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Dunlop, J. S. and McLure, R. J. and Biggs, A. D. and Geach, J. E. and Micha
l
owski, M. J. and Ivison, R. J.
and Rujopakarn, W. and van Kampen, E. and Kirkpatrick, A. and Pope, A. and Scott, D. and Swinbank, A.
M. and Targett, T. A. and Aretxaga, I. and Austermann, J. E. and Best, P. N. and Bruce, V. A. and Chapin,
E. L. and Charlot, S. and Cirasuolo, M. and Coppin, K. and Ellis, R. S. and Finkelstein, S. L. and Hayward,
C. C. and Hughes, D. H. and Ibar, E. and Jagannathan, P. and Khochfar, S. and Koprowski, M. P. and
Narayanan, D. and Nyland, K. and Papovich, C. and Peacock, J. A. and Rieke, G. H. and Robertson, B. and
Vernstrom, T. and van der Werf, P. P. and Wilson, G. W. and Yun, M. (2017) 'A deep ALMA image of the
Hubble Ultra Deep Field.', Monthly notices of the Royal Astronomical Society., 466 (1). pp. 861-883.
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MNRAS 466, 861–883 (2017) doi:10.1093/mnras/stw3088
Advance Access publication 2016 November 30
A deep ALMA image of the Hubble Ultra Deep Field
J. S. Dunlop,
1‹
R. J. McLure,
1
A. D. Biggs,
2
J. E. Geach,
3
M. J. Michałowski,
1
R. J. Ivison,
1,2
W. Rujopakarn,
4
E. van Kampen,
2
A. Kirkpatrick,
5
A. Pope,
5
D. Scott,
6
A. M. Swinbank,
7
T. A. Targett,
8
I. Aretxaga,
9
J. E. Austermann,
10
P. N. Best,
1
V. A. Bruce,
1
E. L. Chapin,
11
S. Charlot,
12
M. Cirasuolo,
2
K. Coppin,
3
R. S. Ellis,
2
S. L. Finkelstein,
13
C. C. Hayward,
14
D. H. Hughes,
9
E. Ibar,
15
P. Jagannathan,
16
S. Khochfar,
1
M. P. Koprowski,
3
D. Narayanan,
17
K. Nyland,
18
C. Papovich,
19
J. A. Peacock,
1
G. H. Rieke,
20
B. Robertson,
21
T. Vernstrom,
22
P. P. van der Werf,
23
G. W. Wilson
5
and M. Yun
5
Affiliations are listed at the end of the paper
Accepted 2016 November 25. Received 2016 November 25; in original form 2016 June 6
ABSTRACT
We present the results of the first, deep Atacama Large Millimeter Array (ALMA)
imaging covering the full 4.5 arcmin
2
of the Hubble Ultra Deep Field (HUDF) im-
aged with Wide Field Camera 3/IR on HST. Using a 45-pointing mosaic, we have
obtained a homogeneous 1.3-mm image reaching σ
1.3
35 µJy, at a resolution of
0.7 arcsec. From an initial list of 50 > 3.5σ peaks, a rigorous analysis confirms 16
sources with S
1.3
> 120 µJy. All of these have secure galaxy counterparts with robust redshifts
(z=2.15). Due to the unparalleled supporting data, the physical properties of the ALMA
sources are well constrained, including their stellar masses (M
∗
) and UV+FIR star formation
rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift
Universe; indeed at z ≥ 2 our ALMA sample contains seven of the nine galaxies in the HUDF
with M
∗
≥ 2 × 10
10
M
, and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off
of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow
us to probe the redshift/mass distribution of the 1.3-mm background down to S
1.3
10 µJy.
We find strong evidence for a steep star-forming ‘main sequence’ at z 2, with SFR ∝ M
∗
and
a mean specific SFR 2.2 Gyr
−1
. Moreover, we find that 85 per cent of total star formation
at z 2 is enshrouded in dust, with 65 per cent of all star formation at this epoch occurring
in high-mass galaxies (M
∗
> 2 × 10
10
M
), for which the average obscured:unobscured SF
ratio is 200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at
z 2.5, and that the star-forming Universe transits from primarily unobscured to primarily
obscured at z 4.
