An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: Spectroscopic redshifts
Alice Danielson, Mark Swinbank, Ian Smail, James Simpson, Catlin Casey, Scott Chapman, Elisabete Da Cunha, Jackie Hodge, Fabian Walter, Julie Wardlow, Dave Alexander, Niel Brandt, Carlos de Breuck, Kristen Coppin, Helmut Dannerbauer, Mark Dickinson, Alastair Edge, Eric Gawiser, Rob Ivison, Alex Karim, Attila Kovacs, Dieter Lutz, Karl Menten, Eva Schinnerer, Axel Weiss, Paul van der Werf
aa r X i v : . [ a s t r o - ph . GA ] M a y Draft version May 11, 2017
Preprint typeset using L A TEX style emulateapj v. 12/16/11
AN ALMA SURVEY OF SUBMILLIMETRE GALAXIES IN THE EXTENDED
CHANDRA
DEEP FIELDSOUTH: SPECTROSCOPIC REDSHIFTS
A. L. R. Danielson, A. M. Swinbank,
Ian Smail,
J. M. Simpson, C. M. Casey,
S. C. Chapman, E. da Cunha, J. A. Hodge, F. Walter, J. L. Wardlow, D. M. Alexander, W. N. Brandt, C. de Breuck, K. E. K. Coppin H. Dannerbauer, M. Dickinson, A. C. Edge, E. Gawiser, R. J. Ivison, A. Karim, A. Kovacs, D. Lutz, K. Menten, E. Schinnerer, A. Weiß, P. van der Werf, Draft version May 11, 2017
ABSTRACTWe present spectroscopic redshifts of S µ m > ∼ µ m detected sources in the Extended Chandra
Deep Field South (the ALMA-LESS survey). We derive spectroscopic redshifts for 52 SMGs,with a median of z = 2.4 ± ∼ z ≥
3. Spectral diagnostics suggest that the SMGs are young starbursts, and thevelocity offsets between the nebular emission and UV ISM absorption lines suggest that many aredriving winds, with velocity offsets up to 2000 km s − . Using the spectroscopic redshifts and theextensive UV-to-radio photometry in this field, we produce optimised spectral energy distributions(SEDs) using Magphys , and use the SEDs to infer a median stellar mass of M ⋆ = (6 ± × M ⊙ for our SMGs with spectroscopic redshift. By combining these stellar masses with the star-formationrates (measured from the far-infrared SEDs), we show that SMGs (on average) lie a factor ∼ z ∼
2. We provide this library of 52 template fits with robust anduniquely well-sampled SEDs available as a resource for future studies of SMGs, and also release thespectroscopic catalog of ∼ Subject headings: galaxies: starburst, submillimetre: galaxies INTRODUCTION Centre for Extragalactic Astronomy, Durham University,Department of Physics, South Road, Durham DH1 3LE, UK Institute for Computational Cosmology, Durham University,South Road, Durham DH1 3LE, UK * Email: [email protected] Department of Astronomy, The University of Texas atAustin, 2515 Speedway Boulevard Stop C1400, Austin, TX78712, USA Department of Physics and Astronomy, University ofCalifornia, Irvine, Irvine, CA 92697, USA Department of Physics and Atmospheric Science, DalhousieUniversity, Halifax, NS B3H 4R2, Canada Research School of Astronomy and Astrophysics, AustralianNational University, Canberra, ACT 2611, Australia Leiden Observatory, Leiden University, P.O. Box 9513, 2300RA Leiden, The Netherlands 0000-0001-5434-5942 Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany Department of Astronomy and Astrophysics and the Insti-tute for Gravitation and the Cosmos, The Pennsylvania StateUniversity European Southern Observatory, Karl Schwarzschild Straße2, 85748, Garching, Germany Centre for Astrophysics Research, Science and TechnologyResearch Institute, University of Hertfordshire, College Lane,Hatfield AL10 9AB, UK Universit¨at Wien, Institut f¨ur Astrophysik,T¨urkenschanzstraße 17, 1180, Wien, Austria National Optical Astronomy Observatory, Tucson, AZ85719, USA Department of Physics and Astronomy, Rutgers University,Piscataway, NJ 08854-8019, USA Astronomy Department, University of Minnesota, MN12345, USA Max-Planck-Institut f¨ur extraterrestrische Physik, Giessen-bachstraße, 85748, Garching, Germany Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel69, D-53121 Bonn, Germany
Submillimeter galaxies (SMGs) with 850 µ m fluxesof S > ∼
10 Gyr ago. Al-though they are relatively rare, their far-infrared lumi-nosities (L IR > × L ⊙ ) imply high star-formationrates ( > ∼
300 M ⊙ yr − ) and so SMGs appear to con-tribute at least 20% of the total cosmic star-formationrate density over z = 1–4 (e.g. Chapman et al. 2005;Barger et al. 2012; Casey et al. 2014; Swinbank et al.2014). If they can maintain their star-formation rates,SMGs also have the potential to consume all their coldgas reservoir within just 100 Myr (e.g. Tacconi et al.2008; Bothwell et al. 2013), and so double their stel-lar masses within their short but intense lifetime (e.g.Hainline et al. 2009; Magnelli et al. 2012). Their abil-ity to form up to 10 M ⊙ of stars within a shortperiod of time makes them candidates of progenitorsof z = 1–2 compact quiescent galaxies (Toft et al. 2014;Simpson et al. 2015a; Ikarashi et al. 2015) as well as lo-cal massive ellipticals (e.g. Lilly et al. 1999; Genzel et al.2003; Simpson et al. 2014). These characteristics sug-gest that bright SMGs represent an essential popula-tion for models of galaxy formation and evolution (e.g.Efstathiou & Rowan-Robinson 2003; Baugh et al. 2005;Swinbank et al. 2008; Narayanan et al. 2009; Dav´e et al.2010; Hayward et al. 2011; Lacey et al. 2015).However, to identify the physical processes that trig-ger the starbursts, measure the internal dynamics of thecold (molecular) and ionised gas, and infer stellar massesfirst requires accurate redshifts. To date, the largestsuch spectroscopic survey of 870 µ m-selected submillime-tre sources was carried out by Chapman et al. (2005) A spectroscopic redshift survey of ALMA-identified submillimetre galaxieswho targeted a sample of 104 radio-identified, SCUBA-detected submillimetre sources spread across seven extra-galactic survey fields. Using rest-frame UV spectroscopywith the Low-resolution Imaging Spectrograph (LRIS)on the Keck telescope, they derived spectroscopic red-shifts for 73 submillimetre sources with a median redshiftof z ∼ z = 3.6).Although the requirement for a radio detection in theseprevious surveys was a necessary step to identify the mostprobable galaxy counterpart responsible for the submil-limetre emission, the radio wavelengths do not bene-fit from the same negative K-correction as longer sub-millimetre wavelengths and indeed, above z ∼ ∼
100 M ⊙ yr − falls below ∼ µ Jy and so below thetypical sensitivity limit of deep radio surveys. Thishas the potential to bias the redshift distribution to z < ∼ µ m surveys, up to 50%of all submillimetre sources are undetected at radio wave-lengths (e.g. Ivison et al. 2005, 2007; Biggs et al. 2011).Some progress can be made by targeting lensed sourceswhose multi-wavelength identifications are less ambigu-ous, and indeed spectroscopic redshifts have been derivedfor SMGs up to z ∼ < ∼ ′′ accuracy without recourse to statistical asssociations atother wavelengths. To identify a sample of SMGs in awell studied field with a well defined selection function,Hodge et al. (2013) undertook an ALMA survey of 122SMGs found in the Extended Chandra
Deep Field South(ECDFS): the “ALESS” survey. This survey followedup 122 of the 126 submillimetre sources originally de-tected with the LABOCA instrument on the AtacamaPathfinder Experiment 12 metre telescope (APEX); theLABOCA ECDFS Sub-mm Survey (LESS) (Weiß et al.2009). Each LESS submillimetre source was targetedwith ALMA at 870 µ m (Band 7). The typical FWHMof the ALMA synthesised beam was ∼ ′′ (significantlysmaller than the LABOCA 19.2 ′′ beam), thus allowingus to directly pinpoint the position of the SMG precisely.From these data, Karim et al. (2013) (see alsoSimpson et al. 2015b) showed that statistical identifi-cations (e.g. using radio counterparts) were incorrectin ∼
30% of cases, whilst the single-dish submillimetresources also suffer from significant “multiplicity”, with >
35% of the single-dish sources resolved into multi-ple SMGs brighter than > ∼ L FIR > ∼ L ⊙ at z ∼
2, and so it appears that a largefraction of the single-dish submillimetre sources oftencontain two (or more) Ultra-Luminous Infrared Galaxies(ULIRGs). Consequently, a new ALESS SMG cataloguewas defined comprising 131 SMGs (Hodge et al. 2013).One of the primary goals of the ALESS survey is toprovide an unbiased catalog of SMGs for which we canderive molecular gas masses, as well as measure spatiallyresolved dynamics of the gas and stars in order to iden-tify the trigering mechanisms that cause the burst of starformation. The first necessary step in this process is to derive the precise spectroscopic redshifts. To this end,we have undertaken a spectroscopic survey of ALMA-identified SMGs using VLT, Keck and Gemini (supple-mented by ALMA) and in this paper we describe theUV, optical and near-infrared spectroscopic follow-up.We use the resulting redshifts to investigate the redshiftdistribution, the environments and typical spectral fea-tures of these SMGs. In addition, we use these preciseredshifts to better constrain the SED fitting from UV-to-radio wavelengths and provide template SEDs for theALESS SMG population.The structure of the paper is as follows. We discussthe observations and the data reduction in §
2, followedby redshift identification and sample properties in §
3. In § § §
6. In the Appendix, we give the ta-ble of ALESS SMG redshifts and provide information onindividual SMGs from the sample.Unless otherwise stated the quoted errors on the me-dian values within this work are determined throughbootstrap analysis and are quoted as the equivalentof 68.3% confidence limits. Throughout the paper weuse a ΛCDM cosmology with H = 72 km s − Mpc − ,Ω m = 0.27 and Ω Λ = 1 - Ω m (Spergel et al. 2003) and aChabrier initial mass function (IMF; Chabrier 2003).Unless otherwise noted, all magnitudes are on the ABsystem. OBSERVATIONS AND REDUCTION
Sample definition
The 870 µ m LESS survey (Weiß et al. 2009) was under-taken using the LABOCA camera on APEX, covering anarea of 0.5 × σ sensitivity of σ µ m ∼ − with a beamof 19.2 ′′ FWHM. In total, we identified 126 submillimetresources above a signal-to-noise of 3.7 σ . Follow-up obser-vations of the LESS sources were carried out with ALMA(the ALMA-LESS, ALESS programme). Details of theALMA observations are described in Hodge et al. (2013)but in summary, the 120 s observations for each sourcewere taken between October and November 2011 inthe Cycle 0 Project main ) catalogue ofthe 99 of the most reliable ALMA-identified SMGs (i.e.lying within the the primary beam FWHM of the best-quality maps). A supplementary (hereafter supp ) cat-alogue was also defined comprising 32 ALMA-identifiedSMGs extracted from outside the ALMA primary beam,or in lower quality maps (Hodge et al. 2013). Whensearching for spectroscopic redshifts, we included boththe main and supp sources, and in § supp sources makes very little quan-anielson et al. 3titative difference to the statistics of the redshift distri-bution.To search for spectroscopic redshifts, we initiated anobserving campaign using the the FOcal Reducer andlow dispersion Spectrograph (FORS2) and VIsible Mul-tiObject Spectrograph (VIMOS) on VLT, but to sup-plement these observations, and in particular to in-crease the wavelength coverage and probability of de-termining redshifts, we also obtained observations withXSHOOTER on VLT, the Gemini Near-Infrared Spec-trograph (GNIRS) and the Multi-Object Spectrometerfor Infra-Red Exploration (MOSFIRE) on the Keck i telescope, all of which cover the near-infrared. As partof a spectroscopic campaign targeting Herschel -selectedgalaxies in the ECDFS, ALESS SMGs were includedon DEep Imaging Multi-Object Spectrograph (DEIMOS)slit masks on Keck ii (e.g. Casey et al. 2012). These ob-servations probe a similar wavelength range to FORS2targeting some of the ALMA-identified SMGs that couldnot be targeted with VLT (due to slit collisions). In to-tal, we observed 109 out of the 131 ALESS SMGs in thecombined main and supp samples. In many cases wehave ALESS SMGs with spectra from five different spec-trographs covering a broad wavelength range and we cancross check the spectroscopic redshifts across all of theinstruments. Next, we discuss the various instrumentsinvolved in our survey. We note that for all observationsdescribed below, flux calibration was carried out usingstandard stars to calibrate instrumental response. VLT FORS2 / VIMOS
