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An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Identifying candidate z~4.5 [CII] emitters

AbstractWe report the results of a search for serendipitous [CII] 157.74$\mu$m emitters at $z\sim4.4$-$4.7$ 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 $z\simeq4.5$ galaxies. Using our sample, we place a lower limit of $>5\times10^{-6}$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 $S_{870\mu{\rm m}}\gtrsim1$mJy lie at $4<z<5$. From stacking the high-resolution ($\sim0.15"$ full-width half maximum) ALMA $870\mu$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 $r_{\rm e} = 1.0\pm0.1$kpc for the continuum (rest-frame $160\mu$m). By fitting the far-infrared photometry for these galaxies from $100$-$870\mu$m, we show that SMGs at $z\sim4.5$ have a median dust temperature of $T_{\rm d}=55\pm4$K. This is systematically warmer than $870\mu$m-selected SMGs at $z\simeq2$, which typically have temperatures around $35$K. These $z\simeq4.5$ SMGs display a steeper trend in the luminosity-temperature plane than $z\le2$ SMGs. We discuss the implications of this result in terms of the selection biases of high redshift starbursts in far-infrared/submillimeter surveys.

Topics: Continuum (set theory) (66%)

Summary (2 min read)

1. Introduction

  • High-redshift (z3) SMGs therefore appear to play a potentially significant role in galaxy evolution; however, their dusty nature and high redshift mean that measuring their spectroscopic redshifts—needed to constrain many of their basic properties—is extremely challenging using ground-based optical/near-infrared spectroscopy.
  • [C II] emission can originate both in photodissociation regions around star-forming regions and also from atomic and ionized gas (e.g., Dalgarno & McCray 1972; Madden et al. 1997; Pineda et al. 2013). [C II] could thus provide information about the volume and extent of the cold gas reservoir and star formation in galaxies.

2.1. ALMA Data

  • These cubes were first continuum-subtracted by subtracting a linear fit to the continuum in the spectrum of each pixel.
  • In one of these SMGs (AS2UDS.0109.1) the authors found a tentative S/N=5.3 emission line corresponding to the same redshift as the detected emission line source (AS2UDS.0109.0).
  • 18 In each collapsed 100 km s−1 slice the authors search in the narrowband image for peaks above 2σ within the ALMA primary beam.

2.2. Multi-wavelength Data

  • The UDS has photometric coverage spanning the optical, near-, mid-, and far-infrared, out to radio wavelengths.
  • This figure also shows a zoomed-in 3″×3″optical/infrared image of each source, with the ALMA continuum contours overlaid.
  • The deblending uses a combination of the ALMA-detected SMGs and Spitzer/MIPS 24 μm and UDS20 radio sources as positional priors for the deblending of the low-resolution SPIRE maps.

3. Results and Discussion

  • The authors provide the source redshifts and line properties in Table 1.
  • The line flux densities are calculated from the Gaussian profile fit to each line.
  • The deblended catalogs for the fields are available from http://astro.dur.ac.
  • The number of line emitters the authors identify from the parent sample of 695 SMGs is consistent with the expectation from the ALESS survey, where two emission line sources were identified from a sample of 99 SMGs (Swinbank et al. 2012).

3.1. Alternative Emission Lines

  • Before the authors discuss the properties of their line-emitter galaxies, they first discuss the identification of the emission lines.
  • 2. [C II] Luminosities and Line Widths Figure 3 shows the FWHM and emission line luminosities of their 10 sources (with those that are potentially lower-redshift or lensed flagged) compared to other studies of high-redshift starforming galaxies and AGNs.
  • To investigate the “[C II] deficit” the authors must first estimate the infrared (rest-frame 8–1000 μm) luminosities of their line emitters.

3.5. z∼4.5 SMGs are Warm

  • Comparing their inferred dust temperatures and far-infrared luminosities in Figure 7, their sample of z∼4.5 SMGs appears to have warmer characteristic dust temperatures at fixed luminosity than inferred for z;2 SMGs and star-forming galaxies (e.g., Magnelli et al.
  • In this section the authors first test the reliability of their measured dust temperatures and then discuss the implications of warm dust temperatures on the selection of high-redshift SMGs.
  • Figure 8 shows the [C II] luminosity function at z∼4.5 as derived from their sample of seven S870>4 mJy continuum-selected [C II] emitters.
  • The authors observations at the bright end of the luminosity function suggest that the model of Lagache et al. (2018) overpredicts the [C II] luminosity function at this redshift unless their sample selection is incomplete, which is possible.

4. Conclusions

  • All of these line emitters are 870 μm continuum-detected sources in their parent survey with S8701 mJy.
  • The extended [C II] emission compared to the dust continuum together with the measured infrared luminosities in these SMGs combines to give a star formation rate surface density measurement of 130±20 M yr −1 kpc−2.
  • I.R.S. also acknowledges support from a Royal Society/Wolfson Merit Award.
  • The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ.