Key words: galaxies: evolution – galaxies: high-redshift – galaxies: starburst – cosmology:
observations – submillimetre: galaxies.
1 INTRODUCTION
A complete understanding of cosmic star formation history, and
the physical mechanisms that drive galaxy formation and evolu-
tion, requires that we connect our UV/optical and infrared/mm
views of the Universe (e.g. Hopkins & Beacom 2006; Dunlop 2011;
Burgarella et al. 2013; Madau & Dickinson 2014). Until the advent
of Atacama Large Millimeter Array (ALMA), these two views have
E-mail: jsd@roe.ac.uk
been largely disconnected, for both technical and physical reasons.
Benefiting from low background and high angular resolution, deep
UV/optical surveys have proved extremely effective at completing
our inventory of unobscured star formation which, certainly at high
redshift, is dominated by large numbers of low-mass galaxies with
individual star formation rates (SFR) 1M
yr
−1
(e.g. McLure
et al. 2013;Bouwensetal.2015;Bowleretal.2015; Finkelstein
et al. 2015; McLeod et al. 2015).
However, UV/optical observations are unable to uncover the most
extreme star-forming galaxies, which, following the breakthroughs
in far-IR/sub-mm astronomy at the end of the last century, are
C
2016 The Authors
Published by Oxford University Press on behalf of the Royal Astronomical Society
862 J. S. Dunlop et al.
known to be enshrouded in dust (Smail, Ivison & Blain 1997;
Barger et al. 1998; Hughes et al. 1998; Eales et al. 1999). Such
objects have now been uncovered in significant numbers through
far-IR/mm surveys with the James Clerk Maxwell Telescope (e.g.
Scottetal.2002; Coppin et al. 2006; Scott, Dunlop & Serjeant 2006;
Austermann et al. 2010; Geach et al. 2013), IRAM (Dannerbauer
et al. 2004; Greve et al. 2004; Lindner et al. 2011), APEX (Weiss
et al. 2009;Smolcicetal.2012), ASTE (Scott et al. 2008, 2010;
Hatsukade et al. 2010, 2011), BLAST (Devlin et al. 2009; Dunlop
et al. 2010)andHerschel (Eales et al. 2010; Oliver et al. 2010;Elbaz
et al. 2011;Lutzetal.2011), with ‘sub-mm galaxies’ now known
out to redshifts z > 6 (Riechers et al. 2013).
While sub-mm surveys for high-redshift galaxies benefit from
a strong negative K-correction [provided by a modified blackbody
spectral energy distribution (SED); Blain & Longair 1993; Hughes
et al. 1993; Dunlop et al. 1994], the high background and/or the rela-
tively poor angular resolution provided by single-dish telescopes at
these wavelengths means that they are only really effective at uncov-
ering rare, extreme star-forming galaxies with SFR > 300 M
yr
−1
(albeit reaching down to SFR > 100 M
yr
−1
in the very deep-
est Submillimetre Common-User Bolometer Array-2 (SCUBA-2)
450/850 µm imaging; Geach et al. 2013; Roseboom et al. 2013;
Koprowski et al. 2016). The existence of such objects presents an
interesting and important challenge to theoretical models of galaxy
formation (e.g. Baugh et al. 2005; Khochfar & Silk 2009;Dav
´
e
et al. 2010; Hayward et al. 2011; Narayanan et al. 2015), but they
provide only 10–15 per cent of the known far-infrared/mm back-
ground (Fixsen et al. 1998; Scott et al. 2012; Geach et al. 2013),
and attempts to complete our inventory of obscured star formation
have had to rely on stacking experiments (e.g. Peacock et al. 2000;
Marsden et al. 2009; Geach et al. 2013; Coppin et al. 2015).
