The SCUBA HAlf Degree Extragalactic Survey (SHADES) - III. Identification of radio and mid-

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Determining an accurate position for a submm galaxy (SMG) is the crucial step that enables us to move from the basic properties of an SMG sample - source counts and 2-D clustering - to an assessment of their detailed, multi-wavelength properties, their con

Mon.Not.R.Astron.Soc.000,000–000(0000)Printed5February2008

A(MNLTEXstyle lev2.2)

TheSCUBAHAlfDegreeExtragalacticSurvey—III.Identi cation

ofradioandmid-infraredcounterpartstosubmillimetregalaxies

R.J.Ivison,1,2T.R.Greve,3J.S.Dunlop,2J.A.Peacock,2E.Egami,4IanSmail,5E.Ibar,2E.vanKampen,6I.Aretxaga,7T.Babbedge,8A.D.Biggs,1A.W.Blain,3S.C.Chapman,9D.L.Clements,8K.Coppin,5,10D.Farrah,11M.Halpern,10D.H.Hughes,7M.J.Jarvis,12T.Jenness,13J.R.Jones,14A.M.J.Mortier,2S.Oliver,15C.Papovich,4P.G.P´erez-Gonz´alez,16A.Pope,10S.Rawlings,17G.H.Rieke,4M.Rowan-Robinson,8R.S.Savage,15D.Scott,10M.Seigar,18S.Serjeant,19C.Simpson,20J.A.Stevens,12M.Vaccari,21,8J.Wagg7,22andC.J.Willott23

UKAstronomyTechnologyCentre,RoyalObservatory,BlackfordHill,EdinburghEH93HJ

ScottishUniversitiesPhysicsAlliance,InstituteforAstronomy,UniversityofEdinburgh,BlackfordHill,EdinburghEH93HJ3AstronomyDepartment,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA

4StewardObservatory,UniversityofArizona,933N.CherryAvenue,Tuscon,AZ85721,USA5InstituteforComputationalCosmology,UniversityofDurham,SouthRoad,DurhamDH13LE6InstituteforAstrophysics,UniversityofInnsbruck,Technikerstr.25,A-6020Innsbruck,Austria7InstitutoNacionaldeAstrof´ sica,OpticayElectr´onica,ApartadoPostal51y216,72000Puebla,Pue.,Mexico8AstrophysicsGroup,BlackettLaboratory,ImperialCollegeLondon,PrinceConsortRoad,LondonSW72BW9InstituteofAstronomy,MadingleyRoad,Cambridge,CB30HA

10DepartmentofPhysics&Astronomy,UniversityofBritishColumbia,Vancouver,BCV6T1Z1,Canada11DepartmentofAstronomy,CornellUniversity,106SpaceSciences,Ithaca,NY14853,USA

12CentreforAstrophysicsResearch,UniversityofHertfordshire,CollegeLane,Hat eldAL109AB13JointAstronomyCentre,660N.A‘oh¯ok¯uPlace,UniversityPark,Hilo,HI96720,USA14CenterforAstrophysicsandSpaceAstronomy,389UCB,Boulder,CO80309,USA15AstronomyCentre,UniversityofSussex,Falmer,BrightonBN19QH16DepartmentodeAstrof´ sicayCC.deAtm´osfera,FacultaddeCC.F´ sicas,UniversidadComplutensedeMadrid,28040Madrid,Spain17Astrophysics,DepartmentofPhysics,DenysWilkinsonBuilding,KebleRoad,OxfordOX13RH

18DepartmentofPhysicsandAstronomy,4129FrederickReinesHall,UniversityofCalifornia,Irvine,CA92697,USA19AstrophysicsGroup,DepartmentofPhysics,TheOpenUniversity,MiltonKeynesMK76AA

20AstrophysicsResearchInstitute,LiverpoolJohnMooresUniversity,TwelveQuaysHouse,EgertonWharf,BirkenheadCH411LD21DepartmentofAstronomy,UniversityofPadova,Vicolodell’Osservatorio3,I-35122,Italy22NationalRadioAstronomyObservatory,P.O.Box0,Socorro,NM87801,USA

23PhysicsDepartment,UniversityofOttawa,150LouisPasteur,PavillonMacDonaldHall,Ottawa,OntarioK1N6N5,Canada

arXiv:astro-ph/0702544v2 30 May 2007

Accepted...;Received...;inoriginalform2007February20

ABSTRACT

c0000RAS

Determininganaccuratepositionforasubmillimetre(submm)galaxy(SMG)isthecrucial

stepthatenablesustomovefromthebasicpropertiesofanSMGsample–sourcecountsand2-Dclustering–toanassessmentoftheirdetailed,multi-wavelengthproperties,theircontributiontothehistoryofcosmicstarformationandtheirlinkswithpresent-daygalaxypopulations.Inthispaper,weidentifyrobustradioand/orinfrared(IR)counterparts,andhenceaccuratepositions,forovertwothirdsoftheSCUBAHAlf-DegreeExtragalacticSurvey(SHADES)SourceCatalogue,presentingoptical,24-µmandradioimagesofeachSMG.Observedtrendsinidenti cationratehavegivennostrongrationaleforpruningthesample.Uncertaintiesinsubmmpositionarefoundtobeconsistentwiththeoreticalexpectations,withnoevidenceforsigni cantadditionalsourcesoferror.Employingthesubmm/radioredshiftindicator,viaaparameterisationappropriateforradio-identi edSMGswithspectroscopicredshifts,yieldsamedianredshiftof2.8fortheradio-identi edsubsetofSHADES,somewhathigherthanthemedianspectroscopicredshift.Wepresentadiagnosticcolour-colourplot,exploitingSpitzerphotometry,inwhichweidentifyregionscommensuratewithSMGsatveryhighredshift.Finally,we ndthatsigni cantlymoreSMGshavemultiplerobustcounterpartsthanwouldbeexpectedbychance,indicativeofphysicalassociations.ThesemultiplesystemsaremostcommonamongstthebrightestSMGsandaretypicallyseparatedby2–6arcsec,～15–50/sinikpcatz～2,consistentwithearlyburstsseeninmergersimulations.

Keywords:galaxies:starburst–galaxies:formation–cosmology:observations–cosmol-ogy:earlyUniverse

2Ivisonetal.

1INTRODUCTION

Observationalcosmologyinthesubmmwavebandhasbeenoneofthefew eldsthatcanclaimtohavebeatenMoore’sLaw(Moore1965),theothernotableastronomicalexceptionbeingtheVirgoconsortium’s‘MillenniumSimulation’(Springeletal.2005).Ithasbene tedenormouslyfromthedevelopmentofbolometerarrayssuchasSCUBA(Hollandetal.1999)andMAMBO(Kreysaetal.1998):thecommissioningofthesegroundbreakingcameras,ontheJamesClerkMaxwellTelescope(JCMT)andtheIRAM30-mtelescope,respectively,yieldedathousand-foldincreaseinmap-pingspeedoversingle-pixeldevicessuchasUKT14(Duncanetal.1990).Adecadeon,thenextgenerationofcamerasexempli edbyLABOCA(Kreysaetal.2003)andSCUBA-2(Hollandetal.2003)willyieldasimilarincreaseinmappingspeedoverexistingarrays.

SCUBAbroughtaboutaradicalshiftinourunderstandingoftheformationandevolutionofgalaxies,withthediscoverythatlu-minous,dustygalaxieswereathousandtimesmoreabundantintheearlyUniversethanatthepresentday(Smail,Ivison&Blain1997;Hughesetal.1998;Bargeretal.1998;Ealesetal.1999).SCUBAwascapableofprovidingonlyapproximatecoordinatessoitwasimmediatelyclearthatthenatureofthesesourceswouldre-mainamysteryuntilmoreaccuratepositionscouldbedetermined–thesubjectofthispaper.Tore nepositionsprovidedbySCUBA,wearereliantonradioobservations;theradioemissionisahigh-resolutionproxyfortherest-framefar-IRemissionobservedinthesubmm(Ivisonetal.1998,2000,2002;Smailetal.2000;Webbetal.2003a;Clementsetal.2004;Dannerbaueretal.2004;Bo-rysetal.2004;Garrett,Knudsen&vanderWerf2005;Vossetal.2006).Althoughlikelytobeinef cientintheeraofSCUBA-2,ra-dioimagingalsoenabledlargesamplesofSMGstobeacquiredbytargetingopticallyfaintµJyradiosources(OFRS)usingSCUBA’sfastPHOTOMmode(Barger,Cowie&Richards1999;Chapmanetal.2002).

Mid-IRimagingwithSpitzerhasalsoprovedusefulforre n-ingSMGpositions(Egamietal.2004;Ivisonetal.2004;Popeetal.2006;Ashbyetal.2006),albeitwithpoorangularresolutionandanimpreciseconnectiontobolometricluminosity.Tobeuseful,suchdataneedtobeclosetothe24-µmconfusionlimit(～50µJy),soradioimagingislikelytoremainthepreferredprocedure.

Radioandsubmm uxdensities,takentogether,aresensitivetoredshift(Carilli&Yun1999;Dunne,Clements&Eales2000;Rengarajan&Takeuchi2001),albeitlimitedtoz<～3bythedepthofradioimagingavailablecurrently.ThisapproachisthesubjectofpaperIVinthisseries(Aretxagaetal.2007).EarlyworkinthisveinconstrainedthemedianredshiftoftheSMGpopulationtobez>～2(Carilli&Yun2000;Smailetal.2000;Ivisonetal.2002).

Thetruetriumphoftheradioidenti cationprocedure,how-ever,hasbeeninidentifyingthecorrectoptical/IRcounterpartssothattheirmorphologies,colours,magnitudes,etc.canbede-terminedunambiguously;moreimportantly,thishasalsoallowedspectroscopiststoplacetheirslitsaccurately,sometimesonappar-entlyblankskywhenopticalcounterpartsweretoofaintforexist-ingimaging(RAB>～26,e.g.LE850.12andSSA13.332–Chap-manetal.2005).Thispainstakingapproachwasslowtopaydivi-dends,withonlyahandfulofredshiftsreportedinitially(Ivisonetal.1998,2000;Bargeretal.1999;Ledlowetal.2002;Knudsen,vanderWerf&Jaffe2003;Simpsonetal.2004).Deeperradioob-servationsalliedwiththelargestexistingsubmmsurveysandtheOFRStechniqueresultedeventuallyintheacquisitionofapprox-imately100spectroscopicredshifts,themajoritybyChapmanetal.(2003,2005).Thishasenabledthedirectdetectionofcolossal

moleculargasreservoirsinarepresentativesampleofSMGs(Nerietal.2003;Greveetal.2005;Tacconietal.2006),followingonfromthepioneeringCOdetectionsofFrayeretal.(1998,1999).ItallowedAlexanderetal.(2005a,2005b)tosuggestthatthebulkoftheSMGpopulationcontainsobscured,oftenCompton-thick,activegalacticnuclei(AGN)viathe rstmeaningfulanalysisoftheirX-rayproperties;itpermittedarigoroustestoftheradio/far-IRrelationathighredshift,viaobservationsnearthepeakofSMGspectralenergydistributions(SEDs)at350µm(Kovacsetal.2006)and, nally,itallowedathoroughanalysisoftheirrest-frameop-ticalphotometricandspectroscopicproperties(Smailetal.2004;Swinbanketal.2004;Takataetal.2006).

Untilnow,themostadventurousblank- eldsurveyshavecov-eredafew×100arcmin2,detectingtypically40galaxies(Scottetal.2002;Webbetal.2003a;Borysetal.2003;Greveetal.2004).Thepropertiesofthesegalaxieswerequicklycharacterisedovertheentireobservablespectralrange(Lillyetal.1999;Ealesetal.2000;Gearetal.2000;Lutzetal.2001;Foxetal.2002;Ivisonetal.2002;Webbetal.2003a,2003b;Waskettetal.2003;Bo-rysetal.2004;Dunlopetal.2004;Popeetal.2005,2006),butitsoonbecameclearthatsomeofthekeyremainingquestions–thedegreeofclusteringandtheroleplayedbyAGN–couldonlybeaddressedbyasigni cantlylargersampleselectedhomogeneouslyfromcontiguoussky.

Despitethesteepslopeofthesubmmnumbercounts(Blainetal.1998,1999),the850-µmconfusionlimit–setataround2mJybytheJCMT’s15-mprimary–dictatesthatwemustmapmoreskyifwearetoobtainlargersampleswithwell-characterisedpositionsand uxdensities.SHADESaimedtodetect200SMGsovertwo0.25-degree2 elds–theLockmanHole(LH;10h52m,+57 .4)andtheSubaru-XMM-NewtonDeepField(SXDF;02h18m, 5 .0).SeeMortieretal.(2005),PaperIofthisseries,foradescriptionofitsmotivationanddesign.SCUBAwasretiredin2005July,beforeSHADEScouldbecompleted,aftertwoyearsplaguedbycryogenicproblems.TheSHADESSourceCatalogue,gleanedfrom800arcmin2andcomprising120SMGsintheLHandtheSXDF,ispresentedinPaperIIofthisseries(Coppinetal.2006).