Our spectroscopic programme aimed to target as manyof the ALESS SMGs as possible using a dual approachwith FORS2 and VIMOS (for a typical SMG redshiftof z ∼ α and UV ISM lineswith VIMOS or [O ii ] λ ≤ ′′ and clearsky conditions (transparency variations below 10%). Ourdual-instrument approach allowed us to probe a largewavelength range using VIMOS LR-Blue grism (4000–6700˚A ) and FORS2 300I (6000–11000 AA ). When de-signing the slit masks, the first priority was always givento the SMGs, but we also infilled the masks with othermid- or far-infrared selected galaxies from the FIDEL Spitzer survey (Magnelli et al. 2009), the HerMES andPEP
Herschel surveys of this field (Oliver et al. 2012;Lutz et al. 2011), S . > µ Jy radio sources and
Chandra
X-ray sources (Lehmer et al. 2005; Luo et al.2008) or optical/near-infrared colour selected galaxies(see Table 3 and Fig. 15).In Fig. 1 we show the spectroscopic coverage of theECDFS from our FORS2 and VIMOS programmes,where the darkest areas demonstrate the areas with thelongest total exposure time and the FORS2 pointingsare overlaid. In total, we recorded 5221 galaxy spectra,
Fig. 1.—
The coverage of our ten VIMOS pointings (greyscale)and 16 FORS2 pointings (blue boxes) in the ECDFS. The ALESSSMG positions are shown as small red circles. VIMOS has fourquadrants separated by small gaps. There is significant overlapbetween the VIMOS pointings, we therefore show the pointingshere with the darkest areas corresponding to the regions with thelongest total exposure time. Our FORS2/VIMOS programme cov-ers 62 out of the 109 targeted SMGs in the ECDFS. targeting 2454 (unique) galaxies.
FORS2
FORS2 covers the the wavelength range λ = 3300–11000˚A and provides an image scale of 0.25 ′′ pix − in thestandard readout mode (2 × ′′ . We used ∼ R = λ / ∆ λ ∼ ∼ ′′ whichcould not be simultaneously observed on a mask. Eachmask was observed in blocks of 3 ×
900 s with each ex-posure nodded up and down the slits by ∼ ′′ to aidsky-subtraction and cosmic-ray removal when the im-ages were combined. Each mask was typically observedsix times (with a range of three to nine times depend-ing on the number of SMGs on the mask and their me-dian brightness), resulting in an on-source exposure time4.5 hrs (with a range of 2.25–6.75 hr).We reduced the data using the spectroscopic reduc-tion package from Kelson (2003) adapted for use withFORS2 data FORS2 pipeline. The pipeline producestwo-dimensional, bias-corrected, flat-fielded, wavelength- A spectroscopic redshift survey of ALMA-identified submillimetre galaxiescalibrated, sky-subtracted images. Individual exposureswere combined in two-dimensions by taking a median ofthe frames and sigma clipping. We then extracted one-dimensional spectra over the full spatial-extent of thecontinuum/emission lines visible, or in the case whereno emission was obvious in the two-dimensional image,we extracted data from the region around the expectedsource position. VIMOS
The VIMOS observations were undertaken in multi-object spectroscopy (MOS) mode. VIMOS consists offour quadrants each of a field-of-view of 7 ′ × ′ witha detector pixel scale of 0.205 ′′ pix − . Each observingblock comprised 3 × ± ′′ along the slit. The exposure time per mask was 3–9 hr,again depending on the number of SMGs on the maskand their average brightness. Slit widths of 1.0 ′′ wereused, for which the typical resolution is R ∼
180 andthe dispersion is 5.3˚A pix − for the LR blue grism withthe OS blue order sorting filter ( ∼ ESOREX pipeline package for VIMOS. Theframes were stacked in two-dimensions before extract-ing the one-dimensional spectra. In a number of cases,the data suffer from overlapping spectra which results ina second order overlapping the adjacent spectrum (thiscan be seen in the VIMOS two-dimensional spectrum ofALESS 057.1 in Fig. 2).
XSHOOTER
To improve the wavelength coverage of our observa-tions, we also obtained XSHOOTER observations of 20ALESS SMGs. XSHOOTER simultaneously observesfrom UV to near-infrared wavelengths covering wave-length ranges of 3000–5600˚A, 5500–10200˚A and 10200–24800˚A for the UV (UVB), visible (VIS) and near-infrared (NIR) arms respectively. Targets were priori-tised for XSHOOTER follow-up based on their K -bandmagnitudes. Our XSHOOTER observations were takenin visitor mode as part of programme 090.A-0927(A)from 2012 December 7–10 in dark time. We observedeach source for ∼ ∼ ′′ . Our observing strategy was4 ×
600 s exposures per source, nodding the source upand down the slit. The pixel scales were 0.16, 0.16 and0.21 ′′ pix − for the UVB, VIS and NIR arms respectively.The slits were all 11 ′′ long and 0.9 ′′ wide for the VIS andNIR arms and 1.0 ′′ wide for the UVB arm. The typicalresolution was R ∼ esorex pipeline package forXSHOOTER. MOSFIRE
We also targeted 36 ALESS SMGs with the MOSFIREspectrograph on Keck i (2012B H251M, 2013B U039M,and 2013B N114M) in H - (1.46–1.81 µ m) and K -band(1.93–2.45 µ m). Observations were taken in clear or pho-tometric conditions with the seeing varying from 0.4–0.9 ′′ . In all cases we used slits of width 0.7 ′′ . The pixel scale of MOSFIRE is 0.18 ′′ pix − and the typical spec-tral resolution for this slit width is R ∼ H -band) and 180 s ( K -band) exposures, withan ABBA sequence and a 1.5 ′′ nod along the slit betweenexposures. Data reduction was completed with mospy . DEIMOS
We targeted 71 of the ALESS SMGs as “mask in-fill” during a Keck ii DEIMOS spectroscopy programmeto measure redshifts for
Herschel / SPIRE sources (pro-gramme 2012B H251). The data were taken on 2012December 9–10 in clear conditions with seeing between1–1.3 ′′ . We used a setup with the 600ZD (600 linesmm − ) grating with a 7200˚A blaze angle and the GG455blocking filter which resulted in a wavelength range of4850–9550˚A. Slit widths of 0.75 ′′ were used and themasks were filled with 40–70 slits per mask. The pixelscale of DEIMOS is 0.1185 ′′ pix − and the typical reso-lution was R ∼ GNIRS
The Gemini Near-Infrared Spectrograph (GNIRS) wasused to target eight ALESS SMGs as (programme GN-2012B-Q-90) between 2012 November 10–15 and De-cember 4–23. The targets were selected based on their K -band magnitude and whether they had a photomet-ric redshift that was predicted to place strong emissionlines in the near-infrared. The instrument was usedin cross-dispersing mode (via the SXD prism with 32lines mm − ), using the short camera, with slit widths of0.3 ′′ , slit lengths of 7 ′′ and a pixel scale of 0.15 ′′ pix − .The wavelength coverage with this setup is 9000–25600˚A,typically with R ∼ ∼ ′′ . Each observing block comprised eight coadds ofthree exposures, resulting in an exposure of ∼ iraf package. ALMA
Spectroscopic redshifts for two of our SMGs,ALESS 61.1 and ALESS 65.1 were determined fromserendipitous detections of the [C ii ] λ µ m line in theALMA band (Swinbank et al. 2012). Although based onsingle line identifications, both redshifts have been con-firmed by the identification of CO(1–0) emission usingATCA (Huynh et al. 2013; Huynh et al. 2017, submit-ted).Once all of the data were collected from the differentspectrographs, we collated the spectra for each ALESSSMG. The instruments used to observe each SMG arelisted in Table 1. ANALYSIS
Redshift identification
To determine redshifts for the sample, the one- andtwo-dimensional spectra (for all ∼ α , C iv λλ iii λ ii λ ii ] λλ α , N ii λ iii ] λλ β (see Tables 2 & 3). The optical / near-infrared counter-parts of the SMGs are often faint and we detect con-tinuum in only ∼
50% of the 52 SMGs for which wedetermine a redshift, (compared to ∼
75% for the radio-identified submillimetre sources in Chapman et al. 2005).The spectra often only contain weak continuum, emis-sion and / or absorption lines, making redshifts difficultto determine robustly. We therefore assign four qualityflags to our spectroscopic data:1. Q = 1 denotes a secure redshift where multi-ple features were identified from bright emis-sion / absorption lines;2. Q = 2 denotes a redshift but derived from one ortwo bright emission (or strong absorption) lines;3. Q = 3 is a tentative redshift based on one (or some-times two tentative) emission or absorption lines.In these cases, we often use the photometric red-shift as a guide to identify the line. These redshiftsare therefore not independent of the photometricredshifts and are thus highlighted in the analysis;4. Q = 4 is assigned to galaxies with no emission linesor continuum detected and so no redshift could bedetermined.Examples of spectra from which Q = 1, 2 & 3 redshiftsare determined are shown in Fig. 2. Since the ECDFShas been the focus of extensive spectroscopic campaigns(although focusing mainly on bright optical/UV-selectedgalaxies) six of our ALMA SMGs have published archivalspectroscopic redshifts, and we highlight these in Table 2. The emission / absorption lines we are using to de-rive redshifts have a range of physical origins within thegalaxies. For example, nebular emission lines arise fromH ii regions and so are expected to trace the systemicredshift, whereas UV ISM lines can trace outflowing ma-terial and so can be offset from the systemic redshiftby several 100 km s − (e.g. Erb et al. 2006; Steidel et al.2010). Ly α emission, which is often used to derive spec-troscopic redshifts, also suffers resonant scattering. Assuch, to derive redshifts for each galaxy we adopt thefollowing approach:1. Wherever possible, systemic redshifts are deter-mined using nebular emission lines such as H α ,[O ii ] λλ iii ] λλ β .If none of these lines are available we use He ii orC iii ] λ Our goal is to provide a quality flag that allows users to gaugethe likely success, (or interpret) follow up observations a source.For example, a non-detection of the CO emission in a Q = 1source should be interpreted as CO faint, whereas a CO non-detection of a Q = 3 source may be due to the faintness of the COemission, or due to a misidentified/spurious redshift.