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An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field:
Identifying Candidate z4.5 [C
II] Emitters
E. A. Cooke
1
, Ian Smail
1
, A. M. Swinbank
1
, S. M. Stach
1
, Fang Xia An
1,2
, B. Gullberg
1
, O. Almaini
3
,
C. J. Simpson
4
, J. L. Wardlow
1
, A. W. Blain
5
, S. C. Chapman
6
, Chian-Chou Chen
7
, C. J. Conselice
3
, K. E. K. Coppin
8
,
D. Farrah
9,10
, D. T. Maltby
3
, M. J. Michałowski
11
, D. Scott
12
, J. M. Simpson
13
, A. P. Thomson
14
, and P. van der Werf
15
1
Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, DH1 3LE, UK; elizabeth.a.cooke@durham.ac.uk
2
Purple Mountain Observatory, China Academy of Sciences, 2 West Beijing Road, Nanjing 210008, Peoples Republic of China
3
School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
4
Gemini Observatory, Northern Operations Center, 670 N. Aohōkū Place, Hilo, HI 96720, USA
5
Physics & Astronomy, University of Leicester, 1 University Road, Leicester LE1 7RH, UK
6
Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 4R2, Canada
7
European Southern Observatory, Karl Schwarzschild Strasse 2, Garching, Germany
8
Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hateld AL10 9AB, UK
9
University of Hawaii, 2505 Correa Road, Honolulu, HI 96822, USA
10
Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
11
Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
12
Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
13
Academia Sinica Institute of Astronomy and Astrophysics, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
14
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
15
Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands
Received 2018 February 6; revised 2018 May 9; accepted 2018 May 12; published 2018 July 10
Abstract
We report the results of a search for serendipitous [C
II] 157.74 μm emitters at z;4.44.7 using the Atacama
Large Millimeter/submillimeter Array (ALMA). The search exploits the AS2UDS continuum survey, which
covers 50 arcmin
2
of the sky toward 695 luminous (S
870
1 mJy) submillimeter galaxies (SMGs), selected from
the SCUBA-2 Cosmology Legacy Survey 0.96 deg
2
Ultra Deep Survey (UDS) eld. We detect 10 candidate line
emitters, with an expected false detection rate of 10%. All of these line emitters correspond to 870 μm 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 [C
II] from z;4.5 galaxies. Using our sample, we place a lower limit
of
510Mpc
63
--
on the space density of luminous (L
IR
;10
13
L
) SMGs at z=4.404.66, suggesting
7
%
of SMGs with
S
1
870 m
m
mJy lie at 4<z<5. From stacking the high-resolution (0 15full-width half
maximum) ALMA 870 μm imaging, we show that the [C
II] line emission is more extended than the continuum
dust emission, with an average effective radius for the [C
II] of
r
1.7
e
0.2
0.1
=
-
+
kpc, compared to r
e
=1.0±0.1 kpc
for the continuum (rest-frame 160 μm).Bytting the far-infrared photometry for these galaxies from 100 to
870 μm, we show that SMGs at z4.5 have a median dust temperature of T
d
=55±4 K. This is systematically
warmer than 870 μm selected SMGs at z;2, which typically have temperatures around 35 K. These z;4.5
SMGs display a steeper trend in the luminosity-temperature plane than z2 SMGs. We discuss the implications
of this result in terms of the selection biases of high-redshift starbursts in far-infrared/submillimeter surveys.
Key words: galaxies: high-redshift submillimeter: galaxies
1. Introduction
Despite their high individual luminosities, ultra-luminous
infrared galaxies (ULIRGs;
L
10
IR
12
>
L
) contribute less than
1% of the local star formation rate density (e.g., Magnelli
et al. 2011; Casey et al. 2012). The situation at higher redshifts,
however, appears to be very different. Measurements of the
redshift distribution of high-redshift ULIRGs, including those
detected at submillimeter wavelengths (so-called submilli-
meter galaxies, SMGs; Smail et al. 1997) show a rapid rise
(1000-fold increase) in their volume density to a peak at
z;2.5 and a decline at high redshifts (e.g., Aretxaga
et al. 2003; Chapman et al. 2005; Wardlow et al. 2011; Casey
et al. 2012; Yun et al. 2012; Simpson et al. 2014, 2017a;
Michałowski et al. 2017).Atz1 SMGs may contribute up to
50% of the star formation rate density (e.g., Peacock
et al. 2000; Chapman et al. 2005; Barger et al. 2012; Casey
et al. 2014; Swinbank et al. 2014; Zavala et al. 2017). SMGs at
higher redshifts (z3) may also hold the key to explaining the
populations of z23 compact, quiescent galaxies now being
detected (e.g., Toft et al. 2014
; Hodge et al. 2016; Simpson
et al. 2017b ). The high stellar masses and apparent old ages of
these galaxies suggest that they formed in rapid, intense bursts
of star formation at z>3 (e.g., Glazebrook et al. 2017;
Simpson et al. 2017b). Such starbursts may be linked to high-
redshift SMGs (e.g., Ikarashi et al. 2015), meaning these
galaxies are an essential element in models of massive galaxy
formation.
High-redshift (z3) SMGs therefore appear to play a
potentially signicant role in galaxy evolution; however,
their dusty nature and high redshift mean that measuring their
spectroscopic redshiftsneeded to constrain many of their
basic propertiesis extremely challenging using ground-based
optical/near-infrared spectroscopy. As a result, the redshift
distribution of SMGs is increasingly incomplete at z 3 (e.g.,
Danielson et al. 2017).
Some progress can be made in identifying z>3 SMGs
using far-infrared photometry to measure their infrared spectral
The Astrophysical Journal, 861:100 (15pp), 2018 July 10 https://doi.org/10.3847/1538-4357/aac6ba
© 2018. The American Astronomical Society. All rights reserved.
1