Inthis,PaperIII,weidentifyradioand/ormid-IRcounter-partsandhenceaccuratepositionsfortheSHADESsampleusing1.4-GHzradioimagingfromtheNationalRadioAstronomyObser-vatory’s(NRAO1)VeryLargeArray(VLA)and24-µmdatafromMIPS(Riekeetal.2004)onboardSpitzer(Werneretal.2004).ThisisthecrucialstepthatallowsustomovefromthebasicpropertiesofanSMGsample–sourcecountsand2-dimensionalclustering–toanassessmentoftheirdetailedpropertiesacrosstheentireacces-siblewavelengthrange,theircontributiontothehistoryofcosmicstarformationandtheirlinkswithpresent-daygalaxypopulations.In§2wedescribethedataexploitedin§3to ndradioandmid-IRcounterpartsforourSMGsample.Weusetheseassociationsin§comparing4todeterminewiththeorythepositionaldevelopeduncertaintyinAppendixassociatedB.In§5withwediscussSMGs,SMGswithmultiple,robustcounterpartsandin§6weexploreiden-ti cationtrends.Finally,in§7and§8weutilisethemagnitudesandcoloursofSMGs,nowrobustlyidenti ed,toconstraintheirred-shiftdistributionandtoidentifyoutliers.Weassume m=0.27, Λ=0.73,H0=71kms 1Mpc 1throughout(Spergeletal.2003).

NRAOisoperatedbyAssociatedUniversitiesInc.,underacooperativeagreementwiththeNationalScienceFoundation.

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

57 45

4035SHADES

)

030LHEX4

002J( N25

OITAN20ILCED15LOCK-3

100500

10 5554

53RIGHT ASCENSION (J2000)

525150

-04 35

4045

SXDF1

)

005002J( N55OITANIL-05 00

CED05SXDF2

SXDF3

10152002 19 30

18 30RIGHT ASCENSION (J2000)

0017 30

0016 30

Figure1.IndividualpointingsforradiomosaicsintheSHADES elds,togetherwithanindicationofthesubmmcoverage.ThediameterofthecirclesistheFWHMoftheVLA’sprimarybeamat1.4GHz.

2OBSERVATIONS2.11.4-GHzradioimaging

Wide- eldradioimageswereobtainedusingtheVLA.TheLHdatausedhere,comprising75hrofintegrationona elddesignatedLOCKMAN-E,weredescribedindetailbyIvisonetal.(2002).Thedatahavesincebeenre-analysedbyBiggs&Ivison(2006),usingthe37-piecemosaicingtechniquedescribedbyOwenetal.(2005),togetherwithadditionalself-calibration.Theresultingimagecov-ersmostoftheprimarybeam,outtoaradiusof23arcmin.Oncecombined,thenoiselevelisunusuallyuniform,4.2µJybeam 1r.m.s.inthecentreofthe eld,witha1.3-arcsecsynthesisedbeam(FWHM).

Wealsoutiliseanewlow-resolutionmap,madebytaperingourLOCKMAN-Edatatogivea4.2-arcsecsynthesisedbeamandthenmosaicingwithB-con gurationdatatakenforseveralnearbypointings:anew eld,11arcmintothesouthwest,designatedLOCK-3,plusarchivaldatafor eldsdesignatedLHEX1,LHEX2,LHEX3andLHEX4,whereLHEX4comprises31hrofintegration,11arcmintothenortheastofLOCKMAN-E.Fig.1illustratesthemosaicofpointings.Thesedata,togetherwithmatched-resolution610-MHzimagingfromtheGiantMetre-waveTelescopeinPune,India,aredescribedbyIbaretal.(inpreparation).

Weobtainednew1.4-GHzdatafortheSXDF,againusingtheVLA,during2003.Manyofthesedatawereaffectedbyinterfer-

c0000RAS,MNRAS000,000–000enceandbyaprolongedfailureofthecorrelator,buttheequiva-lentofaround60hrofnormalintegrationweresalvaged.TheseA-con gurationdatawerecombinedwiththeB-andC-con gurationdatadescribedbySimpsonetal.(2006)resultingina9:3:1ratioofrecordedA:B:Cvisibilities,evenlydistributedinthreepoint-ingsseparatedby15arcmin(seeFig.1).Eachpointingwasimagedasa37-piecemosaic,aswiththeLH.The nalimagewasknit-tedtogetherandcorrectedfortheresponseoftheprimarybeamusingtheAIPStask,FLATN.Theresultingnoiselevelisaround6.3µJybeam 1inthebestregionsofthemap,thoughashighas8.4µJybeam 1nearbright,complexradioemitters,withasynthe-sisedbeammeasuringaround1.7arcsec(FWHM).AswiththeLH,wealsoutilisealow-resolutionmap,taperingourentiredatasettogivea4.2-arcsecsynthesisedbeam.

2.2Opticalimaging

R-bandopticalimagingfortheLHandSXDFwereobtainedusingtheSubaru8-mtelescope.TheLHdataweretakenfromthearchiveandaredescribedinIvisonetal.(2004)andreacha3σdepthof27.7mag;similardataforSXDFaredescribedbyFurusawaetal.(inpreparation),reachinga3σdepthof27.5mag(bothontheVegascale,for2-arcsec-diameterapertures).

2.3Near-andmid-IRimaging

Thenear-andmid-IRdataemployedherewereobtainedusingIRAC(at4.5and8µm)andMIPS(at24µm).Theimagingcov-erstheentireSHADESregionoftheLHtonear-uniformdepthsofσ=0.54,4.4and11µJyat4.5,8and24µm,respectively(Egamietal.,inpreparation),with uxcalibrationaccurateto±10percent,thatisapproximately3×deeperat24µmthanthedatausedbyEgamietal.(2004),Serjeantetal.(2004)andIvisonetal.(2004).IntheSXDF,IRACandMIPSdataareavailablefromtheSpitzerWide-areaInfraRedExtragalactic(SWIRE–Lonsdaleetal.2003)surveyandreachanear-uniformdepthofσ=1.1,7.5and48µJyat4.5,8and24µm(Shupeetal.,inpreparation).Forcomparison,the5-σconfusionlimitat24µm,with20beamspersource,isaround56µJy.

3ASSOCIATIONSBETWEENSUBMMGALAXIESAND

RADIO/MID-IRSOURCESObservationsinthesubmmwavebandaresensitivetocolddustcreatedforthemostpartbysupernovae(SNe)andstellarwinds,re-radiatingenergyabsorbedfromhot,youngstars(Whittet1992).TheradiowavebandisalsosensitivetoSNe–andhencetorecentstarformation–viasynchrotronradiationfromrelativisticelec-trons.Near-IRwavelengthsprobephotosphericemissionfromstarsandinthemid-IR,at24µm,wearesensitivetoemissionfromdustinthecircumnucleartorusofAGNandtothewarmestdustinstar-bursts.ThecorrelationbetweensubmmandradioemissionfromSMGsispoorerthanexpected(fromlocalstudies–e.g.Dunneetal.2000),probablyduetoawiderangeofcharacteristicdusttemperaturesandtotheeffectofradio-loudAGN;nevertheless,predictingtherest-framefar-IRpropertiesofSMGsisbetterac-complishedfromtheradioendoftheSEDthanfromthenear-ormid-IR,addingtothebene tofveryhighspatialresolution(～0.1arcsec)availableatradiowavelengths(Chapmanetal.2004;Muxlowetal.2005;Biggsetal.,inpreparation)andmakingitthe

4Ivisonetal.

Table1.RadiopropertiesofSMGsintheLockmanHoleSHADESSourceCatalogue.

Nickname

Positionat850µmαJ2000δJ2000

′′′

hms105201.417105257.316105238.247105204.171

+572443.04+572105.79+572436.54+572658.85

S850µm(S+,S )/mJy8.8(1.0,1.0)13.4(2.1,2.1)10.9(1.8,1.9)10.6(1.7,1.8)

SNRa

Positionat1.4GHzαJ2000δJ2000

′′′

hms105201.249105257.014105257.084105238.401105238.299105203.691105204.079105204.226

—

105204.013105203.549105300.956

—

105215.636105248.992105129.824105227.579105228.793

—

105228.995105230.717105319.025105319.271105319.067105151.690105150.113105158.018105227.778105228.995

————

105200.445105240.726105240.698105241.453105204.579105257.667105131.305105207.490105208.054105215.989105155.470105213.584105214.202105246.655

—

105124.595105124.342105307.253105306.568105306.933105225.643105201.721105202.070105200.248105159.760105256.561105256.655105256.576105235.138105255.181105245.808

—

105143.488105154.261105252.231105253.121

——

105147.894

+572445.76+572108.31+572102.82+572439.50+572435.76+572707.06+572658.52+572655.46

—

+572524.20+572517.38+572552.06

—

+572504.26+573256.26+572415.19+572512.46+572516.01

—

+572222.42+572209.56+572109.47+572108.45+572116.28+572636.09+572635.73+571800.27+572218.18+572222.42

————

+572040.16+572315.18+572309.96+572320.65+571806.11+573058.71+572040.28+571904.01+571902.58+571619.34+572312.77+573320.81+573328.30+572052.54

—

+572331.08+572336.18+572430.82+572432.65+572427.27+571607.65+571917.00+571923.13+572421.69+572424.94+572352.80+572354.13+572358.62+572516.04+573245.38+573119.86

—

+572435.90+572502.55+573232.39+573240.22

——

+573044.37

bS1

.4GHz

/µJy78.9±4.740.7±5.652.4±5.635.0±5.225.8±4.947.0±5.732.0±5.173.0±5.05σ<2215.0±4.822.2±4.642.6±5.85σ<2252.6±4.725.5±6.319.0±5.444.3±5.119.2±4.55σ<2825.3±4.237.4±4.243.9±7.861.5±7.622.6±7.1106±6115±692.3±4.529.4±4.425.3±4.15σ<275σ<305σ<305σ<2528.5±4.814.5±5.531.4±5.21,050±5020.0±4.563.0±8.223.7±4.9245±1320.0±4.243.0±4.751.0±4.328.7±8.758.4±8.517.4±5.05σ<2014.8±5.441.8±8.724.4±6.713.8±6.520.9±6.25σ<2016.2±4.318.0±4.943.6±4.722.1±4.825.4±5.419.4±5.540.8±5.95σ<2243.7±10.038.7±8.05σ<2122.3±4.922.6±4.845.5±7.431.7±7.45σ<215σ<2121.9±7.2

Submm–radioseparationc/arcsec

3.043.513.513.210.89(9.08)0.813.42—2.34(10.1)4.22—3.653.87(10.3)0.67(9.98)—(11.3)7.741.842.265.742.12(10.9)1.960.81(11.5)————2.753.652.89(9.55)(11.0)(9.99)5.323.524.561.854.217.365.634.27—5.332.161.334.514.35—2.667.343.661.575.014.76(8.33)—(8.53)3.05—(10.1)3.82(10.4)(10.8)——5.52

Notes

LOCK850.01LOCK850.02LOCK850.03LOCK850.04

8.546.836.396.42

LOCK850.05LOCK850.06LOCK850.07LOCK850.08LOCK850.09LOCK850.10LOCK850.11LOCK850.12LOCK850.13LOCK850.14LOCK850.15

105302.615105204.131105301.403105153.862105216.088105248.607105129.531105227.612105132.333105230.110105319.200

+571826.95+572526.34+572554.24+571839.75+572504.11+573258.58+572405.21+572513.08+573134.76+572215.55+572110.64

8.1(2.0,2.1)6.8(1.3,1.3)8.5(1.8,1.9)5.4(1.1,1.2)5.9(1.6,1.6)9.1(2.7,2.9)6.2(1.7,1.8)6.1(1.7,1.7)5.6(2.3,2.9)7.2(1.8,1.9)13.2(4.3,5.0)

4.905.835.305.244.674.534.534.583.894.844.51

LOCK850.16LOCK850.17LOCK850.18LOCK850.19LOCK850.21LOCK850.22LOCK850.23LOCK850.24LOCK850.26

105151.453105158.250105227.693105235.709105256.858105137.551105213.737105200.227105240.950

+572637.00+571800.81+572217.75+573119.05+573038.05+573323.32+573154.11+572038.05+572312.01