TABLE 1Summary of spectroscopic features
Condition Number of galaxiesTotal [ supp ]Total 131 [32]Q = 1 20 [1]Q = 2 11 [3]Q = 3 21 [3]Redshifts measured 52 [7]Not observed 22 [10]Observed but no spec z
57 [15]Ly α
23 [1][O ii ] 10 [3][O iii ] 6 [0]H α
14 [3][O iii ] & H α β Notes : The numbers in brackets represents the number of supp
SMGs included in the total in each row.
2. If no nebular emission lines are detected, we deter-mine the mean of the redshifts from the UV ISMabsorption lines of C ii λ iv λ ii λ v λ ii λ ii .3. If Ly α is the only detected line then the redshiftis determined from a fit to this line, although wecaution that the velocity offset from the systemiccan be up to ∼ − . In most of the galax-ies where a redshift is determined solely from Ly α ,the observations were taken with VIMOS using thelow-resolution ( R ∼ iv λ ∼
30% of the redshifts are deter-mined from a single line and generally these redshifts areallocated Q = 3 unless strong continuum features (suchas breaks across Ly α ) are also identified, which leads toan unambiguous identification and a higher quality flag.Single line redshifts are typically backed up by either con-tinuum breaks across Ly α , the absence of other emissionlines that would correspond to a different redshift, lineprofiles (i.e. asymmetric Ly α profile or identifying thedoublet of [O ii ] λ α ; inthree cases they are determined from H α detections innear-infrared spectra and in five cases they are from de-tections of the [O ii ] doublet.We summarise the main spectroscopic features that wedetect in Table 1 and provide detailed information oneach of the 109 observed SMGs in Table 2.In Fig. 3 we compare our precise spectroscopic mea- A spectroscopic redshift survey of ALMA-identified submillimetre galaxies Fig. 2.—
Example one- and two-dimensional spectra of ALESS SMGs from each spectrograph used. The upper three rows are high quality(Q = 1) spectra while the bottom row shows lower quality examples (Q = 2 and 3 spectra) and we mark identified and potential featuresin all panels, where red dashed lines mark typical emission lines and blue dashed lines mark typical absorption lines. In ALESS 057.1 (anX-ray AGN) the bright continuum below the central strong emission line and continuum is contamination from higher order emission froman adjacent slit on the VIMOS mask. ALESS 037.2 is an example of a Q = 3 redshift where the redshift is determined from narrow H α ,although the apparent ratio of S ii / H α is unusually high. anielson et al. 7 Fig. 3.—
A comparison of our spectroscopic redshifts forALESS SMGs with their estimated photometric redshifts fromSimpson et al. (2014). Overall, the photometric redshifts agreewell with our spectroscopic redshifts with a median ∆ z/ (1 + z spec ) = 0.00 ± H -band mag-nitude distribution comparable to that of a complete sample of z ∼ z ∼ z ∼ surements for the ALESS SMGs to the photometricredshift estimates for these SMGs from Simpson et al.(2014) who determine photometric redshifts for 77 ofthe ALESS SMGs which have 4–19 band photome-try. We flag those sources with spectroscopic red-shifts, but poor photometric coverage and we also high-light the spectroscopic Q = 3 redshifts since their spec-troscopic identification is often guided by the photo-metric redshifts. Nevertheless, even if these Q = 3SMGs are omitted, there is good agreement between thephotometric and spectroscopic redshifts with a median∆ z/ (1 + z spec ) = 0.00 ± σ = 0.1.In four cases, there appear to be significant outliers, with | ∆ z/ (1 + z spec )) | > < ′′ )from the ALMA position and an IRAC source co-incident with the ALMA position. HST imaging(Chen et al. 2015) reveals two galaxies and it ispossible that the blue source is a lens, as confirmedby high-resolution, ∼ ′′ ALMA band 7 follow-upobservations; (Hodge et al. 2016). 3. ALESS 037.2: the Q = 3 spectroscopic redshift issignificantly lower than the z > α and [S ii ] (see Fig. 2; [N ii ],if present would lie under strong sky lines) andthe photometric redshift is poorly constrained andbased on detections in six bands and limits in afurther six. Furthermore, the spectroscopic lineidentifications would not correspond to any com-mon emission lines if the photometric redshift iscorrect.4. ALESS 101.1: this has a Q = 2 redshift based ona single detection of Ly α . It has poor constraintson the photometric redshift with photometric de-tections in only five bands and no detections below J -band. Thus the spectroscopic redshift is signifi-cantly more reliable.For a significant fraction of the ALMA sample targetedin our survey, we were unable to derive a spectroscopicredshift (these are assigned Q = 4 in Table 2). To under-stand whether this is caused by magnitude limits or theirredshifts, we first compare the photometric redshifts ofthe spectroscopic failures to those for the SMGs for whichwe were able to determine a spectroscopic redshift. Themedian photometric redshift of spectroscopic failures is z = 2.4 ± z = 2.4 ± much higherredshifts than those SMGs where we have succeeded inobtaining a redshift. Similarly there does not appearto be any correlation with submillimetre flux: for the52 SMGs with spectroscopic redshifts, the median 870- µ m flux is S µ m = 4 . +0 . − . mJy, whereas those 57 SMGswhere we could not determine a redshift have a median S µ m = 4 . +0 . − . mJy.Next, we test the hypothesis that we were unable tomeasure spectroscopic redshifts for some ALMA SMGssimply due to their faint optical magnitudes. In Fig. 4we show the distributions of the S µ m flux density, R -band and 4.5 µ m magnitudes and 1.4 GHz flux den-sity for the 109 (out of 131) ALESS SMGs that werespectroscopically targeted. The median R -band magni-tude of the ALESS SMGs with spectroscopic redshifts is R = 24.0 ± ∼ R = 25.0 ± µ m is m . µ m = 20.9 ± m . µ m = 21.7 ± R and m . µ m than those for which we wereable to measure a spectroscopic redshift (and also mayhave slightly redder R − m . colours).In Fig. 5 we plot the redshifts of the ALESS SMGs A spectroscopic redshift survey of ALMA-identified submillimetre galaxies Fig. 4.—
Fundamental observable properties of our spectroscopic sample of SMGs, comprising 870 µ m fluxes, R -band and 4.5 mu mmagnitudes and 1.4 GHz fluxes. The distributions are compared to those of the parent population of ALESS SMGs (where the parentsample comprises the 109/131 SMGs that were targeted in our spectroscopic survey). In all panels we show three distributions: for thefull sample (with and without spectroscopic redshifts); the properties of the SMGs with Q = 1, 2 or 3 spectroscopic redshifts and thedistribution for SMGs with photometry but no spectroscopic redshift. As separate boxes we also indicate the proportion of the full andspectroscopic samples which are below the detection limit of the observations in each waveband (these 3 σ detection limits are indicated bydotted lines in each panel). On average, we find that the SMGs for which we were able to determine a redshift are marginally brighter inthe R -band, and m . µ m than those for which we were unable to determine a redshift, however, the likelihood of determining a redshiftis independent of the 870 µ m flux density and so our survey is unbiased in this regard. In addition in the R -band and 1.4 GHz panels wealso show the equivalent distribution for the spectroscopic sample of 73 radio-identified submillimetre sources from Chapman et al. (2005),which exhibit comparable properties to our sample. Note that ALESS 020.1 has a very bright radio flux of ∼ versus their 4.5 µ m apparent magnitudes. At the typicalredshift of SMGs ( z ∼ µ m emission providesthe most reliable tracer of the underlying stellar mass,since it corresponds to rest-frame ∼ µ m ( H -band).As a guide, to crudely test how the 4.5 µ m magnitudedepend on redshift in our sample, we generate a non-evolving starburst track, based on the composite SED forthe ALESS SMGs (shown in Simpson et al. 2014 but up-dated to contain the spectroscopic redshift informationin Fig. 9). This model SED has been normalised to themedian apparent 4.5 µ m magnitude for the spectroscopicand photometric redshift samples at the median redshiftof z ∼ µ m flux with redshiftfor our spectroscopic sample is consistent with this track,although with a spread of ∼ µ m flux with increasing redshift. Smail et al. (2004)(see also Serjeant et al. 2003) also identified a similarlylarge spread in K -band magnitudes for SMGs.Hence we see both a spread in the apparent rest-framenear-infrared luminosities within the SMG population,as well as the fainter optical apparent magnitudes (andredder colours) for those SMGs which we failed to obtainredshifts for and marginally higher photometric redshiftscompared to those for which spectroscopic redshifts weremeasured. Each of these trends are weak, but they dosuggest several factors may be driving the spectroscopicincompleteness: a range in stellar masses for SMGs ata fixed redshift (a demonstration of the diversity of theSMG population), varying levels of strong dust extinc-tion and fainter apparent optical fluxes for SMGs athigher redshifts (due to the K correction and increasingdistance).In terms of the radio-detected sub-sample, from theentire main+supp ALESS catalogue, 53 / 131 ALESSSMGs are radio-detected, and we have targeted 52with spectroscopy, measuring redshifts for 34. Themedian 1.4 GHz flux density of the SMGs with spec- troscopic redshifts is S . = 63 +12 − µ Jy compared to S . = 39 +6 − µ Jy for those without spectroscopic red-shifts (Fig. 4). Thus, SMGs for which we were unableto determine a spectroscopic redshift are fainter at radiowavelengths than those for which we measured a spec-troscopic redshift. SPECTROSCOPIC REDSHIFT DISTRIBUTION
The spectroscopic redshift distribution of the ALESSSMGs is shown in Fig. 6. In total 52 redshifts have beendetermined for the ALESS SMGs: 45 main catalogueSMGs and seven supp catalog SMGs. We also overlaythe probability density function of the photometric red-shift distribution of ALESS SMGs from Simpson et al.(2014), scaled to the same number of sources. The Q = 1& 2 and Q = 1, 2 & 3 distributions are shown as individ-ual histograms to test the effect of including the Q = 3redshifts. The full redshift distribution ranges between z = 0.7–5.0, with a significant (but not dominant) tail at z ≥ Hub-ble