energy distribution (SED, e.g., to identify 500 μm risers;
Dowell et al. 2014). However, the degeneracy in the SED
shape between dust temperature and redshift make the
derived redshifts highly uncertain (e.g., Blain 1999; Béthermin
et al. 2015; Schreiber et al. 2018).
For the subset of optical/near-infrared bright SMGs where
reliable photometric redshifts can be measured, recent studies
have suggested that z4 SMGs are characterized by far-
infrared SEDs that have warmer dust temperatures than SMGs
at z;2 (4050 K compared to 35 K; e.g., Swinbank et al.
2014; Schreiber et al. 2017 ). Although there are potentially
biases in the derived characteristic temperatures due to
selection effects, the higher dust temperatures inferred at
z;4 may also be driven by physical differences in galaxy
properties compared to SMGs at z; 2, for example, by
reecting the size of the dust regions or the star formation rate
of the galaxy. Alternatively, higher star formation efciencies
in z4 SMGs (which may have shorter dynamical times than
SMGs at z;2) may result in the warmer dust temperatures.
However, these results rely on uncertain photometric redshifts
and hence to reliably constrain any evolution in characteristic
dust temperatures with redshift, precise spectroscopic redshifts
for z>3 galaxies are required.
(Sub)millimeter spectroscopy provides one of the most
reliable means to derive redshifts for distant SMGs, especially
at
z3 where the multi-wavelength counterparts are faint or
undetected in the optical/near-infrared. With the advent of the
Atacama Large Millimeter/submillimeter Array (ALMA) it is
now possible to obtain high-resolution imaging and spectrosc-
opy in submillimeter wavebands. This allows us to both
efciently target single-dish submillimeter sources and pre-
cisely locate the counterpart of the SMG, and also to search for
emission lines in the far-infrared to measure spectroscopic
redshifts.
The
2
P
32
2
P
1/2
ne structure line of C
+
at 157.74 μm
(hereafter [C
II]) is one of the primary routes by which
interstellar gas can cool and consequently is typically the
strongest emission line in the far-infrared spectra of star-
forming galaxies. [ C
II] emission can account for up to 2% of
the total bolometric luminosity in a galaxy (e.g., Brauher
et al. 2008), although with one dex of scatter at a xed far-
infrared luminosity (e.g., Díaz-Santos et al. 2013). The scatter
arises due to the complex mix of processes that generate [C
II]
emission. For example, [C
II] emission can originate both in
photodissociation regions around star-forming regions and also
from atomic and ionized gas (e.g., Dalgarno & McCray 1972;
Madden et al. 1997; Pineda et al. 2013). [C
II] could thus
provide information about the volume and extent of the cold
gas reservoir and star formation in galaxies. In particular for
star-forming galaxies, the photodissociation regions can
dominate the [C
II] emission so several studies have shown
the [C
II] emission line correlates with the star formation rate
(e.g., Stacey et al. 1991; Graciá-Carpio et al. 2011; De Looze
et al. 2014).
To date, one of the largest samples of interferometrically
identied SMGs available is the ALMA-LABOCA Extended
Chandra Deep Field-South Survey (ALESS; Hodge et al. 2013),
which identied 99 SMGs, 21 of which are likely to lie at z>4
given their multi-wavelength properties (Simpson et al. 2014).
At z45 [C
II] is redshifted to 870 μm. In two of the
ALESS sources [C
II] was serendipitously detected in the