5.8(1.8,1.9)4.7(1.3,1.3)6.0(1.9,2.1)5.1(2.0,2.4)4.1(2.0,2.5)7.5(3.2,4.2)4.3(1.9,2.4)2.7(1.2,1.2)5.8(2.4,2.9)

4.324.494.273.923.624.003.713.603.93

LOCK850.27LOCK850.28LOCK850.29LOCK850.30LOCK850.31LOCK850.33LOCK850.34LOCK850.35LOCK850.36LOCK850.37LOCK850.38

105203.574105257.001105130.923105207.786105216.055105155.975105213.502105246.915105209.335105124.130105307.104

+571813.46+573107.14+572035.95+571906.59+571621.11+572311.76+573328.14+572056.25+571806.78+572334.86+572431.39

5.0(1.3,1.3)6.4(1.7,1.8)6.7(2.0,2.2)4.7(1.5,1.6)6.0(1.8,2.0)3.8(1.0,1.1)14.0(3.1,3.2)6.1(2.2,2.4)6.3(1.7,1.8)7.5(2.9,3.5)4.3(2.2,2.7)

4.634.674.394.194.344.455.374.124.554.103.63

LOCK850.39LOCK850.40LOCK850.41LOCK850.43

105224.851105202.014105159.861105257.169

+571609.80+571915.80+572423.60+572351.81

6.5(2.2,2.5)3.0(1.1,1.2)3.8(0.9,1.0)4.9(2.1,2.6)

4.203.794.543.80

LOCK850.47LOCK850.48LOCK850.52LOCK850.53LOCK850.60LOCK850.63LOCK850.64LOCK850.66LOCK850.67LOCK850.70

105235.629105256.239105245.531105240.488105143.583105153.906105251.808105138.687105208.998105148.516

+572514.04+573245.82+573121.94+571928.42+572445.97+572505.07+573242.23+572017.24+572355.13+573046.69

3.5(1.7,2.1)5.4(2.1,2.5)3.9(2.2,2.7)4.4(2.3,2.9)3.1(1.7,2.0)3.6(1.2,1.3)5.8(2.5,3.2)4.2(1.9,2.2)2.5(1.5,1.5)3.8(2.2,2.5)

3.543.943.523.623.404.003.873.743.303.52

0.0110.0260.0200.0270.005(0.104)0.0040.014—0.038(0.176)0.032—0.0210.048(0.181)0.002(0.180)—(0.178)0.0680.0090.0090.0710.004(0.059)0.0040.004(0.178)————0.0260.0640.026(0.004)(0.181)(0.091)0.0660.0040.0640.0100.0270.0750.0350.065—0.0780.0130.0110.0750.059—0.0420.0750.0260.0150.0600.067(0.105)—(0.103)0.023—(0.176)0.049(0.124)(0.159)——0.070

z=2.148

z=3.036z=1.48

24µmid

Confusedat24µmz=2.14?

z=2.611

z=1.147z=2.239z=1.95624µmid24µmid24µmid

CandidateidRadio+24µmidz=2.689

z=2.686

Radio+24µmid

24µmid

z=0.68924µmidTrainwreck?24µmid24µmid24µmid

24µmid

a)Rawsignal-to-noiseratio(SNR),beforedeboosting.

b)Integrated uxdensities;fortentativedetections,thesearegiveninitalics.

c)Possiblecounterpartswith8.0–12.5-arcsecoffsetsarelistedinparenthesesforcompleteness.Reliableidenti cations(P 0.05)arelistedinbold.

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

Table1.Cont...

Nickname

Positionat850µmαJ2000δJ2000

′′′

hms105218.618105141.660105315.927105148.516105157.004105145.333105152.104105231.989105307.939105153.302105139.056

+571903.79+572217.63+572645.47+572838.69+572210.07+571738.68+572127.38+571800.40+572839.14+571733.38+571509.81

S850µm(S+,S )/mJy3.9(1.8,2.0)3.5(1.9,2.3)4.4(2.2,2.6)4.7(2.5,3.1)3.2(1.2,1.3)4.5(2.2,2.7)3.1(1.3,1.5)5.3(1.9,2.3)3.1(2.0,2.1)3.4(1.5,1.7)11.2(4.2,5.3)

SNR

Positionat1.4GHzαJ2000δJ2000

′′′

hms105219.086105141.705105141.992105315.439105149.101105157.153105157.665

—

105152.594105151.198105231.523

—

105153.365105138.877

+571857.87+572220.10+572217.52+572637.42+572840.28+572209.58+572212.35

—

+572124.43+572127.29+571751.67

—

+571730.05+571503.90

S1.4GHz

/µJy95.8±4.626.7±4.627.3±4.827.1±7.848.0±6.015.5±4.439.5±7.85σ<2322.4±4.526.3±4.655.2±5.35σ<2884.5±5.319.8±6.3

Submm–radioseparation/arcsec

7.032.502.69(8.96)4.981.305.81—4.947.33(9.51)—3.376.09

Notes

LOCK850.71LOCK850.73LOCK850.75LOCK850.76LOCK850.77LOCK850.78LOCK850.79LOCK850.81LOCK850.83LOCK850.87LOCK850.100

3.693.483.683.663.843.703.654.013.373.644.300.0300.0250.027(0.150)0.0360.0170.050—0.0640.077(0.096)—0.0120.077

Radio+24µmid

24µmidPlausibleid24µmidRadio+24µmid

wavebandofchoicefortheidenti cationofcounterpartsatotherwavelengthsandseveralrelatedobjectives.

Aradiosourcepeakingat 4σwithanintegrated uxden-sityinexcessof3σ,ineitherthehigh-orlow-resolutionimages,whereσisthenoisemeasuredlocally,isconsideredarobustdetec-tion.IntheLHandSXDF,thesurfacedensitiesofallradiosourcesabovethisthresholdare1.9±0.1arcmin 2(Ivisonetal.2005)and1.6±0.1arcmin 2,respectively.Wherearobustdetectiondoesnotexist,welistthosesourcespeakingat 3σwithanintegrated uxdensityinexcessof2σ,thesebeingconsideredtentativedetec-tions.Positionsand uxdensitiesweremeasuredusingJMFITwithmulti-component ts:usuallyaGaussianandasurfacebaseline,withanextraGaussiancomponentforclosemultipleradiosources.Toenableustomakeappropriatecorrectionsforbandwidthsmear-ing–theradio avourofchromaticaberrationwhichcausesthepeak uxdensitytofallasafunctionofdistancefromthepointingcentre–measurementsweremadeinimagesofeachpointingratherthaninthe nal,largemosaic.Incaseswheresourcesappearedinmorethanone400-arcmin2pointing,error-weightedmeanswereobtained.

ForeachSMGwehavesearchedforpotentialradio(1.4-GHz)counterpartsinsideapositionalerrorcircleofradius8arcsec(see§4),alsolistingthosewithin12.5arcsecforcompleteness.Thisrelativelylargesearchareaensuresthatnorealassociationsaremissed.Attheextremedepthsreachedbytheradioimagingre-portedhere,thecumulativesurfacedensityofradiosourcesinthe8-arcsec-radiuserrorcirclesyieldsonerobustsourceineverytensearchareas,thoughnotallwillberegardedassigni cantassocia-tionsasweshallseeshortly.

Wehavealsosearchedforpotential24-µmcounterpartsin-sideapositionalerrorcircleofradius8arcsec,listingthosewithin15arcsecforcompleteness(aslightlylargerradiusthanforthera-diocounterpartstoaccountforthelarger24-µmbeam).

Toquantifytheformalsigni canceofeachofthepotentialsubmm/radioandsubmm/mid-IRassociationswehaveusedthemethodofDownesetal.(1986;seealsoDunlopetal.1989).ThiscorrectstherawPoissonprobability,P,thataradioor24-µmsourceoftheobserved uxdensitycouldlieattheobserveddis-tancefromtheSMG,forthenumberofwaysthatsuchanappar-entlysigni cantassociationcouldhavebeenuncoveredbychance.

Thepositions, uxdensitiesandPvaluesofallLHandSXDFradioand24-µmcounterpartsarepresentedinTables1through4,adoptingthosecounterpartswithin8arcsecwithP 0.05asro-bust.Pvaluesforcounterpartswithlargerseparationsarelistedin

c0000RAS,MNRAS000,000–000

parenthesis,usingsearchradiiof12.5or15arcsecat1.4GHzand

24µm,respectively.Wehavealsosearchedforcaseswherecoin-cidentradioand24-µmcounterpartswithin8arcsechaveP1.4GHzandP24µm 0.10, ndingthreesuchcases.FigsA1andA2con-tain25-arcsec×25-arcsecpostagestampimagescentredontheLHandSXDFSMGpositions,respectively.AlternativenamesusedfortheseSMGsintheliteraturearelistedinTable5.

Ouridenti cations–basedonradioand/or24-µmdata–aresummarisedinTable6.Clementsetal.(inpreparation)andDyeetal.(inpreparation)willpresentindependentidenti cationanalysesinSXDFandLH,respectively,usingopticalandnear-IRcolourswhicharebelievedtoprovideausefulcomplementtodeepradioimaging(e.g.Webbetal.2003b;Popeetal.2005).

Ofthe32identi cationsmadeinonlyonewaveband–equalnumbersineach eld–21areradiocounterparts,mainlyinSXDF.Ofthese21SMGs,onlysevenhavedetectionsat24µmthathavenotmadethegradeviathePstatistic.Ofthe11mid-IR-onlyidenti cations, vehaveradiocounterpartsjustaboveouradoptedP 0.05threshold.

Intotal,we ndrobustcounterpartsfortwothirds(79)ofthe120sourcesintheSHADESSourceCatalogue,entirelyconsistentwithpreviousstudies(Ivisonetal.2002,2005;Popeetal.2006).

4ONTHEUNCERTAINTYINSMGPOSITIONS

SCUBA-2willheraldavastincreaseinthenumberofcataloguedSMGs,coveringtensofsquaredegrees.Radiocoverageofsuchar-easatthedepthemployedherewillnotbetrivialtoacquire,evenintheeraofe-VLAandLOFAR.Itisinteresting,therefore,tospec-ulateaboutwhetherourknowledgeofpanchromaticSMGprop-ertieswillprogressintheabsenceofradiodetections(andhenceaccuratepositionsandcounterpartsatotherwavelengths)forthemajorityofSMGs.Canwedeterminethesigni canceofsubmmdetectionsrequiredtoenablespectroscopicfollow-upwithmodernintegral- eldunit(IFU)spectrometerssuchasKMOSonthe8-mVeryLargeTelescope(Sharplesetal.2006)?

SubmmpositionsfortheSHADESSourceCataloguewerede-ducedby ttingtothebeampatterninanoptimally lteredmap(i.e.aftersmoothingwiththebeam),thenaveragingoverfourinde-pendentreductionsofthesamerawdata(Coppinetal.2006).Onereductionadoptedthecentreofthenearest3-arcsecpixelastheposition,whiletheothersused1-arcsecpixels,soasmallround-ingerroraddstotheuncertainty.Ignoringthisminoreffect,thepositionaluncertaintyshouldbe α= δ=0.6θ(SNR) 1in

6Ivisonetal.

Table2.RadiopropertiesofSMGsintheSXDFSHADESSourceCatalogue.