Ultra Deep Field (UDF) by ASPECS (Aravena et al.2016; Walter et al. 2016) and Dunlop et al. (2016) Giventhe different selection wavelengths, flux limits and samplesizes between the ALESS SMGs and the ALMA / UDFgalaxies, we caution against drawing strong conclusionsabout the differences between these redshift distributions(for a detailed discussion see B´ethermin et al. 2015).Nevertheless, we note that all of these distributions peakat z ∼ ± supp SMGsand those with only Q = 3 redshifts. Karim et al. (2013)demonstrate that up to ∼
30% of the supp sources arelikely to be spurious. However, supp sources which havean optical / near-infrared counterpart have a lower likli-anielson et al. 9hood of being spurious sources. The median redshift ofthe main catalogue SMGs with Q = 1, 2 & 3 redshifts is z = 2.5 ± z = 2.1–3.4,whereas the median redshift of the main + supp cataloguewith Q = 1, 2 & 3 redshifts is z = 2.4 ± z = 2.1–3.0. The median redshiftof the Q = 1, 2 & 3 SMGs in the supp sample aloneis z = 2.3 ± main and supp samples suggests only a 60% likelihood that theyare drawn from different populations. Since the statisticsof the samples do not vary strongly with the inclusion ofthe supp sources, we are therefore confident that includ-ing the supp sources in our analyses is unlikely to biasany of our results.Since most previous SMG redshift surveys have, bynecessity, relied on radio detections to identify probab-listically the likely counterparts, we briefly discuss theproperties of the radio-detected subset of the ALESSSMGs, as this provides a reasonable comparison to pre-vious work. In our sample we targeted 52 of the 53radio-detected SMGs with spectroscopy and measuredredshifts for 34 of them (65%). The median 1.4 GHz ra-dio flux density of the 34 radio-detected ALESS SMGswith spectroscopic redshifts is 63 +12 − µ Jy, as compared to50 +6 − µ Jy for all
52 radio-detected SMGs. In contrast, themedian radio flux density of the 73 radio-detected sub-millimetre sources in Chapman et al. (2005) with spec-troscopic redshifts is 75 +8 − µ Jy. On average, the radio-detected ALESS SMGs with redshifts are ∼
20% fainterat 1.4 GHz than the Chapman et al. (2005) sample andour spectroscopic completeness is ∼
10% lower. We notethat it appears that the Chapman et al. (2005) radio-identified submillimetre sources have a higher AGN frac-tion than our ALESS sample, and indeed up to ∼ ∼ +16 − % for the ALESS SMGs. Typically AGN spec-tra have stronger, more easily identifiable emission fea-tures and thus our ∼
10% lower spectroscopic complete-ness may be due to a lower AGN fraction. DISCUSSION
Although the primary aim of this work is to deter-mine the redshifts of unambiguously identified SMGs tosupport further detailed follow-up (e.g. CO or H α dy-namics, Huynh et al. e.g. 2013), there is also a wealthof information contained within the spectra themselvesconcerning the dynamics, chemical composition, and en-ergetics of these SMGs. Furthermore, the redshifts canbe used as constraints in SED models (e.g. constrain-ing the star-formation history and so the stellar masses)and to investigate the environments in which these SMGreside. Spectral diagnostics
Stacked spectral properties
Stacked spectra are a useful tool to detect weak fea-tures that are not visible in individual spectra and also
Fig. 5.—
A plot showing the distribution of 4.5 µ m apparentmagnitude versus redshift for ALESS SMGs. We see a tendencyfor more distant SMGs to have fainter 4.5 µ m magnitudes and toassess this we plot a line showing the expected variation with red-shift for a galaxy with a fixed, non-evolving luminosity, assumingthe composite ALESS SED from Simpson et al. (2014) (see alsoFig. 9). This track is normalised to the median apparent magnitudein 4.5 µ m at a median redshift of z = 2.4. The data roughly followthis trend, although they exhibit at least an order of magnitudevariation in 4.5 µ m magnitude at a fixed redshift. Those SMGswhich are found to be physically associated (pairs or triples) withother SMGs are highlighted. Those in associations have a marginaltendency to be among the brighter SMGs (and therefore could po-tentially be more massive; see § ± σ ranges given in Simpson et al. (2014) and Table 2. The two ex-treme outliers are identified with their ALESS ID. for determining the average properties of the popula-tion. We therefore produce composite spectra over twodifferent wavelength ranges, one covering Ly α and UVISM lines and one around the [O ii ] λ best redshift in Table 2.Where the sky subtraction leaves significant residuals,the region within ± > α can besignificantly offset in velocity from this systemic redshift(see Fig. 12). In the composite spectrum these spectralfeatures may therefore appear broadened and offset.We first discuss the composite spectra of the regionaround Ly α , 1000–2000˚A, see Fig. 8. We show a compos-ite constructed from just the Q = 1 and 2 spectra whichdisplays strong Ly α and a continuum break at ∼ ii absorption lines andapparently offset Si iv absorption, as well as potentially0 A spectroscopic redshift survey of ALMA-identified submillimetre galaxies Fig. 6.—
The spectroscopic redshift distribution of the SMGs from our survey. Those SMGs with secure redshifts (Q = 1 & 2) are shown,as well as the distribution for all Q = 1, 2 & 3 redshifts. We compare the distribution to the probability density function of the photometricredshifts from Simpson et al. (2014) normalised to the same total number of sources. We also compare to the redshift distribution ofradio-identified submillimetre sources from Chapman et al. (C05 2005). We see very striking differences between the ALESS SMG redshiftdistribution and that for Chapman et al. (2005), both at low and high redshifts, z < ∼ z > ∼ ∼
23% of the SMGs at z > z < ∼
1, raising the possibility that some of the low-redshift radio counterparts to submillimetre sources claimed by Chapman et al.(2005) could be misidentifications. The bin size is ∆ z = 0.2 and the grey shaded box indicates the incompleteness in the Q = 1, 2 & 3sample compared to the parent population of targeted SMGs in the field. weak C iv absorption and emission and O i absorption.If the feature identified as Si iv is real, then it and theweaker C iv features, both of which show blueshifts, maybe indicative of strong stellar winds. To illustrate thetypical strength of the absorption features we also over-lay the composite spectrum of ∼
200 Lyman break galax-ies (LBGs) from Shapley et al. (2003) (the LBG compos-ite shown here corresponds to the quartile of 200 LBGsfrom the Shapley et al. (2003) sample that has the closestmatch in Ly α equivalent width to our ALESS sample).We note that due to the different wavelength ranges ofthe different instruments used and the fact that we de-redshift and stack in the rest-frame, not all the spectrain our stack contribute to the full wavelength range.We also construct a composite from the Q = 3 spec-tra and plot this in Fig. 8. The purpose of this is totest the reliability of the redshifts derived for the Q = 3spectra by searching for weak spectral features which areundetected in the individual spectra, but become visiblein the stacked spectrum due to the improved signal tonoise. In addition to an emission line identified as Ly α (which is frequently the feature used to derive the red-shift for these sources), we see only a potential emission feature which would correspond to C iii ] λ iii ] λ ii ], and potentially also H δ absorption(Fig. 9). In addition, we see in this composite that con-tinuum falls off bluewards of ∼ ∼ >
100 Myr, or in post-starburststellar populations, 0.3–1 Gyr after the strongest star for-mation has ended (Shapley 2011). In the composite, theposition discontinuity is more consistent with the Balmerbreak than a 4000˚A break, as the continuum at 3500–anielson et al. 11
Fig. 7.—
The spectroscopic redshift distribution of the SMGs inour 870 µ m survey compared to that for two faint 1.1-mm selectedsamples in the UDF from Aravena et al. (2016) and Dunlop et al.(2016) (we note that the total number of sources for the distribu-tions shown are not the same). These SMG samples have quitedifferent selection functions and levels of incompleteness and so wedo not draw any strong conclusions from the apparent differencesbetween them, beyond noting that both distributions peak at rel-atively high redshifts, z ∼ z ∼ µ m sample showing a more significanthigh-redshift tail beyond z ∼ . ± . × lower than it is at 3900–4000˚A.To try to place limits on the age of the visible stel-lar populations within the ALESS SMGs, we use theSED templates from Bruzual & Charlot (2003) to pre-dict the spectra expected from a starburst of 100 Myrduration observed at ages of 10 Myr, 100 Myr and 1 Gyr(post-starburst). We redden the model spectra using thereddening law from Calzetti et al. (2000) adopting themedian extinction of A V = 2 for the ALESS SMGs, as de-rived from SED fitting (see § H -band lu-minosity; see § hyper-z fitusing a constant star-formation history, which indicates(as expected) a heavily dust reddened spectrum of theseSMGs. Our best-fit constant star-formation model showsa slightly bluer continuum than that derived using thephotometric redshift sample by Simpson et al. (2014), il-lustrating a modest bias to bluer restframe UV continuuain those SMGs for which we can measure spectroscopic redshifts for. Nevertheless, our spectroscopic compositeSED still display a very red continuum shape and a clearbreak at ∼ UV-to-radio SEDs
Using our sample of spectroscopically confirmed SMGswith extensive UV-to-radio photometry, we employ the magphys
SED fitting code from (see da Cunha et al.2015) to fit the UV-to-radio emission on a galaxy-by-galaxy basis to estimate the dust reddening, far-infraredluminosity and infer the stellar mass for each SMG. Es-timates of these parameters have been made using pho-tometric redshifts, but the addition of spectroscopic red-shifts removes some of the degeneracies between photo-metric redshift, reddening and star-formation histories,to allow more precise estimates to be made. The UV–mid-infrared photometry for the ALESS SMGs is givenin Simpson et al. (2014), whilst the (deblended)