ALMA Band 7 observations at a redshift of z=4.424.44
(Swinbank et al. 2012), placing weak constraints on the
properties of these galaxies and the [C
II] luminosity function
at this redshift. However, with only two sources, a larger
spectroscopic sample is clearly required in order to improve our
understanding of the properties of z4 SMGs.
To increase the sample size of high-redshift SMGs, we have
undertaken the ALMA-SCUBA-2 survey of the Ultra Deep
Survey (UDS) eld (AS2UDS):anALMABand7surveyofall
716 submillimeter sources detected in the UKIDSS UDS eld by
SCUBA-2 on the James Clerk Maxwell Telescope (JCMT; Geach
et al. 2017). This survey has precisely located 695 SMGs (S. M.
Stach et al. 2018, in preparation). Here, we examine the ALMA
datacubes to search for serendipitous emission lines. The
frequency coverage of our data is 336340 and 348352 GHz,
corresponding to z=4.404. 46 and z=4.604.66 for [C
II].We
aim to spectroscopically conrm [C
II] emission line sources at
z;4.5 and thus determine their basic properties such as infrared
luminosity and dust temperature, as well as measure the number
density of SMGs at z>4.
The paper is laid out as follows. In Section 2 we outline the
observations and data reduction. Section 3 presents our results and
discussion. Our conclusions are given in Section 4. Throughout
we use AB magnitudes and assume a ΛCDM cosmology with
Ω
M
=0.3, Ω
Λ
=0.7, and H
0
=70 km s
1
Mpc
1
.
2. Observations, Data Reduction, and Analysis
2.1. ALMA Data
The UDS 0.96 degree
2
eld was observed at 850 μm with
SCUBA-2 as part of the Cosmology Legacy Survey (Geach
et al. 2017) to a depth of σ
850
;0.9 mJy beam
1
, detecting 716
submillimeter sources above 4σ, S
850
;3.6 mJy. We observed all
716 of these submillimeter sources with ALMA at 870 μmin
Band 7 to pinpoint the galaxies responsible for the submillimeter
emission. The data were taken in the period 2013 November to
2017 May (Cycles 1, 3, and 4) with a dual polarization setup. The
full data reduction and catalog will be presented in S. M. Stach
et al. (2018, in preparation). In brief, our observations cover a total
bandwidth of 7.5 GHz split into two sidebands: 336340 GHz and
348352 GHz. The synthesized beam of our observations is
0.150.3 arcsec FWHM, adopting natural weighting. The primary
beam of ALMA is 18 arcsec FWHM, which covers the
SCUBA-2 beam (FWHM14.5 arcsec). This coverage, com-
bined with the higher resolution and greater depth of the ALMA
observations, means that we expect to detect the sources
responsible for the original SCUBA-2 detections in the ALMA
continuum data.
Each pointing was centered on the SCUBA-2 catalog
position and observed for a total of 40 s. A subset of 120
of the pointings were observed in both Cycles 3 and 4 and thus
have a longer total integration time (typically 8090 s). All data
were processed using the Common Astronomy Software
Application (
CASA; McMullin et al. 2007). We construct
cleaned, tapered continuum maps and dirty, tapered datacubes.
For more reliable line detections, we image the cubes without
applying any cleaning to deconvolve the beam. The nal
cleaned, 0
5 FWHM-tapered continuum maps have average
depths of σ
870 μm
=0.25, 0.34, and 0.23 mJy beam
1
for Cycle
1, 3, and 4 data, respectively. S. M. Stach et al. (2018, in
preparation) will present an analysis of the data, catalog
construction, and multi-wavelength properties.
2
The Astrophysical Journal, 861:100 (15pp), 2018 July 10 Cooke et al.

