Barger et al.
2000; Ivison et al. 2002; Chapman et al. 2005) to match the
sub-millimetre sources to their radio bright counterparts.
The limitation of such radio identifications is that the ra-
dio waveband does not benefit from a strong negative K-
correction, so there is a bias against identifying the highest
redshift (z > 2.5–3) SMGs in the radio images. The difficul-
ties with reliably identifying sub-millimetre source counter-
parts contributed in part to the slow advance in our under-
standing of these galaxies in the years following their dis-
covery.
Not withstanding the challenges described above,
the first large-scale spectroscopic redshift surveys of
radio-identified SMGs (Chapman et al. 2005), and
later sub/millimetre interferometrically-selected sam-
ples (Smolˇci´c et al. 2012; Danielson et al. 2017), found
that these galaxies are typically located at redshifts
of z ∼ 2.5. At these redshifts, the sub-millimetre flux
of the sources corresponds to far-infrared luminosities
> 10
12
–10
13
L
, i.e. Ultra-Luminous InfraRed Galaxies
(ULIRGs). However, SMGs have volume densities three
orders of magnitude greater than comparably luminous
local ULIRGs (Chapman et al. 2005). Such high infrared
luminosities indicate star-formation rates (SFR) of the order
100–1000 M
yr
−1
(Magnelli et al. 2012; Swinbank et al.
2014), a star-formation rate high enough that within a few
dynamical times (a hundred million years) the SMG could
form the stellar mass of a massive galaxy M
∗
& 10
11
M
.
Indeed, constraints on the stellar masses of SMGs have
found M
∗
∼ 10
11
–10
12
M
(Borys et al. 2005; Hainline et al.
2011; Micha lowski et al. 2014) making SMGs some of the
most massive galaxies at z ∼ 2. The space density of these
sources and their prodigious star-formation rates means
that SMGs contribute ∼ 20% of the Universal star-formation
density between z = 1–4 (Casey et al. 2013; Swinbank et al.
2014). Being both massive and strongly star-forming
galaxies in the early Universe, SMGs have been proposed
as the progenitors of massive local spheroidal galaxies (e.g.
Genzel et al. 2003; Blain et al. 2004; Cimatti et al. 2008;
Simpson et al. 2014; Toft et al. 2014; Koprowski et al. 2014;
Simpson et al. 2017), potentially following an evolutionary
path where, following their ultra-luminous infrared phase,
the SMG descendants would display both star-formation
and obscured AGN activity, and then appears as a quasi-
stellar object (QSO), until the system completely exhausts
its supply of gas (Coppin et al. 2008; Simpson et al. 2012).
The major advance in studies of SMGs came with
the development of sensitive sub-millimetre interferometers:
initially the Sub-Millimeter Array (SMA) (Younger et al.
2008; Wang et al. 2011; Smolˇci´c et al. 2012) and more re-
cently the Atacama Large Millimetre/sub-millimeter Ar-
ray (ALMA) (Hodge et al. 2013; Simpson et al. 2015a;
Hatsukade et al. 2016; Walter et al. 2016; Franco et al.
2018; Hatsukade et al. 2018; Cowie et al. 2018). Interfero-
metric observations, in particular with ALMA, allow us to
observe SMGs in the sub-millimetre at spatial resolutions
more than an order of magnitude finer than achievable in
single-dish surveys and free from confusion – enabling de-
tections of sources to flux densities more than an order of
magnitude fainter than the single-dish limits.
Deep, blank-field surveys utilising these interferome-
ters have successfully recovered faint, serendipitously de-
tected sources across arcmin
2
regions such as in the Hubble
Ultra-Deep Field and GOODS-South (Aravena et al. 2016;
Walter et al. 2016; Dunlop et al. 2017; Franco et al. 2018;
Hatsukade et al. 2018). These are effective surveys for de-
tecting the fainter examples of the SMG population free from
the potential biases from clustering of sources around bright
detections. However, the modest field of view of interferome-
ters means that such surveys can only cover small areas and
as a result have so far yielded relatively few (10’s) of detected
sources, with only very few of the brightest examples having
S
870
1 mJy. To obtain statistically robust samples of the
brighter SMGs (S
870
>
∼
1–10 mJy), whose properties may be
the most distinct from ’normal’ star-forming galaxies, we re-
quire a hybrid approach – where we exploit the fast mapping
speed of single-dish telescopes to identify numbers of these
relatively rare sources over the large fields needed to yield
large samples – combined with interferometric observations
in the same sub-millimetre waveband to allow us to pre-
cisely locate the counterparts to the single-dish sources. We
first employed this dual-survey approach with the ALMA
LABOCA Extended Chandra Deep Field South survey
(ALESS) (Hodge et al. 2013; Karim et al. 2013). This was
an ALMA Cycle 0 survey of the 122 sub-millimetre sources
detected in the LABOCA/APEX single-dish survey of the
Extended Chandra Deep Field South (LESS: Weiß et al.