Her-schel / SPIRE+PACS, ALMA and radio photometry aregiven in Swinbank et al. (2014) (see also da Cunha et al.2015). For each SMG, we use magphys to fit the pho-tometry at the spectroscopic redshift, and we show thebest-fit SEDs (normalised by their 8–1000 µ m luminosi-ties) in Fig. 10 . These normalised, rest-frame SEDsdemonstrate a large range in the UV- to optical-fluxdensity which is driven by a large spread in the dustattenuation. Indeed, the estimated extinction variesfrom A V ∼ V = 1.9 ± L FIR =(3.2 ± × L ⊙ , both of which are consis-tent with previous estimates (for the same sample) de-rived using photometric redshifts (A V = 1.7 ± L FIR = (3.5 ± × L ⊙ respectively from Simpsonet al. (2014)). In addition, magphys also returns es-timates of the stellar masses (solving for the star for-mation histories and ages) and we derive a median stel-lar mass for our 52 SMGs with spectroscopic redshiftsof M ⋆ = (6 ± × M ⊙ , in agreement with previousestimates for this sample using photometric redshiftsand simple assumptions about the star formation his-tories by Simpson et al. (2014), see also da Cunha et al.(2015). This is also consistent with the stellar masses es-timates for the radio-identified submillimetre sources inthe Chapman et al. (2005) sample ( M ⋆ ∼ × M ⊙ ;Hainline et al. 2011). In Fig. 11 we plot the ALESSSMGs with spectroscopic redshifts on the stellar mass–star-formation rate plane. For comparison, we overlaythe trends proposed for the so-called “main-sequence” ofstar-forming galaxies at z = 1, 2 & 3 and compare theseto the SMGs in the same redshift slices. From this plot,it is clear that the SMGs in our sample lie (on aver-age) a factor ∼ ± × − yr − (see also e.g.Magnelli et al. 2012; Simpson et al. 2014). Velocity offsets between emission / absorption lines The template SEDs are available from: http://astro.dur.ac.uk/$\sim$ams/zLESS/
Fig. 8.—
Composite spectra around the Ly α emission line ( ∼ α emissionline. For comparison the composite spectrum of LBGs from Shapley et al. (2003) is overlaid in red (and offset for clarity). The Q = 3 stackat the bottom was produced to test the validity of the uncertain Q = 3 redshifts by identifying features in their composite spectrum. Thesolid blue line is a running median of the Q = 3 composite. We see apparently significant detections of Ly α and a weak feature which maybe C iii ] λ Fig. 9.—
Left:
The composite spectrum covering restframe 3400–4400˚A of the Q = 1 & 2 ALESS spectra with the X-ray AGN removedfrom the sample. This shows strong [O ii ] emission and potentially H δ absorption, as well as the presence of a spectral break around ∼ Right:
A composite SEDusing the photometry from Simpson et al. (S14, 2014) for those ALESS SMGs with Q = 1, 2 & 3 spectroscopic redshifts. The photometryfor each sources has been de-redshifted and normalised by their rest-frame H -band luminosity. The solid line represents the running medianof 20 points per bin. The shaded region indicates the bootstrap error on the running median. The red curve represents the best fit modelSED assuming a constant star-formation rate to the average photometry for all ALESS SMGs, whereas the green curve is the equivalentmodel fit taken from Simpson et al. (2014). The de-redshifted photometry and limits are shown as grey points and arrows respectively.The vertical dashed lines indicate the Balmer (3646˚A) and 4000˚A breaks. anielson et al. 13 Fig. 10.—
The best-fit rest-frame SEDs for all ALESS SMGswith spectroscopic redshifts. These SEDs have been fitted using magphys (see da Cunha et al. 2008) and are normalised by theirfar-infrared (8–1000 µ m) luminosity. The coloured curves representSEDs for SMGs with Q = 1 & 2 redshifts. They are colour-codedby the logarithm of their ratio of rest-frame S µ m / H flux density(with red denoting a higher ratio). Grey curves represent SEDs forSMGs with Q = 3 redshifts. We see a very large spread in the UVto optical flux density arising from a large spread in the attenu-ation. The colour scale in the upper image shows the 52 SEDsranked by their characteristic dust temperature. These illustratethe wide variety in both the restframe UV/optical/near-infraredand mid-infrared characteristics of SMGs with very similar far-infrared luminosities. Rest-frame UV optical spectroscopic analysis of high-redshift, star-forming galaxies have shown that redshiftsderived from UV ISM absorption lines typically displaysystematic blue-shifted offsets from the systemic (neb-ular) redshifts (e.g. Erb et al. 2006; Steidel et al. 2010;Martin et al. 2012), whilst redshifts determined fromLy α emission often show a systematic offset redward ofthe systemic. These velocity offsets are a consequence oflarge scale outflows (e.g. Pettini et al. 2002; Steidel et al.2010), where the outflows material between the galaxyand the observer absorbs the UV and scatter Ly α pho-tons from the receeding outflow, redshifting them withrespect to the neutral medium within the galaxies. Forsome of the ALESS SMGs we are able to determine neb-ular, UV ISM and Ly α redshifts, allowing us to compareto the results for other star-forming populations.In Table 2 we summarise the lines detected for eachALESS SMG and the redshift associated with fitting toeach line. We show the velocity offsets between theLy α , UV ISM and nebular emission lines in Fig. 12.We also overlay the velocity offsets for the radio-identified counterparts to submillimetre sources studiedby Chapman et al. (2005). Although the same trendis seen in the SMGs and LBGs (Ly α is redshifted andthe UV ISM lines are blueshifted with respect to thesystemic redshift), the SMGs display significantly morescatter, with velocity offsets ranging between ∼ − − for the UV ISM-derived redshifts and be-tween ∼ − − for the Ly α -derived red-shifts, as compared to −
600 to +100 km s − and ∼ +100to +900 km s − respectively for the LBGs in Steidel et al.(2010). The wide variation in the velocity offsets may bedue to a spread in the viewing angle of the winds or thepresence of multiple components (Chen et al. 2015 sug- Fig. 11.—
The stellar mass–star-formation rate plane for ALESSSMGs with spectroscopic redshifts compared to the so-called“main-sequence” of star-forming star-forming galaxies at z = 1,2 & 3. We identify the ALESS SMGs with the best spectroscopicredshifts (Q = 1 & 2) and the points are colour coded by their spec-troscopic redshift. Taken at face value plot suggests that at z ∼ ∼ × higher than the bulk of the star-forming popula-tion at their stellar mass. However, we caution that the stellarmasses of these highly obscured and strongly star-forming galaxiesare systematically uncertain (Hainline et al. 2011). We illustratethe expected conservative uncertainties for the measurements bythe error bars plotted in the lower-right of the panel and stressthat it is possible that the SMGs could be moved systematicallyby comparable amounts on this figure. gest that most SMGs are major mergers and so the spec-tra may have contributions from merging components),or the diversity of conditions within these SMGs, in par-ticular with regard to the strength of large-scale winds.Since the wind must be accelerated by star formation orAGN activity, in Fig. 12 we plot the velocity offsets be-tween lines as a function of bolometric luminosity (wenote that only two SMGs in our sample are X-ray AGN;Wang et al. 2013 and neither of these show Ly α and UVISM lines with extreme offsets from the systemic red-shift). Although there is significant scatter, within theALESS sample the SMGs with lower bolometric lumi-nosity tend to have wind velocities that are lower thanthose of the highest luminosity sources.We note that the outliers in Fig. 12 are ALESS 088.5and ALESS 049.1, with Ly α offset from the systemic by > − . For both ALESS 088.5 and ALESS 049.1the only line available to determine a nebular / systemicvelocity was He ii λ α ). It is im-portant to note that the nebular lines such as H α , [O iii ]and [O ii ] may also be influenced by winds, however thisis more typically observed as line broadening as opposedto a centroid shifting. Environments
One of the key benefits from obtaining spectroscopicredshifts for SMGs is the capability they provide tostudy both the small- and larger-scale environments of4 A spectroscopic redshift survey of ALMA-identified submillimetre galaxies
Fig. 12.—
Top:
Velocity offsets of the UV ISM absorption linesand Ly α from the systemic redshifts (marked by the dashed line)versus bolometric luminosity (L − µ m ) for all ALESS SMGsand the radio-identified submillimetre sources from Chapman etal. (2005), where appropriate lines are detected. The median ofeach sample is marked by a larger symbol. The red and bluedotted lines represent the mean of the distributions of Ly α andISM velocity offsets respectively from the z = 2–3 LBG study from(Steidel et al. 2010) and the full range are shown as error barson the bottom figure. We show a representative error bar forour data derived from the median error on the bolometric lumi-nosity and we estimate a typical redshift measurement error of ∼
100 km s − from fitting the spectral lines. The green points in-dicate offsets measured between lines which can be either nebularor ISM lines and are frequently strongly influenced by winds, suchas C iv λ v λ iii ] λ ii λ ii . Notethat the far-infrared luminosities for the Chapman et al. (2005)sources are derived from their radio fluxes and may be overesti-mated. Bottom:
Histograms of the distributions of velocity offsetsfor Ly α (red), UV ISM lines (blue) and other lines (green). Thehistograms include the SMGs from ALESS and the radio-identifiedsubmillimetre sources in Chapman et al. (2005), and demonstratethat Ly α and the UV ISM lines in SMGs do indeed respectivelypeak redward and blueward of the systemic velocity, as expectedif these systems are driving outflows and winds. these sources. Hence, we next use our spectroscopicredshift sample to search for physical associations be-tween SMGs and between SMGs and other galaxy pop-ulations within the field. Various studies have in-vestigated the environments of SMGs and suggestedthat at least some SMGs reside within overdense envi-ronments (e.g. Chapman et al. 2001; Blain et al. 2004;Chapman et al. 2009; Daddi et al. 2009; Capak et al.2011; Walter et al. 2012; Ivison et al. 2013; Decarli et al.2014; Smolcic et al. 2016). For example, Blain et al.(2004) (see also Chapman et al. 2009) identified an over-density of six SMGs and two radio galaxies at z = 1.99within 1200 km s − of each other in the GOODS-N field.Clustering analysis has also suggested that SMGs clusteron scales of 5–10 h − Mpc − , while pair counting suggestsSMGs have properties consistent with them evolving intothe passive red galaxies at z ∼
1, and subsequently themembers of rich galaxy groups or clusters at z ∼ >
35% of the single dishsources resolved into multiple SMGs (where an SMG isa far-infrared bright galaxy with a 870 µ m flux brighterthan 1 mJy). Simpson et al. (2015b) also showed that thenumber density of S > ∼ ∼
80 timeshigher than that derived from blank-field counts. Aftertaking into account the observational biases in their sam-ple, they proposed that an over-abundance of faint SMGsof this magnitude is inconsistent with line-of-sight pro-jections dominating multiplicity in the brightest SMGs,and strongly suggests that a significant proportion ofthese high-redshift ULIRGs are likely to be physicallyassociated. These SMGs are typically separated by ∼ ′′ which corresponds to ∼ ∼ ′′ ) wherethe SMGs lie within 2000 km s − (although an offset of2000 km s − is larger than the typical velocity dispersionof rich clusters, even at z ∼
0, we broaden our searchwindow to account for potential outflow-driven shifts inthe spectral features used to derive the redshifts of manyof the SMGs (see § z = 0.06–1.25. Onlyin ALESS 067 do we have indirect evidence for an in-teracting pair of SMGs (ALESS 067.1 and ALESS 067.2)based on the morphology of the sources in HST imaging(Chen et al. 2015).Next, we search for physical associated betweenSMGs across the whole ECDFS field (i.e. betweenthe ALMA maps). We identify seven pairs of SMGswithin 2000 km s − of each other, with ALESS 075.2,ALESS 088.5 and ALESS 102.1 also appearing as a triple“association”. These pairs/triples of SMGs have an aver-age offset of ∼ ∼ > z ∼ z = 0.3–anielson et al. 151.0, . From this catalog, we select only secure redshiftsand remove duplicates (we also remove cases in which twosecure but differing redshifts are given from two differentreferences).In Fig. 13 we plot the spectroscopic redshift distri-bution of the ALESS SMGs, together with the fieldpopulation. In those cases where ≥ − , these associations do not often statisticallycoincide with significant over-densities in the backgroundgalaxy population, although the two SMGs at z ∼ ≤ − . The median apparent magnitude at4.5 µ m for these ten SMGs is m . µ m = 20.4 +0 . − . as com-pared to a median of m . µ m = 21.1 +0 . − . for the 42 ALESSSMGs in the parent spectroscopic sample which are notin identified “associations”. We conclude that there isno evidence in the current sample that the SMGs in “as-sociations” are any brighter (and thus potentially moremassive) than those not in “associations”. CONCLUSIONS
In this work we present the results from a redshift sur-vey of ALMA-identified SMGs. Our main conclusionsare: • The redshift distribution for ALESS SMGs withspectroscopic redshifts is centered at z = 2.4 ± z = 0.7–5.0 and an in-terquartile range of z = 2.1–3.0. This is consistentwith the photometric redshift distribution for thesesources, and the median is consistent with previ-ous estimates based on the radio-identified coun-terparts to submillimetre sources (Chapman et al.2005). However, since we do not rely on a radio se-lection, our sample is not biased against higher red-shift SMGs and indeed, 23% of the ALESS SMGswith spectroscopic redshifts lie at z > • We identify velocity offsets up to ∼ − be-tween the redshifts measured from nebular emis-sion lines (i.e. H α , [O iii ], H β and [O ii ]) and thosemeasured from Ly α or UV ISM absorption lines.We conclude that it is likely that the extreme SFRswithin the SMGs (typically ∼ ±
30 M ⊙ yr − )are driving strong galaxy-scale outflows in manyof these systems. • Since many of our spectra of SMGs are too faintto exhibit any obvious emission or absorption fea-tures (continuum is only detected in ∼
50% of thesources), we produce composite spectra over vari-ous wavelength ranges to search for weaker featuresin the “typical” ALESS SMG optical-to-near in-frared spectrum. At rest-frame 1000–2000˚A we seestrong, asymmetric Ly α emission and blueshiftedSi ii and potentially Si iv absorption suggestive ofstrong stellar winds. Our composite spectrum atrest-frame 3400–4400˚A shows a Balmer break, in-dicative of on-going star formation. Comparing ourcomposite to spectral models we suggest that it ismost consistent with a young starburst with an ageof ∼
10 Myr. • We use our precise spectroscopic redshifts to re-duce the uncertainties when modelling the SEDs ofour SMGs using magphys and find a large spreadin the dust attenuation (A V ∼ V = 1.9 ± M ⋆ = (6 ± × M ⊙ andby combining with our estimates of their star-formation rates, we show that SMGs lie (on aver-age) ∼ z ∼ z ∼ ACKNOWLEDGMENTS
We acknowledge the ESO programmes 183.A-0666and 090.A-0927(A). The ALMA observations were car-ried out under programme 2011.0.00294.S. ALRD ac-knowledges an STFC studentship (ST/F007299/1) andan STFC STEP award. AMS gratefully acknowl-edges an STFC Advanced Fellowship through grantST/H005234/1, STFC grant ST/L00075X/1 and theLeverhume foundation. IRS acknowledges support fromSTFC, a Leverhulme Fellowship, the ERC Advanced In-vestigator programme DUSTYGAL 321334 and a RoyalSociety/Wolfson Merit Award. WNB acknowledgesSTScI grant HST-GO-12866.01-A. CMC acknowledges6 A spectroscopic redshift survey of ALMA-identified submillimetre galaxies
Fig. 13.—
Top:
The spectroscopic redshift distribution of SMGs (Q = 1, 2 & 3) compared to the less luminous galaxy populations in thefield. The latter is based on the catalogue compiled by Luo et al. (2011) with the addition of recent redshifts from the full FORS2/VIMOSsurvey (Table 2 & 3) and from Williams et al. (2014). We plot all the galaxies in the ECDFS for which we have spectroscopic redshifts(including the SMGs), we also plot the distributions for just the radio/MIPS sources, as well as the SMGs. We see little correlation betweenthe peaks in the SMG redshift distribution and the general galaxy distribution. The binning is 6000 km s − in all panels. Bottom:
Expandedviews of the redshift distribution around the associations of the ALESS SMG compared to the overall galaxy redshift distribution. Wefind a maximum of three SMGs in our adopted 2000 km s − window, in addition to three pairs of SMGs. The pairs/triples in the SMGpopulation do not obviously coincide with overdensities in the less-active galaxy populations across the field. The colour coding is the sameas Fig. 13 and the top axis indicates velocity relative to the redshift of the pair/triple. support from a McCue Fellowship at the University ofCalifornia, Irvines Center for Cosmology and the Univer-sity of Texas at Austins College of Natural Science. JLWis supported by a European Union COFUND/DurhamJunior Research Fellowship under EU grant agreementnumber 267209. AK acknowledges support by the Col- laborative Research Council 956, sub-project A1, fundedby the Deutsche Forschungsgemeinschaft (DFG). ALMAis a partnership of ESO (representing its member states),NSF (USA) and NINS (Japan), together with NRC(Canada) and NSC and ASIAA (Taiwan), in cooperationwith the Republic of Chile. The Joint ALMA Observa-tory is operated by ESO, AUI/NRAO and NAOJ. REFERENCESAlexander D. M., Brandt W. N., Smail I., Swinbank A. M., BauerF. E., Blain A. W., Chapman S. C., Coppin et al., 2008, AJ,135, 1968 Alexander D. M., Swinbank A. M., Smail I., McDermid R.,Nesvadba N. P. H., 2010, MNRAS, 402, 2211 anielson et al. 17
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ALESS SMGS WITH LITERATURE REDSHIFTS
The following sources are ALESS SMGs with previously measured spectroscopic redshifts:1. ALESS 018.1: is listed as ID 66 in Casey et al. (2011), with a redshift of z = 2.252 derived from an H α detectionwith the Infrared Spectrometer And Array Camera (ISAAC) on the VLT;2. ALESS 057.1: is listed as ID 112a in Szokoly et al. (2004) with a redshift of z = 2.940 derived from detections ofHe ii , O vi and N v with FORS1 / FORS2. It is classed as a QSO with strong high-ionisation emission lines;3. ALESS 067.1: is listed as ECDFS-45 in Kriek et al. (2008) at z = 2.122, derived from emission lines in thenear-infrared spectrum observed with GNIRS;4. ALESS 073.1: is listed as GDS J033229.29 − z = 4.762 was determined via the detection of Ly α andN v using FORS2.5. ALESS 098.1: is identified as ID J033129 in Casey et al. (2011). The redshift, z = 1.4982 is derived through atentative detection of H α , however, it is also spectroscopically-identified in the restframe UV in the same paperand therefore it is given a “secure” status. This redshift is, however, in disagreement with our Q = 1 redshiftof z = 1.3735 derived from fitting to an [O ii ] line in the FORS2 observations, with a tentative detection of H α anielson et al. 19at the same redshift under a sky line in the XSHOOTER near-infrared spectrum. We use our redshift in theanalysis in this work;6. ALESS 122.1: is listed as radio ID 149 in Bonzini et al. (2012). The redshift of z = 2.03 is determined from UVISM absorption features observed with VIMOS. NOTABLE INDIVIDUAL SOURCES
Since we have a wealth of spectroscopic data we can utilise the spectra not only for the purpose of determiningredshifts but also to search for diagnostic features indicative of AGN activity, star formation, strong stellar winds etc.Here we highlight and discuss some of the most notable, high signal-to-noise spectra.