ALMA continuum sources were identied in the continuum
maps as submillimeter sources with a signal-to-noise ratio
(S/N) S/N4.3 within a 0.5 arcsec diameter aperture,
calculated from the aperture-integrated ux and the noise
measured in randomly placed apertures for each map. This S/N
limit provides a 2% false-positive rate, as determined by
inverting the maps. In total we selected 695 ALMA continuum-
detected SMGs brighter than S
870
1 mJy, which will be
discussed in S. M. Stach et al. (2018, in preparation).
To search for emission lines, we use the dirty datacubes, which
were constructed at raw spectral resolution (13.5 km s
1
) and
tapered to 0
5 FWHM resolution to match the continuum maps
by applying a 400 kλ Gaussian uv taper. These cubes were rst
continuum-subtracted by subtracting a linear t to the continuum
in the spectrum of each pixel.
16
These continuum-subtracted
cubes were also used to calculate [C
II] emission sizes in
Section 3.4.
To search for emission lines in the datacubes, we velocity-
binned the 0
5 tapered, continuum-subtracted cubes to 50, 100,
and 200 km s
1
channels and then extracted the spectra at the
position of each AS2UDS continuum-selected SMG. We
search each of these spectra for peaks with S/N2. These
were then ret in the 50 km s
1
channel spectrum using a
Gaussian prole and the integrated S/N calculated within the
FWHM of the line.
Given the non-Gaussian nature of the noise in the ALMA
datacubes, to determine the purity of the sample and hence an
acceptable S/N threshold for our detections, we calculate an
empirical false-positive rate by applying the same procedure to
the inverted, velocity-binned, continuum-subtracted cubes at
the AS2UDS source positions. This false-positive rate is
dependent on the velocity binning of the spectra. We require a
threshold for selection that produces a false detection rate
of 10%.
For a false-positive rate of ten percent in our nal sample,
we take an integrated S/N cut that varies depending upon the
velocity binning with S/N=8.0 for 50 km s
1
channels,
S/N=7.5 for 100 km s
1
channels, and S/N=7.0 for
200 km s
1
channels. We detect signicant emission lines in
10 SMGs above these limits. We plot all 10 line emitters in
Figure 1 and list their properties in Tables 1 and 2.
To ensure we identify all bright line emitters within the
ALMA pointings, we run two additional line searches. First, two
of the SMGs where we have identied an emission line lie in
ALMA maps that contain a second SMG. In these cases we
extract the spectra of this second SMG and search for
(lower-signicance) emission lines at similar frequencies
to the detected emission line. In one of these SMGs
(AS2UDS.0109.1) we found a tentative S/N=5.3 emission
line corresponding to the same redshift as the detected emission
line source (AS2UDS.0109.0). We include this SMG as
a supplementary source
17
and the spectra and optical/
near-infrared thumbnails are shown in Appendix A.Ifconrmed,
this secondary source would be located 70 kpc in projection and
50 km s
1
offset in redshift from the primary source.
Second, we also searched the 695 ALMA cubes for emission
line sources lacking continuum counterparts. In each con-
tinuum-subtracted cube we step through the cube, collapsing in
velocity bins of 100 km s
1
(7 resolution elements), the
centers of which are shifted by 50 km s
1
between slices.
18
In
each collapsed 100 km s
1
slice we search in the narrowband
image for peaks above 2σ within the ALMA primary beam. For
any peak detected we extract and t a Gaussian prole to the
full spectrum and measure the integrated S/N of the line. We
perform the same search on the inverted cubes to calculate a
false-positive rate. Using this method we nd no additional
emission line sources within the ALMA pointings above a
false-positive rate of 50%, corresponding to a line ux limit of
Sdv1Jykms
1
.
2.2. Multi-wavelength Data
The UDS has photometric coverage spanning the optical,
near-, mid-, and far-infrared, out to radio wavelengths.
The UKIRT Infrared Deep Sky Survey (UKIDSS; Lawrence
et al. 2007) UDS data release 11 (DR11) photometric catalog
(O. Almaini et al., in preparation) is based on a deep,
Figure 1. Emission lines detected in the AS2UDS Band 7 datacubes, ranked by their integrated S/N. These are labeled with the SMG ID, the S/N and the
corresponding redshift if the line is [C
II] 157.74 μm. We nd 10 line emitters with integrated S/N ranging from S/N=7.217.3 in the 695 SMGs with
S
870 μm
1 mJy. Fluxes shown are not primary beam-corrected (typically a correction of <10%) and are measured at the position of peak ux within the 0.5 arcsec
tapered dirty cubes. The gray lines show the unbinned data. The black histogram shows the data binned to 100 km s
1
. The red lines show the Gaussian t to the
detected emission line and continuum level. We note that AS2UDS.0243.0 and AS2UDS.0535.0 have photometric properties that suggest the line we detectis
CO(87) or CO(54), respectively, corresponding to z
CO
<2. Further observations are needed to conrm the nature of these emission lines.
16
Second-order polynomial and constant ts were also tested, but the linear t
produced a good t to the data without over-tting.
17
The false-positive rate at this signicance and line width is 50%, so this
source requires further observations in order to conrm it.
18
We also tested channels of 50 and 200 km s
1
with step sizes that were half
the channel sizes, but no additional signicant emission lines were detected.
3
The Astrophysical Journal, 861:100 (15pp), 2018 July 10 Cooke et al.