A key goal, therefore, of deep surveys with ALMA is to close
the depth/resolution gap between UV/optical and far-infrared/mm
studies of the high-redshift Universe, and hence enable a com-
plete study of visible+obscured star formation within the overall
galaxy population. Over the last 2 yr, ALMA has begun to make
important contributions in this area. Most early ALMA programmes
have focused (sensibly) on pointed observations of known objects
(e.g. Hodge et al. 2013; Karim et al. 2013; Ouchi et al. 2013;
Bussmann et al. 2015; Capak et al. 2015; Maiolino et al. 2015;
Simpson et al. 2015a,b; Scoville et al. 2016), including gravita-
tionally lensed sources (e.g. Weiss et al. 2013;Watsonetal.2015;
B
´
ethermin et al. 2016; Knudsen et al. 2016; Spilker et al. 2016).
However, strenuous efforts have been made to exploit the resulting
combined ‘blank-field’ survey by-product to improve our under-
standing of the deep mm source counts (e.g. Ono et al. 2014;Car-
niani et al. 2015; Fujimoto et al. 2016;Oteoetal.2016) albeit with
interestingly different results. More recently, time has been awarded
to programmes that aim to deliver contiguous ALMA mosaic imag-
ing of small regions of sky with excellent multiwavelength support-
ing data (e.g. Hatsukade et al. 2015; Umehata et al. 2015). Such
programmes offer not only further improvements in our knowledge
of the sub-mm/mm source counts, but also the ability to determine
the nature and physical properties (redshifts, stellar masses, SFR) of
the ALMA-detected galaxies. For example, ALMA 1.1-mm imag-
ing of 1.5 arcmin
2
within the CANDELS/UDS field (PI: Kohno) has
provided new results on the 1.1-mm counts, and enabled the study
of several ALMA-detected galaxies (Tadaki et al. 2015; Hatsukade
et al. 2016).
However, to date, no homogeneous ALMA imaging has been
undertaken within the best-studied region of deep ‘blank-field’ sky,
the Hubble Ultra Deep Field (HUDF). On scales of a few arcmin
2
,
the HUDF remains unarguably the key ultradeep extragalactic sur-
vey field and, lying within the GOODS-South field at RA 03
h
, Dec.
−28
◦
, is ideally located for deep ALMA observations. While four of
the six Hubble Frontier Fields
1
provide alternative target fields for
deep ALMA observations, the quality of the optical-near-infrared
data in these fields will never seriously rival that which has already
been achieved in the HUDF. In part this is due to the huge investment
in HST optical imaging in this field made prior to the degradation of
the ACS camera (Beckwith et al. 2006). However, it is also a result
of the more recent investment in imaging with Wide Field C amera
3 (WFC3)/IR on the HST since 2009. Specifically, the combination
of the UDF09 campaign (Bouwens et al. 2010; Oesch et al. 2010;
Illingworth et al. 2013) followed by the UDF12 programme
(Dunlop et al. 2013; Ellis et al. 2013; Koekemoer et al. 2013), has
delivered the deepest near-infrared imaging ever achieved (reaching
30 AB mag, 5σ ) over an area of 4.5 arcmin
2
. As a result of coupling
this multiband HST imaging with the recently augmented ultradeep
Spitzer data (Labb
´
eetal.2015), accurate photometric redshifts,
stellar masses and UV SFR are now known for 3000 galaxies in
this field (e.g. Parsa et al. 2016). For a field of this size, the HUDF
is also uniquely rich in optical/infrared spectroscopic information,
with a combination of ground-based optical spectroscopy and HST
WFC3/IR near-infrared grism spectroscopy delivering redshifts and
emission-line strengths for over 300 galaxies (see Section 2.2). Fi-
nally, the HUDF lies in the centre of the Chandra Deep Field South
(CDFS) 4-ms X-ray imaging (Xue et al. 2011), and has recently
been the focus of a new programme of ultradeep radio imaging
with the JVLA (PI: Rujopakarn).