ALESS 057.1:
This SMG hosts a luminous AGN which is detected in X-rays (Wang et al. 2013). The VIMOSspectrum (Fig. 2) exhibits strong, broad, symmetric Ly α emission, broad N v and C iv emission (FWHM ∼ − )which is significantly blue-shifted ( ∼ − ) with respect to both He ii and Ly α (which have velocities that areconsistent within measurements errors). The C iv emission line also displays a P-Cygni profile. ALESS 066.1:
This SMG is listed as an X-ray AGN at z = 1.310 in Wang et al. (2013). However, our observationsreveal the optical/near-infrared photometry and X-ray emission are dominated by a foreground QSO at z = 1.310 butour near-infrared spectroscopy with MOSFIRE identifies an emission line in K -band slightly to the north of the QSO.At λ = 2.333 µ m this line corresponds to H α at z = 2.5542. Careful analysis of the ALMA and optical imaging revealsthat the SMG is indeed < ∼ ′′ north of the QSO and hence is likely to be lensed by the foreground QSO. ALESS 073.1:
This SMG also hosts a luminous X-ray AGN (Vanzella, E. et al. 2008; Coppin et al. 2009;De Breuck et al. 2014; Wang et al. 2013) and the spectrum (Fig. 2) shows strong, broad N v with aFWHM ∼ − as compared to a relatively narrow and weak Ly α (FWHM ∼
700 km s − ). ALESS 075.1:
We have excellent spectroscopic coverage of this SMG and have strong detections of [O ii ], [O iii ] λ β and H α with XSHOOTER. The H α detection is narrow with FWHM ∼
160 km s − . The [O iii ] emission isnot fit well with a single Gaussian as it is an asymmetric line with a red wing, possibly indicating an outflow (e.g.Alexander et al. 2010). Given the high [O iii ] luminosity and the lack of an X-ray detection, this outflow may beaccelerated by an obscured AGN (i.e. outflows in high-redshift ULIRGs hosting AGN activity; Harrison et al. 2012). ALESS 079.2:
This SMG has strong detections of H α and [N ii ] with XSHOOTER. The one- and two-dimensionalspectra show structured emission (see Fig. 14). In the one-dimensional spectrum the H α and [N ii ] lines are truncatedat their red end and appear to be more extended towards lower velocities. The flux ratio of [N ii ] λ α is consistentwith the ionising radiation arising from H ii regions as opposed to an AGN. ALESS 087.1:
Strong rest-frame UV continuum is detected in this SMG with ISM absorption lines, with reshiftsconsistent with the Ly α emission line. However, the Ly α is significantly offset northwards of the continuum in thetwo-dimensional spectrum. We therefore extract two spectra in Fig. 14 taken from the position of the Ly α and thecontinuum. The Ly α profile is marginally asymmetric with a truncated blue edge. The continuum spectrum showsan obvious break and relatively strong Si iv absorption. Unfortunately, there is very poor photometric coverage of thisSMG (3.6–8 µ m only) so we are unable to say whether the offset Ly α is due to a close companion or an interactionwith another system, or a less-obscured part of a single galaxy. ALESS 122.1:
This SMG has very blue continuum with strong UV ISM absorption lines in both the FORS2 andVIMOS spectra (Fig. 14). There is very strong, broad C iv absorption (FWHM of > − ). The C iv exhibitsa strong, narrow component associated with interstellar absorption and a very broad red component associated withstellar winds. The strength of this redshifted component suggests the presence of a large number of very massivestars ( >
30 M ⊙ ; Leitherer & Heckman 1995). Models show that Si iv is relatively weak for a continuous star formationhistory but yields a strong P-Cygni profile for bursty star formation. Detection of a P-Cygni profile for Si iv is thereforea good indicator that the burst duration is short relative to the age. The Si iv absorption feature is unusually broad( > − ). This is the blueshifted wind absorption. Swinbank et al. (2014) determine L FIR = (6 . ± . × L ⊙ for this SMG which implies a star-formation rate of SFR ∼ ±
70 M ⊙ yr − (using Kennicutt 1998) which is higherthan typical ALESS SMGs, SFR ∼ (310 ±
30) M ⊙ yr − (Swinbank et al. 2014). We note that an AGN may also exhibitstrong C iv absorption and given the very strong continuum and the large width of the C iv in this SMG, it is plausiblethat it may be a broad absorption line (BAL) AGN.0 A spectroscopic redshift survey of ALMA-identified submillimetre galaxies Fig. 14.—
Some of the most notable spectra of SMGs in the sample, featuring evidence of winds, AGN activity, and multiple components.The sky subtraction is poor in some of the spectra and is a particular problem in the near-infrared and in the FORS2 spectrum ofALESS 073.1. The main skylines have been highlighted in grey. a n i e l s o n e t a l. Table 2: ALESS spectroscopic redshift catalog
ALESS ID RA DEC z spec Q spec z a phot M/S b Instruments c Notes(J2000) (J2000)ALESS 001.1 53.310270 -27.937366 4.9540 3 4 . +2 . − . M GMX [O ii ] in M- K ALESS 001.2 53.310059 -27.936562 ... 4 4 . +2 . − . M FVX BLANKALESS 001.3 53.309069 -27.936759 ... 4 2 . +0 . − . M X BLANKALESS 002.1 53.261188 -27.945211 2.1913 3 1 . +0 . − . M DV poss. C iii ] em in DALESS 002.2 53.262800 -27.945252 ... 4 - M D BLANKALESS 003.1 53.339603 -27.922304 4.2373 3 3 . +0 . − . M FMV poss. Ly α em in F+V ALESS 003.2 53.342461 -27.922486 ... 4 1.44 +0 . − . S M BLANKALESS 003.3 53.336294 -27.920555 ... 4 - S M BLANKALESS 003.4 53.341644 -27.919379 ... 4 - S M BLANK
ALESS 005.1 52.870467 -27.985840 ... 4 2 . +0 . − . M DMX BLANKALESS 006.1 53.237331 -28.016856 2.3338 1 0 . +0 . − . M GX cont. from bright sources above SMG; Ly α em ( z = 2 . iv em ( z = 2 . α and [O iii ]5007 in G ( z = 2 . . +0 . − . M DFXS strong cont.; z from H α in X-NIR; He ii in X-VIS ( z = 2 . ALESS 007.2 53.312522 -27.758499 ... 4 - S D BLANK
ALESS 009.1 53.047244 -27.869981 ... 4 4 . +0 . − . M D BLANKALESS 010.1 53.079418 -27.870781 0.7616 1 2 . +0 . − . M FV [O ii ] in V; [O ii ] ( z = 0 . iii ]4959 ( z = 0 . β ( z = 0 . z is mean from [O ii ], [O iii ], H β , possible lensALESS 011.1 53.057688 -27.933403 2.6832 2 2 . +1 . − . M FV Ly α em in V, no cont.ALESS 013.1 53.204132 -27.714389 ... 4 3 . +0 . − . M DG BLANKALESS 014.1 52.968716 -28.055300 ... 4 4 . +2 . − . M VX BLANKALESS 015.1 53.389034 -27.991547 ... 4 1 . +0 . − . M DFGVX BLANK
ALESS 015.2 53.391876 -27.991724 ... 4 - S M BLANK
ALESS 015.3 53.389976 -27.993176 3.4252 3 - M DM Ly α em ( z = 3 . iv em ( z = 3 . ALESS 015.6 53.388192 -27.995048 ... 4 - S M BLANK
ALESS 017.1 53.030410 -27.855765 1.5397 1 1 . +0 . − . M DFMV strong cont.; z from H α in M- H ; Mg ii abs in F ( z = 1 . ALESS 017.2 53.034437 -27.855470 2.4431 3 2.10 +0 . − . S M poss. H α in M-KALESS 017.3 53.030718 -27.859423 ... 4 2.58 +0 . − . S D BLANK
ALESS 018.1 53.020343 -27.779927 2.2520 d . +0 . − . M V cont. in V; archival z from Casey+11ALESS 019.1 53.034401 -27.970609 ... 4 2 . +0 . − . M FV BLANK
ALESS 020.1 53.319834 -28.004431 ... 4 2.58 +0 . − . S DFV cont. in FALESS 020.2 53.317807 -28.006470 ... 4 - S D BLANK
ALESS 022.1 52.945494 -27.544250 ... 4 1 . +0 . − . M FV cont. in F+VALESS 023.1 53.050039 -28.085128 ... 4 4 . +2 . − . M V BLANKALESS 025.1 52.986997 -27.994259 2.8719 3 2 . +0 . − . M V Ly α + break, cont.ALESS 029.1 53.403749 -27.969259 1.438 9 2 2 . +2 . − . M DGMV H α in M- H ALESS 031.1 52.957448 -27.961322 ... 4 2 . +1 . − . M FVX BLANK
ALESS 034.1 53.074833 -27.875910 2.5115 2 1.87 +0 . − . S M broad H α in M- K ALESS 035.1 52.793776 -27.620948 ... 4 - M V BLANKALESS 037.2 53.401514 -27.896742 2.3824 3 4 . +0 . − . M M H α ( z = 2 . ii ] ( z = 2 . ALESS 038.1 53.295153 -27.944501 ... 4 2.47 +0 . − . S D strong cont.+emission lines from contaminating source
ALESS 039.1 52.937629 -27.576871 ... 4 2 . +0 . − . M X poss. faint lines, no cont.ALESS 041.1 52.791959 -27.876850 2.5460 2 2 . +4 . − . M FV strong cont. in F+V; C iii ]1909 em ( z = 2 . ii ]2326 em ( z = 2 . . +0 . − . M DFV possible faint lines, no cont.
ALESS 043.3 53.276120 -27.798534 ... 4 - S D BLANK
ALESS 045.1 53.105255 -27.875148 ... 4 2 . +0 . − . M FV no cont.; poss. Ly α em z = 2 . iv z = 2 . ALESS 046.1 53.402937 -27.547072 ... 4 - S FV faint cont. in F
ALESS 049.1 52.852998 -27.846406 2.9417 2 2 . +0 . − . M DFV strong cont. in F + V; He ii em ( z = 2 . iv em ( z = 2 . A s p ec t r o s c o p i c r e d s h i f t s u r v e y o f A L M A - i d e n t i fi e d s ub m illi m e t r e ga l a x i e s Continued from previous page
ALESS ID RA DEC z spec Q spec z a phot M/S b Instruments c Notes(J2000) (J2000)ALESS 049.2 52.851956 -27.843914 ... 4 1 . +0 . − . M M BLANKALESS 051.1 52.937754 -27.740922 1.3638 3 1 . +0 . − . M FV strong cont. in F+V, [O ii ] ( z = 1 . ∼ ii em ( z = 1 . . +0 . − . M DF strong cont. in F+D; Mg ii em ( z = 1 . z = 1 . d . +0 . − . M FV cont. + Ly α em ( z = 2 . iv em ( z = 2 . ii em ( z = 2 . . +0 . − . M X BLANKALESS 061.1 53.191128 -28.006490 4.4190 1 6 . +0 . − . M A ALMA [C ii ]158 µ m ALESS 062.1 53.150677 -27.580258 ... 4 - S D BLANKALESS 062.2 53.152410 -27.581619 1.3614 1 1.35 +0 . − . S DFV [O ii ] in D+F. [O ii ] doublet resolved in D. ALESS 063.1 53.285193 -28.012179 ... 4 1 . +0 . − . M G poss. faint em linesALESS 065.1 53.217771 -27.590630 4.4445 1 - M AD z from ALMA [C ii µ m, Ly α ALESS 066.1 53.383053 -27.902645 2.5542 1 2 . +0 . − . M FMV H α and [N ii ] in M; lensed?ALESS 067.1 53.179981 -27.920649 2.1230 d . +0 . − . M FVX cont. in F+V; H α , [O iii ]5007 in X-NIR; merging with 067.2ALESS 067.2 53.179253 -27.920749 2.1230 3 2 . +0 . − . M X BLANK but likely merging with 067.1ALESS 068.1 53.138888 -27.653770 ... 4 - M VX BLANKALESS 069.1 52.890731 -27.992345 4.2071 3 2 . +0 . − . M D single line, poss. Ly α with asymmetric profileALESS 069.2 52.892226 -27.991361 ... 4 - M M BLANKALESS 069.3 52.891524 -27.993990 ... 4 - M DM BLANKALESS 070.1 52.933425 -27.643200 2.0918 3 2 . +0 . − . M FX strong cont. in F; poss. Ly α in X-UVBALESS 071.1 53.273528 -27.557831 3.6967 2 2 . +0 . − . M V Ly α ( z = 3 . v em ( z = 3 . d . +0 . − . M DF very broad Ly α and N v em in D+F; Ly α ( z = 4 . v ( z = 4 . . +0 . − . M DFV BLANKALESS 075.1 52.863303 -27.930928 2.5450 1 2 . +0 . − . M FVX very interesting source; strong cont. in V+F; [O iii ]4959 ( z = 2 . iii ]5007 ( z = 2 . iii ], H β ( z = 2 . ii ] doublet ( z = 2 . α ( z = 2 . α in X ( z = 2 . ALESS 075.2 52.865276 -27.933116 2.2944 2 0.39 +0 . − . S DM H α , [N ii ] ( z = 2 . ), [S ii ] ( z = 2 . ) in M- K ALESS 075.4 52.860715 -27.932144 ... 4 2 . +0 . − . M DM BLANKALESS 076.1 53.384731 -27.998786 3.3895 2 - M DFMV [O iii ]5007 + [O iii ]4959 in M; poss. Ly α ( z ∼ . . +0 . − . M D BLANKALESS 079.2 53.090004 -27.939988 1.7693 1 1 . +0 . − . M FVX Strong H α , [N ii ]6548, 6583 in X-NIR; structured lines- 2 componentsALESS 079.4 53.088261 -27.941808 ... 4 - M D BLANKALESS 080.1 52.928347 -27.810244 4.6649 3 1 . +0 . − . M FV poss Ly α in FALESS 080.2 52.927570 -27.811376 ... 4 1 . +0 . − . M D BLANK