2009) and yielded detections of 126 single-dish sources with
deboosted 870 µm fluxes S
870
> 3.6 mJy. This survey sug-
gested that some previous single-dish detections were in
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AS2UDS: Source catalogue and properties 3
reality multiple galaxies blended by the coarse resolution
of the single-dish telescope (Karim et al. 2013) and pre-
vious multiwavelength methods of identification of SMGs,
were failing to correctly locate the counterpart to the sub-
millimetre emission almost half of the time (Hodge et al.
2013; Simpson et al. 2015a,b).
These initial ALMA studies of flux-limited samples have
begun to illuminate the range of characteristics of bright
sub-millimetre galaxies, free from the selection biases which
influenced earlier radio and mid-infrared based studies. In
particular they have highlighted the ∼ 10–20% of the popu-
lation which are effectively undetectable in even the deep-
est optical/near-infrared (Simpson et al. 2014), which rep-
resent either the highest redshift, the least massive or the
most obscured examples of this population. However, the
first ALMA surveys lacked the sample size to identify sta-
tistically significant subsets of the rarest classes of SMGs.
For example there are only two z ∼ 4.4 [Cii]-selected sources
in the ALESS survey (Swinbank et al. 2012; Gullberg et al.
2018), which provide an insight into the properties of the
more distant examples of sub-millimetre galaxies. Similarly,
ALESS yielded just ten X-ray detected AGN–SMG systems,
which can be used to probe the co-evolution of super-massive
black holes in strongly star-forming galaxies (Wang et al.
2013). To improve the statistical strength of the conclu-
sions about these rarer subclasses of sub-millimetre galaxies,
larger surveys are needed in extragalactic survey fields with
the deepest supporting data necessary to detect the faintest
examples of this population.
Driven by this need, we have just completed a larger
study, nearly an order-of-magnitude larger than ALESS,
which exploits the wide-field sub-millimetre mapping of key
extragalactic survey fields undertaken by the SCUBA-2 Cos-
mology Legacy Survey (S2CLS: Geach et al. 2017). We focus
in this project on the S2CLS 850-µm map of the ∼ 1 degree
diameter UKIDSS Ultra Deep Survey (UDS) Field, which
was the largest, uniform area mapped by S2CLS. The S2CLS
UDS map has a median sensitivity of σ
850
= 0.9 mJy over
an area of 0.96 degrees
2
, with 716 sources catalogued above
a 4-σ detection limit (corresponding to a 2 % false posi-
tive rate) of S
850
∼ 3.5 mJy. We began our investigation of
this sample with a pilot ALMA study of a subset of thirty
bright SCUBA-2 detected sources in Cycle 1 (Simpson et al.
2015a,b, 2017). We then expanded the study during Cy-
cles 3, 4, and 5 to complete the ALMA 870-µm observa-
tions of all 716 > 4σ sources. This yields AS2UDS – the
ALMA SCUBA-2 UDS survey – the largest, homogeneously
selected, sample of SMGs to date with 708 detections, a five-
fold increase over the previous largest similarly robust sam-
ple. The first results from this survey have already been pre-
sented: number counts and rates of multiplicity (Stach et al.
2018), the serendipitous detection of high redshift [Cii] emit-
ters (Cooke et al. 2018), and the use of this survey as a
training set for machine learning algorithms to identify the
multiwavelength counterparts to single-dish submillimetre
sources (An et al. 2018). We present a full analysis of the
multiwavelength properties of this sample in Dudzeviˇci¯ut˙e
et al. (in prep.) and in Gullberg et al. (in prep.) we discuss
the information available on the sizes and morphologies of
the dust continuum in these sources from our highest reso-
lution ALMA observations.
In this paper we present the final catalogue for the
AS2UDS survey
1
. In our analysis we compare results
from our new large sample to previous studies. To sim-
plify these comparisons we have limited them in general
to flux-limited samples from: 1. larger unbiased blank-
field surveys at 850 µm (as there is evidence of differ-
ences compared to populations selected in the far-infrared
and millimetre, e.g. Smolˇci´c et al. 2012; Koprowski et al.
2014; Scudder et al. 2016; Ikarashi et al. 2017); 2. with deep
(< 1 mJy rms) interferometric identifications in the same
waveband as any initial single-dish selection, if appropri-
ate (c.f. Barger et al. 2014; Umehata et al. 2014; Hill et al.
2018); 3. and which are not explicitly lensed, owing to the
potential selection effects and variable flux limits as well
as uncertainties from cluster lenses and especially galaxy-
scale lensed samples (e.g. Weiß et al. 2013; Fujimoto et al.