Nickname

SXDF850.01SXDF850.02SXDF850.03SXDF850.04SXDF850.05SXDF850.06SXDF850.07SXDF850.08SXDF850.09SXDF850.10SXDF850.11SXDF850.12SXDF850.14SXDF850.15SXDF850.16SXDF850.17SXDF850.18SXDF850.19SXDF850.20SXDF850.21SXDF850.22SXDF850.23SXDF850.24SXDF850.25SXDF850.27SXDF850.28

SXDF850.29SXDF850.30SXDF850.31SXDF850.32SXDF850.35SXDF850.36SXDF850.37SXDF850.38SXDF850.39SXDF850.40SXDF850.45SXDF850.47SXDF850.48SXDF850.49SXDF850.50SXDF850.52SXDF850.55SXDF850.56SXDF850.63SXDF850.65SXDF850.69SXDF850.70SXDF850.71SXDF850.74SXDF850.76SXDF850.77SXDF850.86SXDF850.88SXDF850.91SXDF850.93SXDF850.94SXDF850.95SXDF850.96SXDF850.119

Positionat850µmS850µmSNRa

Positionat1.4GHzSb1

.4GHz

Submm–radioPNotes

αJ2000δJ2000

(S+,S )αJ2000δJ2000

separationhms ′′′

/mJyhms ′′′

/µJy/arcsec

021730.531 045936.9610.4(1.5,1.4)7.35021730.629 045936.7054.3±9.71.490.005021803.509 045527.2410.1(1.6,1.6)6.62021803.556 045527.5566.2±10.90.770.001021742.144 045628.228.8(1.5,1.6)5.95021742.128 045627.6777.2±9.30.600.001021738.621 050337.474.4(1.7,2.0)3.88021738.680 050339.46185±122.180.002021802.876 050032.758.4(1.7,1.9)5.35021802.858 050030.91574±101.860.001021729.769

050326.81

8.2(2.2,2.2)

4.72

021730.224 050325.3766.6±12.76.950.034021729.926 050322.0147.4±10.85.340.033021729.753 050318.5092.9±9.6(8.31)(0.044)021738.921 050523.727.1(1.5,1.6)5.16021738.878 050528.0341.2±11.34.360.029021744.432 045554.726.0(1.8,1.9)4.39021744.137 045548.7252.0±9.57.450.042021756.422 045806.746.4(2.0,2.1)4.35021755.772 045814.3146.0±10.5(12.3)(0.110)021825.248 045557.217.7(2.6,3.1)4.24021824.975 045602.85149±126.970.017021825.797 045551.3147.4±10.4(10.1)(0.094)021725.117 045937.444.5(1.9,2.2)3.81021725.101 045933.7756.8±10.03.680.018021759.369 050503.745.7(1.7,1.8)4.34021759.294 050504.0442.0±10.81.160.004021819.256 050244.214.8(1.9,2.1)3.93021818.748 050249.2530.4±11.4(9.11)(0.109)021819.018

050248.90

40.0±11.15.890.040021815.699 045405.226.2(1.6,1.6)4.76—

—

5σ<37——021813.887 045741.744.8(1.7,1.8)4.10021813.805

045743.22

36.5±8.81.920.011021754.980 045302.837.6(1.7,1.7)5.25—

—

5σ<39——021757.790 050029.756.4(2.0,2.2)4.30021757.591 050033.6940.8±9.04.940.034021828.149 045839.214.3(1.8,2.1)3.79021827.782

045837.17

95.9±10.15.860.020021744.182 050215.974.4(2.0,2.2)3.78—

—

5σ<34——021742.803 050427.715.2(2.0,2.2)3.99021742.499

050424.50

690±505.560.002021800.379 050741.506.2(2.3,2.6)4.08—

—

5σ<36——021742.526 050545.475.2(1.7,2.0)4.12021742.455 050545.8871.3±10.11.140.002021734.578 050437.715.1(2.0,2.3)3.93021734.696 050439.1835.3±10.32.300.014021734.749

050430.47

42.3±12.07.680.047021812.120 050555.744.0(2.1,2.5)3.58—

—

5σ<38——021807.861 050148.495.6(2.0,2.3)4.08021807.934 050145.38316±123.300.002021807.043

045915.50

4.8(2.2,2.7)

3.76

021806.920 045912.7296.7±10.43.340.009021806.831 045917.5296.2±9.63.760.011021806.419 045920.0557.9±9.0(10.4)(0.085)021816.468 045511.825.3(1.8,1.9)4.15021816.484 045508.66245±93.170.003021740.305 050116.225.7(2.0,2.2)4.14021740.020 050115.3229.3±11.34.350.037021736.301 045557.466.0(1.7,2.0)4.37021735.856

045555.10

55.9±11.87.070.039021722.888 050038.106.0(2.4,3.0)3.96—

—

5σ<40—–-021800.888 045311.245.3(1.8,2.1)4.06021800.867

045305.71

45.1±11.35.540.035021832.272 045947.215.4(1.8,1.9)4.20—

—

5σ<38——021724.445 045839.934.5(2.2,2.6)3.71021724.569 045841.2940.9±9.22.300.013021825.427 045714.713.8(2.3,2.7)3.49021825.176

045719.70

49.8±18.26.250.037021750.595 045540.164.0(1.7,2.1)3.69—

—

5σ<37——021729.669 050059.213.6(1.5,1.6)3.78021729.625

050058.57

40.3±9.50.920.003021829.328 050540.7121.9(6.2,6.8)4.92—

—

5σ<40——021733.887 045857.713.0(1.6,1.9)3.39

021734.363 045857.23175±117.150.015021734.400 045859.7643.1±10.17.950.048021733.616

045858.21

64.2±13.24.090.018021724.621 045717.687.6(2.5,2.9)4.28——5σ<39——021820.259 045648.473.3(2.0,2.2)3.43—

—

5σ<35——021802.858 045645.495.3(2.0,2.5)3.93021802.827 045647.8038.8±12.72.360.014021804.896 050453.743.2(1.8,2.1)3.41021805.118 050452.1289.3±11.13.690.011021804.972 050501.0288.8±10.37.370.029021752.190 050446.503.9(2.2,2.7)3.52021751.865

050446.96

41.7±13.84.880.033021750.679 050631.823.6(2.2,2.5)3.47——5σ<40——021745.802 045750.494.1(1.7,2.1)3.73——5σ<38——021807.935 050403.244.3(1.9,2.4)3.70——5σ<27——021751.395 050250.823.6(2.1,2.4)3.49——5σ<38——61.4±10.4µJy,13.0′′toSSW021811.199 050247.164.0(1.9,2.3)3.64——5σ<29——021821.235 045903.224.1(1.9,2.4)3.66—

—

5σ<35——24µmid

021758.732 045428.833.3(1.8,2.1)3.45021758.729 045433.4138.9±12.74.580.032021755.781 050621.824.4(2.0,2.4)3.73021756.308 050624.9184.2±13.1(8.46)(0.049)021736.432 050432.153.0(2.0,2.1)3.35021735.951 050425.9743.8±10.7(9.48)(0.093)021736.175

050433.26

34.0±9.94.000.047021817.184 050404.703.6(1.9,2.2)3.54—

—

5σ<37——021800.994 050448.494.5(2.1,2.5)3.74021801.494

050443.74

40.8±9.3(8.85)(0.091)54.5±10.8µJy,12.6′′toESE

021734.808 045723.933.5(2.1,2.5)3.43——5σ<35——021733.082 045813.483.1(2.0,2.1)3.36——5σ<28——021740.079 045817.734.1(1.8,2.1)3.75——5σ<41——021741.715 045833.703.4(1.9,2.2)3.47—

—

5σ<35——021800.000 050212.754.7(2.1,2.5)3.79021800.238 050216.8337.5±8.45.410.03985.3±9.9µJy,12.7′′toNNW

021756.345

045255.24

4.5(2.1,2.5)

3.73

021756.205 045303.3671.9±8.7(8.39)(0.056)021756.005

045251.96

38.0±9.7

6.06

0.043

a)RawSNR,beforedeboosting.

b)Fluxdensitiesfortentativedetectionsaregiveninitalics.

c)Possiblecounterpartswith8.0–12.5-arcsecoffsetsarelistedinparenthesesforcompleteness.Reliableidenti cations(P 0.05)arelistedinbold.

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

Table3.Mid-IRpropertiesofSMGsintheLHSHADESSourceCatalogue.

Nickname

Positionat24µmIDa

S24µmOffPb

αJ2000δJ2000

/µJy-set

hms ′′′

′′

LOCK850.01105201.30+572446.11934217±163.200.024LOCK850.02105257.07+572102.919460545±313.510.010LOCK850.03

105238.66+572443.71745173.7±21.37.900.196105238.31+572439.520054183±333.000.026105238.31+572434.820603175±231.810.012LOCK850.04

105204.21+572655.615970261±733.270.020105204.04+572658.315971179±681.190.006105203.67+572707.037071,104±33(9.10)(0.026)LOCK850.05105302.86+571823.91192158.6±15.13.640.107LOCK850.06

105204.12+572525.81192275.1±12.70.550.005105203.51+572517.117409379±18(10.5)(0.107)105205.19+572522.920753540±48(9.22)(0.060)LOCK850.07105300.97+572552.25670341±214.050.021LOCK850.08105153.69+571834.91811481±255.050.021LOCK850.09105215.73+572501.713577159±733.760.043105215.65+572504.513578466±743.560.012LOCK850.10105247.39+573257.91608865.9±11.1(9.82)(0.429)105248.27+573251.01760479.6±10.8(8.05)(0.313)LOCK850.11

105129.16+572406.88740112±573.390.053105129.39+572410.38741177±515.220.063105129.81+572416.38742111±17(11.3)(0.349)LOCK850.12105227.60+572512.43757263±190.690.001LOCK850.13105131.45+573129.111931240±17(9.09)(0.137)105131.77+573141.211932172±147.880.110LOCK850.14105230.72+572209.45560188±167.880.102105229.06+572221.85563103±13(10.5)(0.343)LOCK850.15105319.26+572108.33834353±202.390.009105318.99+572115.6383670.4±12.15.240.141LOCK850.16105151.67+572636.03626314±242.020.008LOCK850.17105158.48+571801.21338764.2±26.11.900.040105157.96+571759.917315239±182.520.015LOCK850.18105229.06+572221.85563103±13(11.8)(0.381)LOCK850.19

105236.09+573119.613661118±153.120.045105235.52+573111.717536242±197.510.076105235.06+573123.717539221±367.000.075LOCK850.21105256.79+573037.9283297.9±14.10.570.004105257.80+573035.32833124±18(8.07)(0.218)LOCK850.22105137.09+573316.92895402±217.410.045105136.68+573332.82896377±20(11.8)(0.127)LOCK850.23105212.83+573200.52722116±17(9.70)(0.288)105214.71+573154.71751657.3±11.37.860.213LOCK850.24105200.45+572039.71842455±212.450.007LOCK850.26105241.13+572319.823975.9±12.77.920.193105240.66+572309.75601195±163.290.029LOCK850.27105203.45+571819.31984106±155.930.117105204.77+571805.91986196±13(12.3)(0.247)LOCK850.28105257.69+573058.613901252±14(10.2)(0.154)LOCK850.29105131.65+572040.818689111±147.630.149LOCK850.30105207.68+571904.12004233±192.630.016LOCK850.31105215.96+571619.23434467±192.060.005LOCK850.33105155.40+572312.91917104±144.790.091LOCK850.34

105213.66+573321.3293293.5±12.06.960.153105214.21+573327.9293384.9±16.75.700.134105213.97+573332.82934128±195.990.101LOCK850.35

105246.46+572056.815351.0±12.73.720.124105245.94+572051.415952161±14(9.26)(0.206)105246.42+572106.615953110±38(11.1)(0.346)105246.91+572106.115954108±34(9.85)(0.309)105247.94

+572101.3

1955575.0±11.4(9.71)(0.393)LOCK850.36———5σ<60——LOCK850.37105124.60+572331.01870250±175.420.047105124.27+572341.417334126±166.640.116LOCK850.38105307.06

+572431.6

5682260±160.410.001LOCK850.39———5σ<60——LOCK850.40105201.54+571915.9199491.9±15.03.840.077105203.07+571923.5199785.2±14.0(11.5)(0.422)LOCK850.41

105200.24+572421.513508475±373.710.013105159.81+572425.113509651±461.560.002105159.27+572413.317394108±15(11.4)(0.358)105200.19+572415.317395212±22(8.72)(0.147)LOCK850.43105256.64+572351.45780261±244.300.031105256.61+572358.05781456±357.660.042LOCK850.47

105234.85

+572504.6

17453107±16(11.3)(0.359)

a)UsedtoidentifysourcesinFig.A1.

b)Pwascalculatedusingasearchradiusof8arcsec.Forpossiblecounterpartswith8–15-arcsecoffsets,Pwascalculatedusingasearchradiusof15arcsec—thesevaluesarelistedinparentheses.Reliableidenti cations(P 0.05)within8arcsecarelistedinbold.

c0000RAS,MNRAS000,000–000Table3.Cont...