ALESS 080.5 52.923654 -27.806318 1.3078 3 - S D tentative [O ii ] + [Ne iii ]ALESS 081.1 52.864805 -27.744336 ... 4 1.70 +0 . − . S V BLANK
ALESS 082.1 53.224989 -27.637470 ... 4 2 . +3 . − . M DFV BLANKALESS 084.1 52.977090 -27.851568 3.9651 3 1 . +0 . − . M DFM Ly α ( z = 3 . v ( z = 3 . . +0 . − . M DF cont. in F; poss faint linesALESS 087.1 53.212016 -27.528187 2.3086 1 3 . +0 . − . M FV Ly α em ( z = 2 . iv abs ( z = 2 . ii abs ( z = 2 . α offset from cont.ALESS 088.1 52.978175 -27.894858 1.2679 1 1 . +0 . − . M FVMX [O ii ] ( z = 1 . ii ]3726,3729 visible in X-VISALESS 088.2 52.980797 -27.894529 2.5192 3 - M DM C ii ]2326 em ( z = 2 . iv em ( z = 2 . . +0 . − . M DFV strong cont. in V, poss break; Ly α em ( z = 2 . ii ( z = 2 . . +0 . − . M D C iii ] em ( z = 2 . α em ( z = 2 . ALESS 089.1 53.202879 -28.006079 0.6830 3 1.17 +0 . − . S F bright [O ii ] + cont a n i e l s o n e t a l. Continued from previous page
ALESS ID RA DEC z spec Q spec z a phot M/S b Instruments c Notes(J2000) (J2000)ALESS 094.1 53.281640 -27.968281 ... 4 2 . +0 . − . M DV BLANKALESS 098.1 52.874654 -27.956317 1.3745 d . +0 . − . M DFMVX [O ii ] ( z = 1 . α under sky in X-NIRALESS 099.1 53.215910 -27.925996 ... 4 - M D BLANK ALESS 101.1 52.964987 -27.764718 2.7999 2 3.49 +03 . − . S V Ly α ALESS 102.1 53.398333 -27.673061 2.2960 3 1 . +0 . − . M FV cont. in V, Ly α ( z = 2 . iii ] ( z = 2 . ALESS 106.1 52.915187 -27.944236 ... 4 . +0 . − . S DM BLANK
ALESS 107.1 52.877082 -27.863647 2.9965 3 3 . +0 . − . M VM Ly α em ( z = 2 . iv em ( z = 2 . ii ], [O iii ] in MALESS 107.3 52.878013 -27.865465 ... 4 2 . +1 . − . M D BLANKALESS 110.1 52.844411 -27.904784 ... 4 2 . +0 . − . M FMV BLANKALESS 110.5 52.845677 -27.904005 ... 4 - M DM BLANKALESS 112.1 53.203596 -27.520362 2.3154 1 1 . +0 . − . M FGV Ly α em ( z = 2 . α ( z = 2 . iii ]5007 ( z = 2 . β em ( z = 2 . . +0 . − . M FV strong cont in F+V, [O ii ] doublet in F ( z = 1 . α em ( z = 3 . . +1 . − . M FV BLANKALESS 116.2 52.976826 -27.758735 ... 4 4 . +1 . − . M F BLANKALESS 118.1 52.841347 -27.828161 2.3984 3 2 . +0 . − . M DFV strong cont in F+V, Ly α abs + break, C iv em ( z = 2 . . +0 . − . M V BLANKALESS 122.1 52.914768 -27.688792 2.0232 d . +0 . − . M FV very strong blue cont. and abs. lines. V: C ii ] abs ( z = 2 . iv abs ( z = 2 . ii em ( z = 2 . iv and Si ii blended abs.; C iii ] ( z = 2 . ii ii ii . +0 . − . M FV poss faint linesALESS 126.1 53.040033 -27.685466 ... 4 1 . +0 . − . M V BLANK
TABLE 2Notes: The 22 ALESS SMGs not targeted in our spectroscopy programme (and without redshifts from literature) are not listed here. The supp SMGs are shownin italics. z spec = − means we could not determine a spectroscopic redshift. a Photometric redshifts from S14. Those SMGs without a photometric redshifthave poor photometric constraints (detections in < bands). b M = main catalog, S = supp catalog. c F = VLT/FORS2, V = VLT/VIMOS, X = VLT/XSHOOTER,M = Keck/MOSFIRE (Band H or K ), D = Keck/DEIMOS, G = Gemini/GNIRS. d These redshifts are for the six sources which also have literature spectroscopicredshifts described in §
3. The quality flag (Q) for the spectroscopic redshifts is Q = 1 for secure redshifts; Q = 2 for redshifts measured from only one or twostrong lines; Q = 3 for tentative redshifts measured based on one or two very faint features; Q = 4 for those sources which were targeted but no redshift couldbe determined.
ANCILLARY REDSHIFTS
When designing the slit masks, we in-filled the unused portions masks (not targeting the high-prioroty SMGs) withother candidate high-redshift galaxies, in particular with mid-, far-infrared or radio selected galaxies. Here, we providethe details of the galaxies targeted.The ID for each galaxy relates to the input catalogue from which a target was selected. These are summarised as:
Statistically Robust or Tentative candidate LESS SMG multiwavelength counterparts from Biggs et al.(2011) (see also Wardlow et al. 2011) but which were later shown by ALMA observations to be incorrect IDs(Hodge et al. 2013).
Robust or tentative IDs for LESS sources with signal-to-noise of SNR = 2.7–3.7 σ in the original LESSmap. These IDs for “faint SMGs” are derived using 1.4 GHz radio emission (Biggs et al. 2011) but have not yet beenconfirmed (or ruled out) by ALMA. Galaxies in the LESS submillimetre error circles which have photometric redshifts that are consistent withthe ALESS photometric redshifts (Wardlow et al. 2011). µ m-selected galaxies from the Spitzer
FIDEL survey without pre-existing spectroscopic redshifts(Magnelli et al. 2009).
X-ray sources from the 2 Ms or 4 Ms surveys (e.g. Lehmer et al. 2005; Luo et al. 2008).
Galaxies from the
Herschel / SPIRE images which peak at 350 µ m (and which have been identified anddeblended using the 24 µ m positions as priors; Roseboom et al. 2010). Individual redshifts for these sources will bepublished in Oliver et al. (in prep), although we include the redshift distributio in Fig. 15. Galaxies from the
Herschel / SPIRE images which peak at 250 µ m or 350 µ m (and which have beenidentified and deblended using the 24 µ m positions as priors; Roseboom et al. 2010. Individual redshifts for thesesources will be published in Oliver et al. (in prep), although we include the redshift distributio in Fig. 15. Optically faint radio galaxies (OFRGs) from the JVLA 1.4 GHz survey of this field. These radio sourcesare typically brighter than > µ Jy at 1.4 GHz but have optical magnitudes fainter than I AB = 22. Optically (colour) selected galaxies. These comprise a mix of z ∼ α emitting galaxies, BM/BXgalaxies and Lyman break galaxies at 1 . < z < . Galaxies which were not in any of the other prior catalogs but which could still be placed on the masks. B - or V -band drop-out galaxies (i.e. candidate z > ∼ z > ∼ “b” suffix denotes a secondary galaxy that happened to lie on the slit, but is notthe primary target.We also note that the catalogs are not unique (a galaxy could be an ALMA source that is also in the FIDEL 24 µ mcatalog, a radio catalog, a BX/BM and also a Chandra
X-ray source). In those instances, the object will only appearonce in the table, but under the ID from which it was selected for slit placement (i.e. there are no RA / Dec repeats).As in Table 2, the instrument IDs are denoted by F = VLT / FORS2, V = VLT / VIMOS, X = VLT / XSHOOTER,M = Keck / MOSFIRE, D = Keck /, DEIMOS, and G = Gemini / GNIRS. The quality flag (Q) for the spectroscopicredshifts is Q = 1 for secure redshifts; Q = 2 for redshifts measured from only one or two strong lines; Q = 3 fortentative redshifts measured based on one or two very faint features; Q = 4 for those sources which were targeted butno redshift could be determined. The redshift distribution for each of these sub-samples is shown in Fig. 15.anielson et al. 25
Fig. 15.—
Spectroscopic redshift distributions for the various galaxy population targeted during the spectroscopic campaign. In eachpanel, we show the redshift distribution for all galaxies, but also show the histograms for the best quality (Q = 1) spectra, and those withQ = 1 & 2. The number of galaxies with spectroscopic redshifts (and the median redshift) are also given in the panels.
Top Left : Redshiftdistribution for
ALL galaxies targeted;
Top Right:
Redshift distribution for 24 µ m selected galaxies from the FIDEL survey; Middle Left:
Redshift distribution for optically faint radio galaxies (OFRGs);
Middle Right:
Redshift distribution for the LBGs, BX/BMs and Ly α emitters; Bottom Left:
Redshift distribution for
Chandra
X-ray sources.
Table 3. Spectroscopic redshifts for the full sample
ID RA DEC z spec
Q Inst ID RA DEC z spec
Q Inst(J2000) (J2000) (J2000) (J2000)101 53.30820 -27.93445 4.6892 1 F 104 53.26036 -27.94606 1.9469 3 VF106 52.90094 -27.91398 2.3484 3 VMF 107 52.89957 -27.91209 ... 4 VMF108 52.89780 -27.90952 ... 4 VF 109 52.90089 -27.91278 3.0159 2 V110 52.87580 -27.98573 1.4135 1 F 112 52.87865 -27.98229 0.4342 1 F113 53.23814 -28.01708 1.3648 3 VF 114 53.23651 -28.01645 ... 4 VF116 53.31593 -27.76045 0.7516 1 VF 117 53.02072 -27.51948 0.9610 2 VF118 53.01840 -27.52046 0.7283 3 VF 119 53.04730 -27.87038 ... 4 F280 53.08039 -27.87200 ... 3 V 122 53.19980 -27.90448 3.1977 3 V123 53.20365 -27.71445 ... 4 VF 123b 53.20339 -27.71603 2.8382 2 V124 52.96913 -28.05492 ... 4 V 127 53.07793 -27.62877 ... 4 V131 53.03317 -27.97311 0.9607 3 VF 133 53.37387 -27.57901 1.2382 2 V