K-band-selected catalog down to a 3σ depth of K=25.9 mag,
with additional imaging in U, B, V, R, I, J, H, K, and
Spitzer/IRAC.
To derive the photometric properties of our sample, we
match our sources to the UDS DR11 using a search radius of
0.6 arcsec, giving a false-match rate of 3.5% (An et al. 2018).
Three-color 5×5thumbnails of the 10 candidate line
emitters are shown in Figure 2. This gure also shows a
zoomed-in 3 ×3optical/infrared image of each source, with
the ALMA continuum contours overlaid. We use these
thumbnails to assess the multi-wavelength properties of the
line emitters, in particular to determine if the optical/infrared
photometry is contaminated by nearby galaxies.
The UDS20 project (V. Arumugam et al., in preparation)
imaged the UDS at 1.4 GHz using the Very Large Array. The
total area coverage is 1.3 degrees
2
, with the 160 hr
integration resulting in an rms noise of 6 μJy beam
1
across
the full eld. A full description of the radio data will be
presented in V. Arumugam et al. (in preparation).
The UDS also has coverage with the Herschel Space
Observatory Photoconductor Array Camera and Spectrometer
(PACS; Poglitsch et al. 2010) and Spectral and Photometric
Imaging REceiver (SPIRE; Grifn et al. 2010) at 100, 160,
250, 350, and 500 μm. The resolution of the far-infrared
Herschel wavebands (1535 arcsec) requires the data to be
deblended in order to obtain the photometry of our SMGs.
For the deblending we follow the method described in
Swinbank et al. (2014).
19
The deblending uses a combination
of the ALMA-detected SMGs and Spitzer/MIPS 24 μm and
UDS20 radio sources as positional priors for the deblending of
the low-resolution SPIRE maps. To deblend the SPIRE maps
we use a Monte Carlo algorithm that ts the observed ux
distribution with beam-sized components at the position of
each source in the prior catalog. This is then iterated toward
solutions that yield the range of possible uxes associated with
each source. To ensure that we do not over deblend, the
method is rst applied at 250 μm. Any sources in the prior
catalog that are detected at 250 μm above 2σ are then used as
the prior list for the 350 μm deblending, and similarly those
detected above >2σ at 350 μm are then used in the 500 μm
deblending. There are averages of 2.4, 2.0 and 1.9 priors within
the FWHM of the beam centered at the ALMA position (i.e.,
15 arcsec, 25 arcsec and 35 arcsec at 250 μm, 350 μm, and
500 μm respectively). By attempting to recover false positives
injected into the maps we derive 3σ detection limits of 7.0, 8.0,
and 10.6 mJy at 250, 350, and 500 μm, respectively (see
Swinbank et al. 2014 for details). The ALMA sources
are included at all wavelengths so as not to bias their SEDs.
We discuss the Herschel uxes and far-infrared SED ts in
more detail in Section 3.3.
3. Results and Discussion
We identify emission lines in 10 AS2UDS continuum sources:
three with integrated S/N>7.0inthe200kms
1
channel
spectra, six with integrated S/N>7.5 at 100 km s
1
,andone
with integrated S/N>8.0 at 50 km s
1
.Figure1 shows the
spectra of these sources binned to 100 km s
1
channels (we also
show the data at the native resolution). We provide the source
redshifts and line properties in Table 1.Thelineux densities are
calculated from the Gaussian prole ttoeachline.
Table 1
Table of Emission Line Candidates and Line Properties
Source ID
a
R.A. Decl. S
870
b
ν
obs
c
z
[C
II
]
d
FWHM
[C
II
]
e
Sdv
[C
II
]
S/N
f
(J2000)(mJy)(GHz)(km s
1
)(Jy km s
1
)
AS2UDS.0002.1 02:18:24.24 05:22:56.9 7.4±0.5 338.707 4.611±0.009 220±50 1.3±0.3 8.7
AS2UDS.0051.0 02:19:24.84 05:09:20.8 6.3±0.4 350.571 4.421±0.006 770±80 4.0±0.4 10.5
AS2UDS.0104.0 02:16:22.73 05:24:53.3 5.6±0.3 350.447 4.423±0.007 530±60 4.9±0.6 17.3
AS2UDS.0109.0 02:16:18.37 05:22:20.1 5.5±0.7 348.715 4.450±0.007 440±40 4.5±0.4 11.3
AS2UDS.0208.0 02:19:02.88 04:59:41.5 4.0±0.7 338.445 4.615±0.009 290±40 2.2±0.3 8.1
AS2UDS.0232.0 02:15:54.66 04:57:25.6 4.6±
0.3 349.140 4.443±0.008 340±90 0.9±0.2 7.2
AS2UDS.0243.0
g
02:16:17.91 05:07:18.9 4.3±0.3 350.939 L 560±70 3.2±0.4 15.5
AS2UDS.0535.0
h
02:18:13.30 05:30:29.1 2.4±0.5 339.301 L 310±20 3.6±0.2 12.1
AS2UDS.0568.0 02:18:40.02 05:20:05.6 1.2±0.3 351.701 4.404±0.009 340±60 2.8±0.5 10.3
AS2UDS.0643.0 02:16:51.31 05:15:37.2 2.2±0.4 338.512 4.614±0.007 390±110 1.5±0.4 7.2
Median values
i
LL4.4±0.6 L 4.45±0.03 370±50 3.0±0.1 10.4±1.1
Supplementary catalog
j
AS2UDS.0109.1 02:16:19.04 05:22:23.2 2.6±0.6 348.653 4.45±0.01 270±40 1.6±0.2 5.3
Notes.
a
Source IDs, coordinates, and 870 μm ux densities come from the full AS2UDS catalog presented in S. M. Stach et al. (2018, in preparation).
b
The continuum ux densities are primary beam-corrected and were measured in 1 arcsec diameter apertures in the 0.5 arcsec FWHM-tapered maps.
c
Observed frequencies correspond to the peak of the detected emission line.
d
Redshifts are derived assuming the detected emission line is [C II].
e
The FWHM and ux density (and their respective uncertainties) of each line are measured from a Gaussian t to the emission line.
f
S/N measurements come from integrating the spectrum across the line between ν
obs
0.5×FWHM and ν
obs
+0.5×FWHM.
g
AS2UDS.0243.0 has optical, near-infrared, and radio properties that may indicate the line we detect is CO(87), corresponding to z
CO
=1.63±0.01.
h
AS2UDS.0535.0 has optical and near-infrared properties that may indicate the line we detect is CO(54), corresponding to z
CO
=0.70±0.01.
i
Uncertainties on median values are the standard error.
j
Supplementary sources are those within the same ALMA map as a detected line emitter (but not detected above our S/N threshold), which appear to have a low-
signicance emission line at a similar frequency to their detected companion.
19
The deblended catalogs for the elds are available from http://astro.dur.ac.
uk/~ams/HSOdeblend.
4
The Astrophysical Journal, 861:100 (15pp), 2018 July 10 Cooke et al.