Nickname

Positionat24µmID

S24µmOffP

αJ2000δJ2000

/µJy

-set

hms ′′′

′′

LOCK850.48105256.03+573242.318826203±173.900.035105255.37+573246.52010585.2±13.77.030.165LOCK850.52105246.16+573120.218804561±865.360.019105245.76+573120.620079310±352.280.009LOCK850.53105240.29+571924.413519168±154.330.050LOCK850.60

105143.50+572435.81941150±15(10.2)(0.247)105143.90+572443.61351287.8±12.03.490.070105143.08+572452.21351382.5±13.47.440.176105143.81+572454.913514109±15(9.12)(0.282)LOCK850.63

105153.43+572506.2192553.0±13.04.010.130105154.27+572502.71931236±173.780.029105155.24+572459.3193279.1±12.0(12.2)(0.461)LOCK850.64

105251.67+573248.7274088.5±12.56.560.150105252.57+573248.9274153.1±11.8(9.06)(0.454)105252.32+573233.012103425±25(10.1)(0.089)LOCK850.66105139.57+572027.11336571.2±12.1(12.2)(0.484)LOCK850.67105208.07+572348.02044102±14(10.4)(0.340)105208.87+572356.32045108±141.560.017LOCK850.70105147.88+573044.62571106±125.530.108LOCK850.71105219.10+571857.33487181±207.570.100105219.53+571904.8348854.3±16.07.460.212LOCK850.73105141.92+572218.61855278±192.320.011LOCK850.75

105315.19+572645.95713147±175.960.089105315.02+572653.25714150±16(10.7)(0.260)105315.52+572637.116059262±18(8.99)(0.124)LOCK850.76105149.12+572840.12512592±265.070.016LOCK850.77

105156.99+572208.4360251.7±13.11.670.042105157.57+572213.43603154±155.660.080105156.23+572212.3360855.4±13.56.650.199LOCK850.78105143.93+571744.9173485.6±14.7(13.0)(0.462)LOCK850.79105151.22+572127.8188492.8±13.17.160.158105152.63+572124.41892292±185.200.037LOCK850.81105231.52+571751.6173533,667±51(9.59)(0.007)LOCK850.83105307.17+572840.02815344±256.260.041LOCK850.87105153.36+571730.51975399±222.920.011LOCK850.100

105138.76

+571504.7

1623118±135.650.101

thelimitwherecentroidinguncertaintydominatesoversystematicastrometryerrorsandforuncorrelatedGaussiannoise.Here, αand δarether.m.s.errorsinR.A.andDec.,respectively,θistheFWHMofthesubmmbeamandSNRisthesignal-to-noiseratioaftercorrectionfor uxboosting(seeAppendixBforderivations).

Wecanuseourradioassociations,whichshouldprovidenear-perfectpositions,tocheckwhethertheuncertaintiesinsubmmpo-sitionareconsistentwiththistheoreticalexpectation,giventhesizeoftheJCMT’sbeamandtheSNRofthe850-µmsources.

ForaGaussiandistributionofdistributionofradialoffsets(re r2errorsinR.A.andDec.,the

/2σ2

)peaksatσ(= α= δ).Thispeakboundsonly39.3percentofsources,with68percentoftheanticipatedradial√offsetslyingwithin1.51σ(closeto,butnotpreciselyequalto

α2+δ2.ThevalueofσobservedinFig.

2isapproximately3arcsecsoouradoptedsearchradiusof8arcsec(§3)correspondsto～2.5σandshouldthusinclude～95percentofallgenuinera-dioidenti cations;moreover,sincethetypicaldeboostedSNRofthesubmmsourcesis～3(Coppinetal.2006),thetheoreticalex-pectationisalsoσ～3arcsec(fromequation2ofAppendixBforθ=14.5arcsecandSNR=3).Itisclear,therefore,thattheob-serveddistributionofradialoffsetsfortheradioidenti cationsisatleastcomparablewiththeoreticalexpectations.

8Ivisonetal.

Table4.Mid-IRpropertiesofSMGsintheSXDFSHADESSourceCata-logue.

Nickname

Positionat24µmSa24µm

Off

αJ2000δJ2000

/µJy

-set

hms ′′′

′′

SXDF850.01021729.59 045936.6485±47(14.1)(0.109)SXDF850.02021803.54 045526.9313±470.580.001SXDF850.04021738.69 050339.2488±472.010.005SXDF850.05021802.83 050031.0956±471.880.002SXDF850.06

021729.77 050319.6873±477.210.017021729.91 050333.3179±476.820.060021730.15 050324.2532±476.260.023SXDF850.07021738.86 050529.1325±475.460.031SXDF850.08021743.98 045552.1221±477.260.056SXDF850.10021825.61 045559.2153±475.780.057021824.88 045603.3177±47(8.21)(0.132)SXDF850.11021725.16 045935.0195±472.520.017SXDF850.12021758.60 050503.8397±47(11.5)(0.101)SXDF850.14021819.58 050232.2256±47(13.0)(0.161)021818.77 050249.0240±47(8.70)(0.110)SXDF850.16021814.41 045749.0247±47(10.7)(0.136)SXDF850.17021755.11 045250.5174±47(12.5)(0.195)SXDF850.19021827.83 045836.7536±475.400.019SXDF850.21021742.54 050425.86,844±474.370.001SXDF850.24021734.87 050432.7381±476.640.034SXDF850.27021807.93 050144.8334±473.830.018SXDF850.28

021806.32 045914.3176±47(10.9)(0.175)021806.43 045920.3477±47(10.4)(0.075)021806.87 045912.4877±474.040.007SXDF850.29021816.49 045508.2971±473.630.005SXDF850.30021740.00 050115.1523±474.690.016SXDF850.31

021736.75 045610.4452±47(14.6)(0.120)021735.83 045556.7594±477.100.025021736.37 045603.4251±476.030.043SXDF850.32021722.58 050044.4168±477.800.066SXDF850.35021800.86 045306.6215±474.660.036SXDF850.36

021831.92 045959.1162±47(13.0)(0.205)021831.95 045953.2177±477.690.065021833.04 045941.4181±47(12.9)(0.195)021831.86 045937.3182±47(11.7)(0.181)SXDF850.37021724.41 045842.0183±472.140.015SXDF850.45021830.11 050535.4157±47(12.8)(0.206)SXDF850.47021734.37 045859.9298±477.560.048021733.72 045858.7250±472.690.015SXDF850.52021805.09 050452.7151±473.080.029SXDF850.56021751.23 050630.5299±47(8.34)(0.086)SXDF850.69021751.77 050258.6157±47(9.59)(0.168)021751.06 050302.8724±47(13.0)(0.068)SXDF850.71021821.28 045858.8404±474.470.019SXDF850.76021756.32 050625.5183±47(8.86)(0.140)SXDF850.77021736.02 050428.2726±477.320.021021736.51 050425.6295±476.650.042SXDF850.86021816.66 050400.0208±47(9.13)(0.131)SXDF850.88021801.54 050442.1446±47(10.4)(0.079)SXDF850.91021734.24 045714.3203±47(12.9)(0.184)SXDF850.94021740.26 045824.0187±476.830.059021739.24 045813.1198±47(13.4)(0.192)SXDF850.96021800.40 050201.5478±47(12.7)(0.097)SXDF850.119

021756.20 045302.1784±477.200.019021755.65 045258.0202±47(10.8)(0.158)021756.24

045250.9

275±474.620.029

a)Objectsmissinghere,butlistedin

Table2,haveupperlimitsof5σ<235µJyat24µm.

Wecanquantifythismorepreciselyintwoways.First,wecanusethedistributionofradialoffsetsforallradioidenti ca-tioncounterpartsandattempttocorrectstatisticallyforbackgroundcontamination:thedashedlineinFig.2representsthedistributionandabsolutelevelofarandomlydistributedradiopopulationwiththecountsseenintheLHradioimage(§2.1).Thenumberofradioidenti cationswithina6-arcsecradiusofthesubmmpositionsisseentoexceedtherandomlevelbyalmosttwoordersofmagnitude,whichgivesus(additional)con dencethatthevastmajorityoftheradioidenti cationsaretrulyassociatedwiththeSMGs.The nite

Table5.AlternativenamesfortheSHADESSourceCatalogue.

SHADES8-mJyaMAMBObBolocamcChapmandLOCKJ—LOCK–LE850.–LE1200.–

LE1100.–

SMMJ–105201+572443850.010100514105201.25+572445.7

105257+572105850.02–00401–

105238+572436850.030200108105238.30+572435.8

105204+572658850.0414003––105204+572526850.0604–––105153+571839850.0827104––105216+572504850.0929042––

105227+572513850.121600616105227.58+572512.4105230+572215850.140601005105230.73+572209.5105151+572637850.1607096–105151.69+572636.0105158+571800850.1703011–105158.02+571800.2105227+572217850.18–009–105227.77+572218.2

105200+572038850.2432–––105203+571813850.27–00704–105130+572036850.2911–––

105207+571906850.3012––105207.49+571904.0105155+572311850.3318012–105155.47+572312.7

105202+571915850.4021–––

105159+572423850.410801417105200.22+572420.2

105148+572838

850.76

–

a)Scottetal.(2002).

b)Greveetal.(2004);Ivisonetal.(2005).c)Laurentetal.(2005).d)Chapmanetal.(2005).

Figure2.HistogramsofpositionaloffsetsbetweenthepositionsoftheSMGsandthoseofthecounterparts(left:radio;right:24µm),inR.A.(α,thickblue),Dec.(δ,red)andbothtogether(black).Thedashedlinesshowtheexpecteddistributionandabsolutelevelforarandomlydistributedpopu-lationwiththeaveragecountsseenintheLHandSXDFimages.ThedottedlinesshowGaussian tswithσ=3.2arcsecwhichwereconstrainedtobecentredatα=δ=0arcsec.

searchradiuswithinwhichwehavehuntedforradiocounterpartsexplainswhytheobservednumberofcounterpartsfallsbelowthatpredictedforarandompopulationintheoutermostbinsofFig.2.NotethatFig.2usesalltheradioidenti cations,ratherthanjustthosewiththelowestPvalues,soanybiaspresentisdueonlytothe nitesearchradiiusedto ndradioemittersforthisanalysis(12.5arcsec).

Havingcorrectedtheobserveddistributionsfortheexpectedunrelated‘ eld’radiosources(thoseinthebackgroundandfore-ground),aGaussian tcentredatα=δ=0arcsec,showninFig.2,yieldsaFWHMof7.5±0.7arcsec(7.4±0.6arcsecifthecentroidisunconstrained).Thistranslatesinto α= δ=FWHM/2.354=3.2arcsec.Ourcorrectionfortheexpected‘ eld’sourcesshouldhavedealtwithanybroadeningduetoradiosourcesunrelatedtotheSMGs.ThemedianSNRoftheradio-detectedsam-pleusedinthisanalysisis3.0,aftercorrectionforMalmquist-typebias,whichimpliesthat α= δ=0.66θ(SNR) 1,adoptingθ=14.5arcsec,i.e.10percenthigherthanexpected.

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

Table6.Identi cationsummary.

NicknameRobustidenti cation?

LOCK850.01 SXDF850.01 LOCK850.02 SXDF850.02 LOCK850.03 SXDF850.03 LOCK850.04 SXDF850.04 LOCK850.05SXDF850.05 LOCK850.06 SXDF850.06 LOCK850.07 SXDF850.07 LOCK850.08 SXDF850.08 LOCK850.09 SXDF850.09LOCK850.10 SXDF850.10 LOCK850.11SXDF850.11 LOCK850.12

SXDF850.12 LOCK850.13SXDF850.14 LOCK850.14SXDF850.15LOCK850.15 SXDF850.16 LOCK850.16 SXDF850.17LOCK850.17 SXDF850.18 LOCK850.18 SXDF850.19 LOCK850.19 SXDF850.20LOCK850.21 SXDF850.21 LOCK850.22 SXDF850.22LOCK850.23SXDF850.23 LOCK850.24 SXDF850.24 LOCK850.26

SXDF850.25LOCK850.27SXDF850.27 LOCK850.28SXDF850.28 LOCK850.29 SXDF850.29 LOCK850.30 SXDF850.30 LOCK850.31 SXDF850.31 LOCK850.33 SXDF850.32LOCK850.34

SXDF850.35 LOCK850.35SXDF850.36LOCK850.36SXDF850.37 LOCK850.37 SXDF850.38 LOCK850.38 SXDF850.39LOCK850.39SXDF850.40 LOCK850.40 SXDF850.45LOCK850.41 SXDF850.47

LOCK850.43 SXDF850.48LOCK850.47SXDF850.49LOCK850.48 SXDF850.50 LOCK850.52 SXDF850.52 LOCK850.53 SXDF850.55

LOCK850.60SXDF850.56LOCK850.63

SXDF850.63LOCK850.64SXDF850.65LOCK850.66SXDF850.69LOCK850.67 SXDF850.70LOCK850.70 SXDF850.71 LOCK850.71 SXDF850.74 LOCK850.73 SXDF850.76LOCK850.75SXDF850.77

LOCK850.76 SXDF850.86LOCK850.77 SXDF850.88LOCK850.78SXDF850.91LOCK850.79 SXDF850.93LOCK850.81SXDF850.94LOCK850.83 SXDF850.95LOCK850.87 SXDF850.96 LOCK850.100

SXDF850.119

indicatesarobust(P 0.05)radioidenti cation. indicatesarobustidenti cationat24µm.

coincidentradioand24-µmemission(bothP 0.1)yieldsreliableidenti cation. indicatesmultiplerobust(P 0.05)identi cations.

closevisualinspectionofthedatarevealsmorethanonegoodidenti cation.