2016; Arancibia et al. 2018). Thus most of our comparisons
are made to the ALESS survey (Hodge et al. 2013), Super-
GOODS (Cowie et al. 2018) and the various ALMA sur-
veys in GOODS-S (Walter et al. 2016; Dunlop et al. 2017;
Franco et al. 2018; Hatsukade et al. 2018).
In §2 we describe the target selection for our ALMA
survey and data reduction across the different ALMA Cycles
and the wealth of multi-wavelength archival data available
in this field, most notably from the UKIDSS UDS DR11 cat-
alogue (O. Almaini et al. in prep.). §3 describes the source
detection algorithm, the simulated maps used for estimating
completeness and flux boosting derivations. In §4 we present
the first results from our magphys SED fitting (Dudze-
viˇci¯ut˙e et al. in prep.): the photometric redshift distribu-
tion of our sample and comparisons with previous surveys.
In addition we present the selection of active galactic nu-
clei (AGNs) from our catalogue through archival X-ray ob-
servations of the field and IRAC colour-colour selection. §5
presents our main conclusions. We assume a cosmology with
Ω
m
= 0.3, Ω
Λ
= 0.7, and H
0
= 70 km s
−1
Mpc
−1
. All magni-
tudes are in the AB system and errors are calculated from
bootstrap analysis unless otherwise stated.
2 OBSERVATIONS AND DATA REDUCTION
2.1 Sample Selection
AS2UDS is a high-resolution, sub-millimetre interferomet-
ric follow-up survey of the SCUBA-2 850 µm sources se-
lected from the S2CLS map of the UDS field (Figure 1).
The parent single-dish survey covers an area of 0.96 deg
2
,
with noise levels below 1.3 mJy and a median depth of
σ
850
= 0.88 mJy beam
−1
with 80% of sources identified
in regions of the map with σ
850
= 0.86–1.02 mJy beam
−1
(Geach et al. 2017). Across four ALMA cycles (1, 3, 4, and 5)
we observed all 716 > 4σ sources from the SCUBA-2 S2CLS
map, corresponding to observed flux densities S
850
≥ 3.4 mJy
(see: Figure 1).
In Cycle 1 (Project ID: 2012.1.00090.S), 30 of the
brightest sources from an early version of the SCUBA-
2 UDS map (data taken before 2013 February) were ob-
served in ALMA Band 7 (Simpson et al. 2015a,b, 2017).
This early version of the SCUBA-2 map had a depth of
only σ
850
∼ 2.0 mJy
−1
and subsequent integration time in
1
Catalogue can be found at http://astro.dur.ac.uk/AS2UDS
Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/stz1536/5511908 by University of Durham user on 06 June 2019
4 Stach et al.
Figure 1. A sample of the multi-wavelength coverage of the AS2UDS sample which motivates this extra-galactic field being chosen
for high resolution ALMA follow-up. The background shows the S2CLS 850 µm UDS map from which the parent sample is extracted
(Geach et al. 2017). The red circles are the primary beams for AS2UDS targetting the 4 σ detections in the S2CLS map. The black outline
shows the K -band coverage that forms the footprint for the UKIDSS UDS catalogue (O. Almaini et al. in prep.). The Spitzer /IRAC
CH1/3 and CH2/4 coverage is shown in blue/purple respectively, the HST CANDELS F160W in white, VLA 1.4 GHz in grey, and the
X-UDS Chandra coverage in yellow (Kocevski et al. 2018).
the S2CLS survey scattered three of these sources below our
final sample selection criteria (> 4σ), leaving 27 of these orig-
inal single-dish detected sources in our final sample. The re-
maining 689 single-dish sources in the final S2CLS catalogue
were observed across ALMA cycles 3 and 4 (Project ID:
2015.1.01528.S and 2016.1.00434.S, respectively). To cross
calibrate the data, a fraction of these sources were observed
twice, once in Cycle 3 and 4. In addition, in Cycle 5 ten of
the brightest SCUBA-2 sources which returned ‘blank’ maps
from the Cycle 3 and 4 ALMA observations were re-observed
at greater depth (Project ID: 2017.1.01492.S), these maps
will be discussed further in §4.3.
2.2 Data Reduction
Our ALMA targets were all observed in Band 7 (central
frequency 344 GHz ∼ 870 µm). At this frequency the FWHM
of the ALMA primary beam (17.
00
3) covers the FWHM of the
SCUBA-2 beam (14.
00
7). Cycle 1 observations were carried
out on 2013 November 1 (Simpson et al. 2015a), Cycle 3
between 2016 July 23 and August 11, Cycle 4 between 2016
November 9 and 17 and 2017 May 6, and Cycle 5 on 2018
August 24.
These ALMA Band 7 continuum observations were
150 second integrations in Cycle 1 using 26 dishes, 40 second
integrations in Cycle 3 and 4 (with 45–50 dishes), and
285 second integrations using 44 dishes in Cycle 5, with the
7.5 GHz bandwidth of the four spectral windows centred at
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