The aim of the work presented here was to exploit this unique
data base by using ALMA to construct the deepest homogeneous
mm-wavelength image obtained to date on the relevant scales. As
described in detail in the next section, 20 h of ALMA observations
were approved in C ycle 1, and rolled over into Cycle 2, to enable us
to complete a 1.3-mm mosaic covering the full 4.5 arcmin
2
imaged
with WFC3/IR, seeking to reach an rms depth of σ
1.3
30 µJy (PI:
Dunlop). We chose to undertake this first deep ALMA image of
the HUDF at 1.3 mm (rather than at shorter wavelengths) for three
reasons. First, in practice it maximizes sensitivity to higher redshift
dusty star-forming galaxies at z > 3. Second, it is at these longer
wavelengths that the resolution of single-dish surveys is undoubt-
edly poorest, and hence the imaging most confused. Third, this
decision aided the feasibility of the observations in early ALMA
cycles, with only 45 pointings required to complete the mosaic, and
both nighttime and daytime observations being acceptable. Astro-
physically, we sought to reach detections 4–5 times deeper than can
be achieved with the deepest single-dish surveys (corresponding to
SFR 25 M
yr
−1
out to the very highest redshifts), and to exploit
the uniquely complete HUDF galaxy data base in deep stacking
experiments.
Data taking for this project commenced in 2014, and was com-
pleted in summer 2015, and in this paper we present the first results.
We present and discuss the properties of the ALMA map, the sources
uncovered within it, and the implications for our understanding of
cosmic star formation and galaxy evolution. The remainder of this
paper is structured as follows. In Section 2, we describe the ALMA
observations, explain how the data were reduced and provide a sum-
mary of all the key multiwavelength supporting data in the field. In
Section 3, we explain how sources were extracted from the ALMA
map, and then, in Section 4, describe how cross-identifications with
1
http://www.stsci.edu/hst/campaigns/frontier-fields/
MNRAS 466, 861–883 (2017)
A deep ALMA image of the HUDF 863
the HST sources in the field enabled us to clean the source list to a
final sample of 16 robust ALMA sources. In Section 5, we consider
the implications of the number of sources we have detected, aided by
the results of source injection and retrieval simulations, and compare
our results to other recent estimates of deep mm number counts. In
Section 6, we derive the physical properties of the sources we have
detected, and explore the implications for star formation in galaxies
at z = 1–3. Then, in Section 7, we present the results of stacking
the 1.3-mm signal on the positions of known galaxy populations in
the HUDF, and consider the consequences for the mm-wavelength
background and for the ratio of obscured/unobscured star formation
over cosmic history. Finally, we discuss the astrophysical implica-
tions of our findings in Section 8, and summarize our conclusions in
Section 9. Throughout, all magnitudes are quoted in the AB system
(Oke 1974; Oke & Gunn 1983), and all cosmological calculations
assume a flat cold dark matter cosmology with
M
= 0.3,
=
0.7, and H
0
= 70 km s
−1
Mpc
−1
.
2DATA
2.1 ALMA observations and data reduction
The ALMA observations of the HUDF were taken during two sep-
arate observing seasons – the first nine execution blocks (EBs) in
2014 July–September and the remaining four in 2015 May. As the
primary goal of these observations was to produce a continuum map
of the HUDF, the correlator was configured to process the maximum
7.5-GHz bandwidth in the form of four 1875 MHz-wide spectral
windows, each with 128 dual-polarization channels. However, the
velocity resolution of 40–45 km s
−1
is still sufficient to resolve spec-
tral lines that are typically observed in high-redshift star-forming
galaxies. The correlator averaging time was 2 s per sample.
The HUDF was observed using a 45-pointing mosaic, with each
pointing separated by 0.8 times the antenna beamsize. This mosaic
pattern was observed twice per EB, except for one which termi-
nated after only 20 pointings of the first mosaic pass. However, no
problems were found with these data and they were included in the
final map. The amplitude and bandpass calibrator for each EB was
the unresolved quasar J0334−401, this also serving as the phase
calibrator during the second observing season. Although relatively
far (12.
◦
4) from the HUDF, the phase solutions varied smoothly
over the course of each EB, and maps made from these data demon-
strated that phase referencing had indeed been successful. In the
first season, the phase calibrator was J0348−2749 that is only 3.