This

procedurewasreplicatedforthe24-µmcounterpartslistedinTables3and4,correctingforblank- eld,backgroundsourcedensitiesof4.5and1.2arcmin 2tolimitsof50and150µJy,respectively.Theresult,shownintherightpanelofFig.2,isawiderdistribution,borneoutbythebest- tGaussian:aFWHMof10.7±1.0arcsec,whenconstrainedtobecentredatα=δ=0arcsec,or α= δ=4.5arcsec.Thelowaccuracyofthe24-µmpositionsrelativetothosedeterminedat1.4GHzcanaccountformostoftheextrawidth.

c0000RAS,MNRAS000,000–000Figure3.Left:Cumulativedistributionofradialoffsetsbetweentheradioandsubmmpositionsforthe62statisticallysecure(P 0.05)radioidenti- cations.Thedashedlineshowsthepredicteddistribution(1 e r2/2σ2

)assumingthatthepositionaluncertaintyinR.A.orDec.isgivenbyσ=0.6θ/SNR,asdiscussedinAppendixB,wherewehaveusedthemeanSNRsforthesample.AKStestyieldsaprobabilityof0.57thatthedataareconsistentwiththemodel.Right:Thesamepredictedprobabilitydis-tribution(dashedline),thistimecomparedwiththecumulativedistributionforall83sourceswithcandidateradioidenti cations(i.e.includingthoseforwhichP>0.05).Thepoor tintheright-handplot–aKStestyieldsaprobabilityof0.0003thatthedataareconsistentwiththemodel–demon-stratestheimportanceofusingthePstatisticto lterthecandidatelistofassociations.

Asasecondwayofquantifyingthisapproach,wecancon-sideronlythesubsetof‘robust’radioidenti cations(P 0.05)

onthebasisthatthisshouldprovidethemostsecuremeasureofthetruedistributionofuncertaintyinthesubmmpositions.Thera-dialoffsetdistributionforthissubsetof62sourcesisshownintheleft-handpanelofFig.3,whereitiscomparedwiththepredicted

cumulativedistribution(1 e r2/2σ2

),usingσ=0.6θ(SNR) 1asdiscussedinAppendixB.Forthiscalculationwehaveadoptedθ=14.5arcsecandSNR=3.17(theaverageSNRforthedeboosted850-µm uxdensitiesofthese62sources).Itisclearfromthisplotthatthepredicteddistributionisinexcellentagreementwiththatobservedforthissecuresubsetofidenti edsources;indeed,aKolmogorov-Smirnov(KS)testyieldsa57-per-centprobabilitythatthedataareconsistentwiththemodel.Forcompleteness,theright-handpanelinFig.3showsthesamepredictedprobabilitydis-tribution,thistimecomparedwiththecumulativedistributionforall83sourceswithcandidateradioidenti cations,i.e.includingthoseforwhichP>0.05.ThesameKStestnowyieldsaproba-bilityoflessthan0.1percent.Theseplotsgivecon dencethattheradialoffsetdistributionofsecureidenti cationsisconsistentwiththatexpectedgiventheJCMT’sbeamandthedeboosted850-µm uxdensitiesoftheSHADESsourcesandthatthereisnoaddi-tionalsigni cantsourceofastrometricerrorinthesubmmmaps.TheyalsodemonstratetheimportanceofusingthePstatisticto lterthecandidatelistofassociationsforrobustidenti cations.

Inconclusion,thereisnoevidenceforsigni cantadditionalsourcesofpositionalerror.ForanSMGdiscoveredinasubmmsurveywhereaMalmquist-typebiascorrectionhasnotbeenap-plied,wecanparameteriseitspositionaluncertaintyasfollows.Itspositionhavingbeendeterminedaftersmoothingwiththebeam,a

circleofradius0.91θ(SNR2app 1/2

,forpower-lawcountsoftheformN(>f)∝f (2β

,βhas+a4))

68percentchanceofcontainingthesubmmemitter(whereSNRappistherawSNR,un-correctedfor uxboosting–seeAppendixB),or0.91θ(SNR) 1ifacorrectionhasbeenapplied.Thesecorrespondtoconventional1-σerrorcircles.

10Ivisonet

al.

Figure4.Deboostedsubmm uxdensityversustheangularseparationofthecounterpartsforSMGswithmorethanonerobustradioidenti cation(P 0.05)within8arcsecofthe850-µmposition.Pointsdenoteradiodoubles(D)andtriples(T).Theaverageofthesingle,unresolvedorbarelyresolvedradiocounterpartsisrepresentedbyadashedline.Thepaucityofdataatverylowandhighseparationsisduetoour nitespatialresolutionontheonehandandtoouruseofa nitesearchareaandthePstatisticontheother.Thehistogramshowsthedistributionofangularseparations,scaledarbitrarily,formultipleidenti cationsfoundintheMonte-Carlosimulationsdescribedin§5.

Wereturnnowtoourinitialmotivationforthisstudyofposi-tionaluncertainty,namelythefeasibilityofaspectroscopicredshiftdistributionforSMGsbasedonKMOSnear-IRspectroscopyofanunbiasedsample.SuchaprogrammecouldaffordtoloseoneSMGduetopositionalerrorduringasingledeploymentofthe24KMOSIFUs.EachIFUcovers2.8×2.8-arcsec2so,leavingroomforsee-ingeffects,werequire2σ～2.5arcsectoensurethat95.6percentofSMGsfallwithinthecentral5arcsec2ofeachIFU.Ourpa-rameterisationsuggeststhatthislevelofaccuracyrequiresanSMGsamplecutatSNR 20.AdoptingthesourcecountsofCoppinetal.(2006),asourcedensityof～2200deg 2–suf cienttoemployall24KMOSIFUs–wouldrequirethatweprobethe3-mJySMGpopulation;this,inturn,wouldrequirethatwedelvewellbelowthe850-µmconfusionlimittoensureSNR 20,orthatweutilisepositionsdeterminedusingthe450-µmdatathatareacquiredsi-multaneouslybySCUBA-2.OptimalexploitationofKMOSmayrequiresharingtheIFUswithotherprogrammesinallbutthedeep-estSCUBA-2survey elds.

5MULTIPLERADIOCOUNTERPARTS

AnumberofSMGswithmore

thanonerobust(P 0.05)radiocounterpartareapparentinTables1–2andFigsA1–A2:sevenintheLHand veintheSXDF.Thistendencyfor～10percentofSMGstohavemultipleradioidenti cationswasnotedpreviouslybyIvisonetal.(2002)andPopeetal.(2006).TheprobabilityofanSMGpossessingtwostatisticallysigni cantradiocounterpartswasquanti edbyplacing106fakesourcesintotherealLHandSXDFradio eldsandcountingthenumberofP<0.05radiocoun-terparts–asimpleMonte-Carloapproach.ThisrevealedthatthecalibrationofthePstatisticissecure,withP=0.05yielding5.05spuriousassociationsforevery100fakeSMGs.Multiplerobustcounterpartsarefarrarer,however.Forevery100fakeSMGsthesimulationssuggestthatonly0.22willhavemorethanonesecure

Figure5.Histogramofdeboostedsubmm uxdensityforthefullSHADESSourceCatalogue.Cross-hatchedareasrepresentthe12SMGswithtwoormoreradiocomponentswithin8arcsecofthe850-µmposition,associatedrobustlywiththeSMG(P 0.05);single-hatchedareasrepresentthesevenSMGswithmultiple,signi cant24-µmidenti cations.FiveSMGshavemultiple,signi cantradioand24-µmidenti cations.

radioidenti cationbychance,a guredominatedbydoubles,soat rstsighttheobservedtendencyformultiplerobustradiocoun-terpartsishighlysigni cant.However,weknowthataroundhalf(65)oftheSHADESSMGshavearealassociationwitharadioemitter,or59afteraccountingforthesixspuriousidenti cationsweexpect(0.05×120),soshouldwebesurprisedto ndadozenSMGswithmultipleradioidenti cations?Oftheradio-identi edSHADESSMGs,5percentwillbespuriouslyassociatedwithan-otherradiosource.Wethusexpectthreemultipleidenti cationswhereasweseeadozen:asigni cantdifference.

Lookingatthisanotherway,thefractionofradio-identi edSMGswithmultipleradiocounterpartsis18.5±5.3percent(12/65),15.4±4.9percent(10)withseparationsbelow6arcsec.Howfrequentaresuchcasesamongstthegeneralradiopopu-lation?TheproportionofradiosourcesintheSHADES eldswithradiocompanionswithin4,6,8and10arcsecare(cumula-tively)1.2±0.3,3.9±0.5,7.1±0.6and10.3±0.7percent(Pois-sonuncertainties).ThenumberofSMGswithseparationsbelow10arcsec,andparticularlybelow6arcsec,isthussigni cant.Inter-estingly,brightSMGsmakeuponeinsevenofallradiomultipleswithseparationsbelow6arcsec.

Whatcausesthismultiplicity?Atleastthreemechanismscouldberesponsible:AGN-drivenjets;physicalinteractions;andconfusion.

Discriminatingbetweenthesemechanismsisextremelydif -cult.The rst–jets–couldberevealedviatheirmorphologyortheirradiospectralindex,buttodateneitherpropertyhasbeenprobedforasigni cantsample.Thespectroscopicevidencere-quiredtorevealthesecondpossibility–aphysicalassociation–isavailableonlyrarelyintheSHADES elds,althoughanumberoflinked,multiplesystemswithfew-arcsecseparationsandnear-identicalredshiftshavebeendocumentedelsewhere(Ivisonetal.1998,2000;Ledlowetal.2002;Nerietal.2003;Smailetal.2003a;Chapmanetal.2005;Tacconietal.2006)whichleaveslittledoubtthatmanySMGswithmultipleradioidenti cationsareinteraction-drivenstarburstswithseparationsoften(orafewtens)ofkpc.

Fig.4showsaplotofsubmm uxdensityversusangularsep-

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

Figure6.Toprow:fromlefttoright,plotsofthecumulativeradio-identi edfractionfortheLHSMGsample( lledcircles)andtheSXDFSMGsample(opencircles)againstsubmmSNRbefore uxdeboosting,submmSNRafterdeboosting,850-µm uxdensityafterdeboostingand850-µmnoiselevel.Middlerow:thesameplots,butforsourceidenti cationat24µm.Bottomrow:thesameplots,allowingforidenti cationsat24µm,1.4GHz,orcoincidentweakemissionatbothassummarisedinTable6.

arationforthoseSHADESSMGswithmorethanoneradiocoun-terpartandweseenocontradictionoftheprevioustrend:twothirdsofthemultipleidenti cationshaveseparationsof2–6arcsec.How-ever,ourdataandourapproachbiasusagainst ndingsystemswithsmallerandlargerseparations,ascanbeseenbythedistributionofseparationsfoundforfakeSMGswithmultipleradiocounter-partsduringourMonte-Carlosimulations(Fig.4).High-resolutionradioimagingfromMERLINhasprovidedexamplesofmultiple,discreteradiosourcesseparatedby0.2–2arcsec(Chapmanetal.2004;Biggsetal.,inpreparation),thoughtheyarerare.

ThesizeoftheSHADESsurveyprovidesauniqueopportu-nitytoprobethethirdmechanism–confusion.Thesteepnessofthesubmmcountsmayyieldexampleswheretwoormorefaint,unrelatedSMGsshareasightlineandthusconspiretocreateaseeminglybrightSMG.ThereisapproximatelyoneSMGinthe2<S850µm<4mJy uxdensityrangeforevery4.3arcmin2ofsky,accordingtothedifferentialcountspresentedbyCoppinetal.(2006).Wethusexpect185±50suchsourcesintheSHADES elds.Theprobabilityofa2–4-mJySMGlyingwithin7arcsecofanothersourceis～1percent,sowecouldexpecttoseetwooftheseamalgamatedsourcesat uxdensitiesbetween4and8mJyintheSHADESsample.This uxdensityrangeaccountsfor62.5

c0000RAS,MNRAS000,000–000

percentofthefullsample,sowemightexpectaroundthreesuch

sourcesintotal(perhapsrathermoreifweincludedamalgamationsoffarmorecommon,faintersources).Ofthesethree,twoshouldhavearealradioidenti cation;onemayhaveseveral.Thedif cultywefaceinexploringthissmallsubsetofamalgamatedsourcesisinknowingwhichoftheSHADESSMGstheyare.Onepredictionmightbethattheyareexpectedtohavefaintercounterpartsatotherwavelengths,buteventhismaybepremature(Serjeantetal.2007).WemustcontentourselveswiththeknowledgethattheyshouldberevealedviaSCUBA-2450-µmimaginginthenearfuture.WithoutspectroscopicdatawecannotdeterminewhetherphysicalinteractionsorconfusionmakeupthemajorityoftheSMGswithmultipleidenti cations,letalonewhetherbrightSMGsarespecialcaseswheretwomassivecomponentsaremerging,assuggestedbySmailetal.(2003b).Themediandeboostedsubmm uxdensityoftheSHADESSourceCatalogueis5.0mJy;theerror-weightedmean850-µm uxdensityofSMGswithmorethanoneradiocounterpartis5.8±0.4mJy;thatforacomparisonsam-ple,the48SMGswithasingleP 0.05radiocounterpart,is5.4±0.2mJy,sothesimplestapproachyieldsnoevidenceofadifferencebetweenSMGswithsingleandmultipleidenti cations.Fig.5showsthedistributionofdeboostedsubmm uxdensityfor

12Ivisonetal.

thewholeSHADESSourceCatalogueandforthosesourceswithmultiplerobustcounterpartsat1.4GHzor24µm.Takingtheme-dianSHADES uxdensityasourthreshold,eightmultipleiden-ti cationslieaboveand11liebelow,respectively(sixapieceus-ingonlytheradio).However,asour uxdensitythresholdrisesto10mJysothefractionofsourceswithmultipleidenti cationsrisesfrom15/111to4/9(or8/111to4/9usingonlytheradio);evenig-noringthehighprobabilitythatoneoftheremaining vebrightsourcesmaybespurious(SXDF850.45)andthatanotherhassev-eralpossiblecounterparts(Lock850.34–Table6),thisisasignif-icanttrend.Itisplausiblethatthesesourcesareexamplesofcon-fusion(i.e.amalgamatedsources)butwenotethatthephysicallylinkedsystemsreportedtodateareoftensimilarlybright.