The number of line emitters we identify from the parent
sample of 695 SMGs is consistent with the expectation from
the ALESS survey, where two emission line sources were
identied from a sample of 99 SMGs (Swinbank et al. 2012).
3.1. Alternative Emission Lines
Before we discuss the properties of our line-emitter galaxies,
we rst discuss the identication of the emission lines. Within
the ISM of dusty star-forming galaxies, the brightest emission
line in the rest-frame far-infrared is expected to be [C
II] λ157
μm. At observed frame 870 μm this would correspond to
z4.5. [C
II] dominates the cooling of the ISM for
temperatures T<100 K, and as noted earlier, may contribute
up to two percent of the bolometric luminosity (e.g., Smail
et al. 2011). However, there may be contamination from other
emission lines in our sample such as [N
II] λ122 μmatz6.1,
[O
I] λ145 μmatz4.9, [N II] λ205 μmatz3.1, or
high-J
12
CO at z=0.32.7 (4<J
up
<11). In typical sources
the [C
II] emission line is expected to be 10 times brighter
than these other lines (e.g., Brauher et al. 2008), so we expect
contamination to be modest given the shallow depth of the
current ALMA data.
We investigate potential contamination using the multi-
wavelength data available in the UDS eld. The photometric
properties of our emission line SMGs are given in Table 2.
Most sources are very red or undetected in the optical/near-
infrared, which is consistent with them being z>4 dusty
galaxies. In addition, only two have detections at 1.4 GHz,
again consistent with the majority being at z?3 (Chapman
et al. 2005). A discussion of each of the individual line emitters
is given in Appendix B.
Galaxies at z4.5 are not expected to be detected in the
optical B-band due to the Lyman limit at 912 Å redshifting to
5000 Å. In Figure 2 we show the high-resolution ALMA
870 μm continuum emission contoured over a B-band (or
K-band) image of each galaxy. Half of the ALMA detections
do not have a B-band counterpart and/or have photometric
redshifts consistent with a z>4 galaxy. The other ve line
emitters have B-band counterparts that are offset by 1 arcsec
from the ALMA continuum emission. In these cases, we have
agged the photometry and note that this may indicate lensing
of the submillimeter source by a foreground galaxy. These ve
sources are listed in italics in Table 2 and by circle symbols in
all gures where the sources are individually plotted.
On the basis of their multi-wavelength properties, three of
these ve sources with nearby B-band counterparts appear to be
potentially lensed high-redshift [C II] emitters, as the B-band
emission is not spatially coincident with the submillimeter
emission. We crudely estimate that the lensing of these sources
may affect our measured uxes by a factor of 1.52;
however, with the current data we are unable to estimate more
precise magnication factors.
We next estimate the line luminosities of the two sources
where the submillimeter emission is spatially coincident
(within 0
5) with a B-band detection: AS2UDS.0243.0 and
AS2UDS.0535.0, assuming these correspond to high-J
12
CO
lines. We compare these luminosities to other studies of
high-J
12
CO emission lines to see whether it is plausible that
these lines are high-J
12
CO rather than [C II].
The photometric redshifts of AS2UDS.0243.0 and
AS2UDS.0535.0 are reported in Table 2. At these redshifts the
emission lines would correspond to CO(87) at z=1.63±0.01
with
L
1.5 10
CO 8 7
8
-()
L
for AS2UDS.0243.0 and
CO(54) at z=0.70±0.01 with
L
0.2 10
CO 5 4
8
-()
L
for AS2UDS.0535.0. These luminosities are approximately an
order of magnitude brighter than what is found in typical local
ULIRGs (e.g., Arp 220; Rangwala et al. 2011) or AGN-
dominated sources (e.g., Mrk 231; van der Werf et al. 2010).
However, recent studies have found comparably luminous
sources at higher redshifts (z>2, e.g., Barro et al. 2017;Yang
et al. 2017). It is therefore possible that these two sources lie at
Table 2
Photometric Properties of Line Emitters
Source ID V
a
K 4.5 μm S
250
S
350
S
500
S
1.4 GHz
z
phot
Potential
(mag)(mag)(mag)(mJy)(mJy)(mJy)(μJy) Contamination?
b
AS2UDS.0002.1 26.85±0.25 23.97±0.06 22.76±0.02 31±435±543±7 <80 L Y: lens?
AS2UDS.0051.0 >27.47 23.32±0.04 21.93±0.02 <9 <11 <12 <80 L N
AS2UDS.0104.0 L 24.03±0.06 23.50±0.08 <9 <11 <12 <80 L N
AS2UDS.0109.0 L 22.88±0.03 22.56±0.07 <911±3 <14 <80 L N
AS2UDS.0208.0 26.11±0.10 22.94±
0.01 21.28±0.01 < 9 <12 <12 <80 L Y: lens?
AS2UDS.0232.0 LL22.42±0.01 24±420±4 <12 <80 L N
AS2UDS.0243.0 23.21±0.01 20.68±0.01 20.17±0.01 32±5 <17 <17 1220±30
1.58
0.05
0.05
-
+
Y: low-z CO?
AS2UDS.0535.0 25.95±0.09 23.44±0.04 22.98±0.07 12±313±3 <15 <80
0.80
0.03
0.03
-
+
c
Y: low-z CO?
AS2UDS.0568.0 >27.8 24.36±0.06 23.39±0.05 <18 <16 <12 <80 3.5±1.0 N
AS2UDS.0643.0 >27.8 24.30±0.03 21.80 ± 0.01 14±311±3 <13 105±18
4.4
1.1
0.6
-
+
Y: lens?
Median values
d
26.11±0.33 23.44±0.13 22.49±0.10 24±213±2 <13 <80 LL
Supplementary catalog
AS2UDS.0109.1 L 24.02±0.07 23.28±0.13 <9 <11 <12 <80 L N
Notes.
a
Photometry and redshifts are taken from the UDS DR11 catalog (O. Almaini et al., in preparation) and the UDS20 radio catalog (V. Arumugam et al., in
preparation). Ellipses indicate no photometric coverage.
b
Italics indicates that the SMG has a nearby source that may contaminate the photometry. Some or all of these sources may also be lensed (nal column); see
Section 3.1.
c
AS2UDS.0535.0 has a secondary peak in its photometric redshift distribution at z =4.63 (see Appendix B).
d
Uncertainties on median values are the standard errors.
5
The Astrophysical Journal, 861:100 (15pp), 2018 July 10 Cooke et al.