◦
5
from the target. The array configuration varied greatly during the
observations, with the first season generally using baselines twice
as long as required to achieve the requested angular resolution. A
summary of the observations is given in Table 1.
All data reduction was carried out using
CASA and followed stan-
dard procedures. First, the data from the second season needed to
be corrected for incorrect antenna positions that had been used dur-
ing correlation. Other a priori calibrations included application of
system temperature tables and water vapour radiometer phase cor-
rections. The latter were particularly large and time variable for the
second season, presumably as a result of observing during the day.
Very little data needed to be flagged, a notable exception being the
outer four channels of each spectral window which have very poor
sensitivity. After the removal of the frequency response of each
antenna using the bandpass calibrator, amplitude and phase correc-
tions were calculated as a function of time for the flux and phase
calibrators. The flux scale was then set with reference to the regu-
Table 1. Summary of the ALMA observations of the HUDF.
The date of each EB is given along with the approximate max-
imum baseline length and the average amount of precipitable
water vapour (PWV).
Observing Maximum PWV
date baseline (m) (mm)
2014 July 18 650 0.43
2014 July 29 820 1.04
2014 August 17 1100 0.94
2014 August 18 1250 1.51
2014 August 18 1250 1.45
2014 August 27 1100 1.35
2014 August 28 1100 1.20
2014 August 28 1100 1.25
2014 September 1 1100 1.08
2015 May 16 550 0.65
2015 May 16 550 0.80
2015 May 17 550 1.00
2015 May 17 550 1.80
larly monitored flux density of J0334−401 and the gain solutions
interpolated on to the HUDF scans.
A continuum mosaicked image of the calibrated data was pro-
duced using the task
CLEAN. To enhance mapping speed, the data
were first averaged in both frequency and time to produce a data
set with 10 frequency channels per spectral window and a time
sampling of 10 s. The data were naturally weighted for maxi-
mum sensitivity, but the relatively large array configurations still
produced a s ynthesized beam (589 × 503 mas
2
) that was signif-
icantly smaller than the circular 0.7-arcsec beam that had been
requested. As this would potentially lead to problems with detect-
ing resolved sources, we experimented with various u, v tapers in
order to find the best combination of angular resolution and mo-
saic sensitivity. A 220 × 180 kλ taper, with position angle (PA)
oriented to circularize the beam as much as possible, produced a
beam close to that requested (707 × 672 mas
2
) and a final mo-
saic sensitivity as measured over a large central area of the map of
34 µJy beam
−1
. As the detected source flux densities were very
weak, and the synthesized beam sidelobes very low, no deconvo-
lution (cleaning) was performed. The resulting image is shown in
Fig. 1. Finally, to aid checks on data quality, and source reality,
we also constructed three alternative 50:50 splits of the ALMA 1.3-
mm image, splitting the data in half by observing date, sideband and
polarization.
2.2 Supporting multifrequency data
2.2.1 Optical/near-infrared imaging
The key data set that defined the area that we aimed to cover
with the ALMA 1.3-mm mosaic is the ultradeep near-infrared
imaging of the HUDF obtained with WFC3/IR on HST via the
UDF09 (e.g. Bouwens et al. 2010; Bunker et al. 2010; Finkelstein
et al. 2010, 2012;McLureetal.2010; Oesch et al. 2010) and UDF12
(e.g. Dunlop et al. 2013; Ellis et al. 2013; Illingworth et al. 2013;
McLure et al. 2013; Finkelstein et al. 2015) programmes. As de-
scribed in Koekemoer et al. (2013), the final UDF12 WFC3/IR
imaging reaches a 5σ detection depth of 29.7 mag in the Y
105
filter,
and 29.2 mag in the J
125
, J
140
and H
160
filters (total magnitudes, as
derived from small-aperture magnitudes assuming point-source cor-
rections). These unparalleled near-infrared data, covering an area
4.5 arcmin
2
, are complemented by what remains the deepest ever
MNRAS 466, 861–883 (2017)
864 J. S. Dunlop et al.
Figure 1. The ALMA 1.3-mm map of the HUDF, with the positions of the 16 sources listed in Table 2 marked by 3.6-arcsec diameter circles. The border of
the homogenously deep region of near-infrared WFC3/IR imaging obtained through the UDF09 and UDF12 HST programmes is indicated by the dark-blue
rectangle. The ALMA image, constructed from a mosaic of 45 individual pointings, provides homogeneous 1.3-mm coverage of this region, with a typical
noise per beam of σ
1.3
35 µJy.