Weconcludethattheincidenceofveryhigh uxdensityandcounterpartmultiplicityareweaklylinkedandthatthecaseforapreferredseparationbetweenmultiplecounterpartsisplausiblebutnotproven.Inparticular,wenotethatalmosthalfofthebrightestnineSMGs–all>10mJy–havemultipleradiocounterpartsandthatallhaveseparationsintherange2–6arcsec,or20–70kpcattheirlikelyredshiftsandataninclinationof45 tothesky,per-hapsenablingef cientgasfuelingforcentralstarburstsorAGNviaoverlappinggalacticdisks—seethequalitativediscussionandillustrations(particularlyFigs11–13)inthemergersimulationsofSpringel,DiMatteo,&Hernquist(2005)whereaparticularlyin-tenseburstofactivityoccurson rstpassageforsystemsthatlackprominentbulges,withgalaxyseparationsof～30kpcforthesub-sequentfewtensofMyr.

6RADIOANDMID-IRIDENTIFICATIONTRENDSANDSUBMMSAMPLEREFINEMENTFollowingIvisonetal.(2002),weseektoexploittheclearpre-dictionthatspuriousSMGswilllackradioormid-IRcounterparts.Genuinesourcescan,ofcourse,evaderadioormid-IRdetection–becausetheylieatextremeredshift,forexample(seeIvisonetal.2005)–butgeneraltrendsintheidenti cationratemaybeevident.InthissectionwethereforeexplorewhatcanbelearnedabouttheSMGswithoutcounterparts.

Fig.6showsthecumulativeidenti cationrateforSMGsintheLHandSXDF eldsasafunctionofsubmmSNR(beforeandafter uxdeboosting),deboostedsubmm uxdensityandsubmm uxuncertainty.6.1Radiotrends

Lookingattheradioidenti cationtrendsasfunctionsofsubmm uxdensityandnoise,weseetherecoveryratetailingoffatthefaintest uxdensitylimits(<5mJy)inSXDF,whereastherateisremarkably atforfainter uxdensitiesintheLH eld.Both eldsshowimprovingidenti cationratesasthesubmmnoisedeclines,despitethedeboostingproceduresoutlinedinPaperII–aworryingtrend,thoughweshouldbearinmindthatsearchingforidenti -cationswithina xedradiusmustactasabiasagainstlow-SNRsources.Forthehighestvaluesofsubmm uxdensityandnoiseweseesimilaritieswithtrendsdiscussedbyIvisonetal.(2002)forthe8-mJySurvey,i.e.thebrightestsourceineach eldliesinaregionwithhighnoise,andneitherhasarobustradiocounterpart.

TheSXDFradioidenti cationrateversusrawsubmmSNRshowsasteepdeclinebelowanSNRof4;after uxdeboostingthiseffectismitigatedsomewhat,withmatchingtrendsintheSXDFandLH elds.Itisnoteworthythattheoverallradiorecoveryrate

inSXDFisover10percenthigherthanintheLH eld,despitetheshallowerdepthoftheSXDFradioimaging.Weattributethistothreeeffects,eachofwhichwebelievecontributestotheunexpect-edlylowLHidenti cationrate: rst,theLHradioimageisasinglepointing,designedoriginallytoidentifySMGsinthesmall8-mJySurvey eld(cf.amosaicofthreeinSXDF),sotheperniciousef-fectofbandwidthsmearingwillbeevidentforasigni cantlylargerfractionoftheSHADES eldinLHthaninSXDF;second,al-thoughitisclearlyusefultoworkwiththebestpossibleradiodata,deepimaginginevitablyyieldsmorefaint,unrelated,backgroundsources,causingPvaluesforrelativelybrightcounterpartstoriserelativetothosecalculatedforalowersourcedensity;third,itispossible(thoughithasyettobeshownunambiguously–Ivisonetal.2002;Chapmanetal.2004;Muxlowetal.2005)thatasignif-icantfractionoftheemissioninsomeSMGsisresolvedbyhigh-resolutionradiodata.Thattheseeffectsaresigni cant,collectively,isdemonstratedbythesigni cantlyhigherSMGidenti cationrateintheshallower,lowerresolutionSXDFdata;inaddition,sevenLHSMGs(LOCK850.10,.34,.37,.38,.40,.77and.100)aredetectedrobustlyonlyinthenoisier,low-resolutionradioimage,thoughwenotethatinseveralcasesthe4.2-arcsecFWHMimagealonedoesnotallowustodifferentiatebetweenplausiblespectroscopictar-gets.Thereareseverallessonshere:ensureinterferometricdatacontainanadequatefractionofshortspacings–asynthesisedbeamwith1.5–2arcsecFWHMprovidesagoodcompromiseforidenti -cationofFIR-luminousgalaxies;wherenecessary,i.e.whentheareaofinterestissimilartothatoftheradiointerferometer’spri-marybeamandthespectralresolutionispoor(δλ/λ<1000),obtaindatainacompactmosaicofpointingsratherthanasingle,deeppointing.

6.2Mid-IRtrends

Thetrendofoverallrecoveryrateisreversedinthemid-IR,theLHyieldinga20percenthigheridenti cationratethantheSXDF.Thereasonisobvious,however:itisduetothesubstantialextradepthoftheLHSpitzer24-µmdata(σ=11versus47µJy).OnlyoneSMGisidenti edsolelyonthebasisofitsmid-IRemissioninSXDFcomparedwithtenintheLH.Forboth eldsthedeclineatlowdeboostedSNRislessmarkedthantheradiotrend.Againstsubmm uxdensityandnoise,the24-µmidenti cationtrendsforboth eldsmatchthoseatradiowavelengths(withtheaforemen-tioned20percentoffsetfortheSXDFsources);theverybrightestsourcesagainlackrobustcounterparts.

6.3Overalltrends

ThelowerrowofplotsinFig.6showtheoverallidenti cationtrends–thefractionofsourcesidenti edat1.4GHzand/or24µm,includingthethreecasesmentionedin§3whereweakradioand24-µmcounterpartsarecoincident(oneofwhichisthebrightestLHsource,LOCK850.34).

Theidenti cationtrendsaresimilarforthetwoSHADES elds:identi cationisessentiallycompleteaboveadeboostedsubmmSNRof～4withanabruptstepdownto60–70percentthereafter;also,successratesimproveasthesubmmnoisedeclines.TheSXDFidenti cationratetailsoffbelowadeboostedsubmmSNRof2.5andatsubmm uxdensitiesbelow5mJy.ThismaybeduetothelimiteddepthoftheSXDFradioand24-µmimagingratherthananyde ciencyoftheSXDFcatalogue,butwenotethatitisastrongtendency.

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

HG4.1S/mµ42S z

Figure7.RatioofS24µm/S1.4GHzasafunctionofredshift,z,forSHADESsourceswithrobustcounterparts( lledcircles:LH;emptycir-cles:SXDF).Thosewithoutspectroscopicredshifts–themajority–areplottedarbitrarilyatz=2.3.ThetracksofArp220,Mrk231andNGC6240areshowntogetherwithasampleoffaintradiosourcesinSXDF(smalldots–Ibaretal.,in

preparation).

Figure8.Log10S24µm/S850µmversuslog10S1.4GHz/S850µmforSHADESSMGswithbothmid-IRandradioidenti cations( lledcircles),withonlyradioidenti cations(squares)andwithonlymid-IRidenti ca-tions(opencircles).Arepresentativeerrorbarisshown,lowerleft.TheredshiftparameterisationofChapmanetal.(2005)isshownasahorizontalbaratlog10S24µm/S850µm=0(see§7.2).

Summarisingtheseplots,thebestavailablecomplementarydataintheLH–equivalenttothoseavailableintheGreatObserva-toriesOriginsDeepSurvey(GOODS)northern eld–allowsustoidentifyrobustlyovertwothirdsofSMGstocurrentsubmmdetec-tionlimits.Theobservedtrendsinidenti cationrategivenostrongrationaleforrejectinganysourcesfromtheparentSHADESSourceCatalogue,althoughaslightquestionmarkisthrownoversomeofthelowestSNRsources.

c0000RAS,MNRAS000,000–0007CONSTRAINTSFROMSPECTRALINDICES

7.1S24µm/S1.4GHz

Sincethespectralslopesat24µmand1.4GHzaresimilar,itmayproveinstructivetoexaminethebehaviourofS24µm/S1.4GHzasafunctionofredshift,asshowninFig.7.Weexpectthisplottobe-trayAGNcontributionstotheradio uxdensityinso-called‘radio-excessAGN’(Drakeetal.2003;Donleyetal.2005)or,conversely,‘mid-IR-excessAGN’whichhaveQSO-heateddustbutlittleornoAGN-relatedemissionintheradio.Forstar-forminggalaxiesthisratioistightlyconstrainedouttoz=1(Appletonetal.2004).GalaxieswithlowvaluesofS24µm/S1.4GHz,i.e.thosewithstrongradiowithrespectto24-µmemission,areunlikelytobedominatedbystarformation.

TheSHADESSMGsshareapproximatelythesamedistribu-tionofS24µm/S1.4GHzvaluesastheotherradiosourcesinSXDF(Ibaretal.,inpreparation).Fig.7showstheredshifttracksofArp220,NGC6240andMrk231–archetypalultraluminousIRgalaxieswithincreasingdegreesofAGNcontribution.MeasuredvaluesofS24µm/S1.4GHzfortheSHADESSMGsareconsistentwithanyoftheseSEDsbutMrk231isthepreferredtemplate,implyinganAGNcontributiontothemid-IRluminosity.Onlyatz<1couldthemostextremeSMGbeclassi edcon dentlyashavingaradioexcess.7.2S850µm/S1.4GHz

Hughesetal.(1998)andCarilli&Yun(1999)pointedoutthevalueofS850µm/S1.4GHzasanindicatorofredshiftforSMGs,atleastforz<3.Smailetal.(2000)andIvisonetal.(2002)werethe rsttoemploythetechniqueforsigni cantsamplesofSMGs, ndingmedianredshifts,z>～2.

Chapmanetal.(2005)foundthattherelationshowedalargedispersionfortheirsampleofradio-identi edSMGswithspectro-scopicredshifts,indicativeofarangeofSEDs.Theynotedthatapurelysubmm-selectedsampleshouldshowanevenwiderrangeofS850µm/S1.4GHzthantheirradio-identi edSMGs,sincetheneedforanaccurateradiopositionbiasesthesampleinredshiftandtem-perature.

Thesurprisingly attrendidenti edbyChapmanetal.,un-correctedforaprobableredshift-dependent～0.3dexshiftat-tributabletotheirradioselectioncriteria,wasparameterisedasS850µm/S1.4GHz=11.1+35.2z.Thisparameterisationwasnotintendedasacarefulphotometricredshifttechnique–ther.m.s.scatterinredshiftis～1,afterall–butlikelyremainsthebestwaytoestimatethemedianredshiftofradio-identi edSMGsamples.Applyingthistooursampleof65SMGswithrobustradiocoun-terpartsyieldsamedianredshiftof2.8,withaninterquartilerangeof1.3–3.8,somewhathigherandbroaderthanthespectroscopicredshiftdistributionreportedbyChapmanetal.(medianz=2.2,interquartile1.7–2.8,beforetheirsmallcorrectionfortheradiose-lectionfunction).TheChapmanetal.parameterisationisnotap-propriateforSMGswithoutradioidenti cations,butfortheentireSHADESsample(adoptingthelimitsinTables1–2forthoselack-ingformaldetections)itindicatesamedianredshiftof3.3.