Figures (10)
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Frequently Asked Questions (1)
Q1. What have the authors contributed in "An alma survey of the scuba-2 cosmology legacy survey ukidss/uds field: identifying candidate z ∼ 4.5 [c ii] emitters" ?

The authors report the results of a search for serendipitous [ C II ] 157. 74 μm emitters at z ; 4. 4–4. 7 using the Atacama Large Millimeter/submillimeter Array ( ALMA ). From stacking the high-resolution ( ∼0 15full-width half maximum ) ALMA 870 μm imaging, the authors show that the [ C II ] line emission is more extended than the continuum dust emission, with an average effective radius for the [ C II ] of r 1. 7 e 0. 2 0. 1 = + kpc, compared to re=1. 0±0. 1 kpc for the continuum ( rest-frame 160 μm ). By fitting the far-infrared photometry for these galaxies from 100 to 870 μm, the authors show that SMGs at z∼4. The authors discuss the implications of this result in terms of the selection biases of high-redshift starbursts in far-infrared/submillimeter surveys. The authors detect 10 candidate line emitters, with an expected false detection rate of 10 %. 5 10 Mpc 6 3 > ́ on the space density of luminous ( LIR ; 10 13 L ) SMGs at z=4. 40–4. 66, suggesting 7  % of SMGs with S 1 870 m  m mJy lie at 4 < z < 5.