optical imaging obtained with ACS on HST (Beckwith et al. 2006).
This provides imaging in the B
435
, v
606
, i
775
and z
850
filters, reaching
5σ detection depths of 29.7, 30.2, 29.9 and 29.8 mag, respectively.
More recently, the CANDELS programme (Grogin et al. 2011)
has provided deep i
814
data across the HUDF (reaching 29.8 mag,
5σ ) as part of the CANDELS-DEEP imaging of GOODS-South
(Koekemoer et al. 2011; see also Guo et al. 2013).
The core HST imaging data set is extended to shorter wavelengths
by the inclusion of deep VLT VIMOS imaging in the U-band (reach-
ing28mag,5σ ; Nonino et al. 2009), and to longer wavelengths
by the deepest ever K
s
-band imaging obtained with HAWK-I on
the VLT through the HUGS survey (Fontana et al. 2014), which
reaches K
s
= 26.5 mag (5σ ). Imaging longward of 2.2 µmhas
been obtained with Spitzer, with new ultradeep IRAC imaging of
the HUDF at 3.6 and 4.5 µm being provided by our own stack of
the available public data described by Labb
´
e et al. (2015) (see also
Ashby et al. 2013, 2015). This reaches deconfused 5σ detection
depths of 26.5 mag at 3.6 µmand26.3 mag at 4.5 µm.
Galaxy detection and photometry in the deep HST imaging
data set was undertaken using SE
XTRACTOR v2.8.6 (Bertin &
Arnouts 1996) in dual image mode with H
160
as the detection im-
age, and the HST photometry homogenized through appropriately
scaled apertures at shorter wavelengths. The ground-based (U and
K
s
)andSpitzer photometry was extracted by deconfusing the data
using HST positional priors both using the method described in
McLure et al. (2011, 2013), and independently using
TPHOT (Merlin
et al. 2015).
The resulting optical–near-infrared catalogue contains 2900
objects with 12-band photometry (see for example Parsa et al. 2016).
2.2.2 Mid/far-infrared/sub-mm imaging
Longward of 4.5 µm, the original GOODS Spitzer imaging (PID
104; PI Dickinson) provides the deepest available data at 5.6, 8.0
(from IRAC: Fazio et al. 2004) and 24 µm (from M IPS). The
24 µm imaging has been augmented and incorporated within the
Spitzer Far-Infrared Deep Extragalactic Legacy
2
survey (Magnelli
et al. 2009) and reaches a 5σ detection limit of S
24
30 µJy.
Data at longer far-infrared wavelengths are provided by Herschel
(Pilbratt et al. 2010), and we utilize here the final public image prod-
ucts from three major guaranteed-time surveys. PACS (Poglitsch
et al. 2010) images at 100 and 160 µm, reaching rms depths of
0.17 and 0.42 mJy respectively are provided by a combination of
the data obtained through the GOODS-Herschel (Elbaz et al. 2011)
and the PACS Evolutionary Probe (Lutz et al. 2011) surveys, while
SPIRE (Griffin et al. 2010) images at 250, 350 and 500 µm, reaching
5.86, 6.34 and 6.88 mJy respectively (including confusion noise) are
provided by the Herschel Multitiered Extragalactic Survey (Oliver
et al. 2010, 2012).
Because the Herschel (and especially the SPIRE) imaging has
such low angular resolution compared to the ALMA imaging, care
2
PI M. Dickinson, see http://www.noao.edu/noao/fidel/
MNRAS 466, 861–883 (2017)