ThedifferencebetweenthedistributionreportedhereandthatofChapmanetal.(2005)isquitemarked,butcanbeexplainedbyavarietyofeffects:spectroscopicbias; eld-to- eldvariations;strongclusteringoftheSMGpopulation(Blainetal.2004);ouradoptionofdeboosted uxdensitiesforallSHADESsources(alargeproportionoftheChapmanetal.sampleislikelytohavesuf-feredasubmm uxdensityboostofoneformoranother);and,not

14Ivisonetal.

least,thedif cultyofmeasuringaccurateandconsistentradio uxdensitiesusingdatawithdifferentuvcoverage.

Fig.8showsalog–logplotofS24µm/S850µmversusS1.4GHz/S850µmforSHADESSMGs,withdifferentsymbolsrepresentingidenti cationsmadeindifferentwavebands(radioplusmid-IR;mid-IRonly;radioonly).Aswehavediscussed,S1.4GHz/S850µmissensitivetoredshift(andtemperature)andtheChapmanetal.parameterisationisshownasahorizontalbar.Itisapparentthat24-µm uxdensityiscorrelatedsigni cantlywithredshift,asexpectedfortheKcorrectionatthatwavelength.TheSMGwithS1.4GHz/S850µm>0.1isSXDF850.21,themostobvi-ousexampleofalocalgalaxyinthesample(z=0.044,Simpsonetal.2006;seeAppendixA,Fig.A2).

8THEDIAGNOSTICPOWEROFMID-IRCOLOURIvisonetal.(2004)usedacolour-colourplottoexploitthestrongdiagnosticpotentialofthemid-IRfordiscriminatingbetweengalaxiesdominatedbystarburstsandAGN.Keyspectralindicesforhigh-redshiftgalaxiesareavailablebetween3.6and24µmsincetherest-frame～3–10µmslopeforstarburstsissteeperthanforAGN,witha atterregionbetween1and3µm,whereasAGNexhibitpower-lawspectracoveringrest-frame～0.2–10µm(e.g.Mrk231).

Fig.9showsS24µm/S8µmversusS8µm/S4.5µm.Weexpectthelow-S8µm/S4.5µmportion–theleftside–tobeoccupiedbyz>～0.7starbursts,representedherebytheredshifttrackofArp220.High-redshiftstarburstsareexpectedinthelowerleftre-gionofFig.9,butspectralfeaturesinArp220’sSEDyieldsev-eralkinkswhichlimitthediagnosticpoweroftheplot;power-lawAGN,representedinFig.9byMrk231,trackleft-to-rightwithin-creasingredshiftacrossthelowerthirdoftheplot,returningtotheleftonlyz>～4.TheredshifttrackofNGC6240–aclassi-calCompton-thickAGNdisplayingmid-IRPAHfeaturesindistin-guishablefromthoseofastarburstgalaxy–overlapssigni cantlywiththecolour-colourspaceoccupiedbyArp220,atz～0.4andatmuchhigherredshifts,butmostoftheconfusingoverlapoccurswhereweexpectNGC6240-typeSEDsatz～0.6andMrk231-likeSEDsatz>6.

DoSMGsstandoutfroma24-µm-selectedSpitzersampleincolour-colourspace?Fig.9showsanindependentgalaxysam-pleselectedat24µmintheLH,atdepthscommensuratewithourSpitzeridenti cations,andwecanseethatthedataareclus-teredalongthetrackoccupiedbyArp220-likeSEDsforz 0.7,withasigni cantnumberofsourcesalongthetrackde nedbyaMrk231-likeSED.SMGsaresimilarlypositionedanddonotstandoutclearlyfrom24-µm-selectedgalaxies.However,thehatchedareasofFig.9–thosecolourcombinationswherewemightex-pectto ndSMGswiththehighestredshifts(z>～4)–arewellpopulatedwithSMGs.ThefractionofSMGsintheseregionsissigni cantlylargerthanforthecontrolsample:we ndonly14percentofthe4,457mid-IR-selectedgalaxiesinthehatchedregions.BasedontheChapmanetal.parameterisationofS850µm/S1.4GHz,theirmedianredshiftishigherthanthatoftheradio-detectedfrac-tionofSHADES,3.2versus2.8,althoughwenotethatsomeofthebestz<～1candidatesalsofallintheseregions,e.g.SXDF850.52.Nevertheless,itseemssensiblethatanysearchforahigh-redshiftpopulationofSMGsshouldbaseitstargetselectiononacombina-tionoftheS850µm/S1.4GHz,S1200µm/S850µm(Ealesetal.2003;Greveetal.2004),S24µm/S8µmandS8µm/S4.5µmcolours.

9CONCLUDINGREMARKS

Wehavedeterminedthemostlikelyradioand/ormid-IRidenti ca-tions,andhenceaccuratepositions,fortheSHADESSourceCata-loguepresentedbyCoppinetal.(2006).Wehaveidenti edrobustcounterpartstoovertwothirdsofthissample(54and46percentat1.4GHzand24µm,respectively),presentingoptical,24-µmandradioimagesofeachSMG.

Employingthesubmm/radio uxdensityratioasanindicatorofredshift,guidedbytheChapmanetal.(2005)parameterisation,we ndamedianredshiftof2.8fortheradio-identi edsample,somewhathigherthanthespectroscopicmedian.

Wepresentadiagnosticcolour-colourplot,basedonSpitzerdata,inwhichweidentifyregionscommensuratewithSMGsatveryhighredshift.

Wefurtherexploitouridenti cationstoshowthat:for pruningobservedthetrendsparentinSHADESidenti cationsamplerate(cf.giveIvisonnostrongetal.2002);rationaleexpectations, uncertaintieswithinnosubmmevidencepositionforsigni cantareconsistentadditionalwiththeoreticalsourcesofpositionalerror;

than signi cantlywouldbeexpectedmoreSMGsbychance,havemultipleindicativerobustofphysicalcounterpartsasso-ciations.ThesemultiplesystemsaremostcommonamongstthebrightestSMGsandaretypicallyseparatedby2–6arcsec,～15–50/sinikpcatz～2,consistentwithearlyburstsseeninmergersimulations.

ACKNOWLEDGEMENTS

AWBacknowledgessupportfromtheAlfredP.SloanFoundationandtheResearchCorporation.ISacknowledgessupportfromtheRoyalSociety.CSandSRacknowledge nancialsupportfromthePPARC.IAandDHHacknowledgesupportfromCONACYTgrants39548-Fand39953-F.

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c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

APPENDIXA:POSTAGESTAMPIMAGES

Thissectionpresents25×25-arcsecpostagestampimagesofeachSMGintheSHADESSourceCatalogueaswellasadescriptionofthemostunusualexamples.

FigsA1andA2showgreyscaleR-bandopticaldataintheleft-handpanels,whereavailable,andgreyscale24-µmdataintheright-handpanels.SuperimposedontheR-bandimagesarehigh-resolution(1.3arcsecFWHMfortheLH,1.7arcsecFWHMforSXDF)radiocontours,plottedat 3,3,4...10,20...100×σ,whereσwasmeasuredinsource-freeregionsaroundeachSMGandisquotedinthelower-rightcornerofeachimageinunitsofµJybeam 1.Superimposedonthe24-µmdataarelow-resolution(4.2arcsec,FWHM)radiocontours,plottedat 3,3,4...10,20...100×σ,whereσwasmeasuredinsource-freeregionsaroundeachSMGandisagainquotedinthelower-rightcornerofeachimage.Brokencrossesmarkthepositionsofall24-µmsourcesbrighterthan150µJyfoundwithin15arcsecofSMGpositionsinSXDF–theirpositionsarelistedinTable4.Thelargecentralcirclesin-dicate2σpositionaluncertaintieswhereσ=0.6θ/SNRandde-boostedSNRvalueshavebeenadopted(Coppinetal.2006).Asshownin§4,thereisan86.5percentprobabilitythatthesecirclescontainthesourceofsubmmemission.Forcounterpartidenti ca-tionwesimplyusearadiusof8arcsec(or12.5arcsecfortheradio,15arcsecat24µm,tobemorecomplete).

Solidboxesindicaterobustidenti cations,whereP 0.05basedontheradioor24-µmcounts,oracombinationofthetwo.Dashedboxesindicatetentativeassociations.

Casesworthyofcomment

SomeoftheSMGspresentunusualcombinationsofobservedchar-acteristicsandwecommentonthemhere.

LOCK850.06:Betrayedatboth24µmand1.4GHz,butinvisibleoptically.

LOCK850.07:AsLOCK850.06,thoughwithanopticalcounter-partwithin1arcsec;possiblytypicalofthecompositeblue-redpairsnotedbyIvisonetal.(2002).

LOCK850.08:Anopticalcounterpartlikelyliesbehindthediffrac-tionspike.Anidealtargetforadaptive-optics-(AO-)assistedstud-ies,exploitingthebrightstartothenorth.

LOCK850.11:Thisapparentlyobvious24-µmidenti cationjustfailstoqualifyasa‘robust’counterpartbecauseitcomprisestwofaintersources.Weviewtheseaslikelycounterparts.Theyareco-incidentwithadisturbedopticalgalaxywhichshouldbetargetedspectroscopically.

LOCK850.14:Thenearestradioemitterdoesnotqualifyasaro-bustidenti cationbuthasanexcellentspectroscopicredshiftinthecatalogueofChapmanetal.(2005).

LOCK850.15:Acomplexsystemwithasmanyasthreeplausibleidenti cations,suggestiveofacolossalmerger.

LOCK850.16:DescribedindetailbyIvisonetal.(2002,2005).LOCK850.18:Anobvious–thoughfaint–radioidenti cation,yetthereisnosignof24-µmoropticalemission.

LOCK850.19:Astraightforward24-µmidenti cationwithsup-portfromfaintradioemission.

LOCK850.21:Asolid24-µmidenti cation;24-µmanddistortedopticalemissiontothesouth-eastmayberelatedphysically.

LOCK850.23:Faint24-µmandradioemissionpointtoafaintop-ticalcounterpart(circledinFig.A1);wellworthtargetingspectro-scopically,thoughnotformallyarobustidenti cation.

LOCK850.29:Faintradioand24-µmemissionyieldaformal

c0000RAS,MNRAS000,000–000identi cation;thedoubleopticalgalaxyseemstobeoffsettothe

northeastandyetitresemblesmanySMGs;itshouldbetargetedspectroscopically.

LOCK850.30:Amultipleradioidenti cation.TheweakestradiocomponentremainsstubbornlyaboveP=0.05;thebrightestra-dioemitterwasreportedbyIvisonetal.(2002)tohaveaninvertedradiospectrum(seeBertoldietal.2000forotherexamplesofthisphenomenon).The24-µmemissionappearstoliebetweenthera-diocomponents.Inoneobviousinterpretationtheradioemissionmayemanatefromlobespoweredbyacentral,blackhole-andstar-forminggalaxy.

LOCK850.34:Amultitudeofmultiplecounterparts.Anopportu-nityfordetailedstudyofapotentiallycomplex,interactingsystem.LOCK850.37:Robustbutdistinctidenti cationsat24µmand1.4GHz.Challenging,optically.

LOCK850.48:Aseeminglystraightforwardidenti cation,yetapotentiallycomplexsystem.

LOCK850.52:Anextendedcounterpartat24µm,barelyvisibleinthehigh-resolutionradioimageandyetobviousandextendedinthelower-resolutionmap;extraresolutionavailableintheLHhasclearlyhinderedtheidenti cationprocess.Theopticalcounterpartmustbepartofanextensivesystem,presumablylargelyobscured.LOCK850.53:Atypicalcounterpartconsistingoftwoopticalgalaxies,betrayedbytheir24-µmemission.

LOCK850.60:Severalplausibleidenti cationsat24µm,theclos-estofwhichjustfailstoqualifyasarobustcounterpart.LOCK850.63:AnotherplausibleAOtarget.

LOCK850.67:OpticallyfaintSMG,blankat1.4GHz,givenawaybyits24-µmemission.

LOCK850.70:Aclassicopticalpairbetrayedat24µmandbyweakradioemission.

LOCK850.77:AsLOCK850.34:apairofpairs.

LOCK850.79:AnotherSMGwithseveralplausibleidenti ca-tions,thoughonlyoneoftheseisstatisticallyrobust.

LOCK850.87:Opticallyinvisible,yetbrightat24µmand1.4GHz.

SXDF850.01:Opticallyinvisible,yetbrightat1.4GHz.

SXDF850.02:Theradiomorphologyresemblesthebaseofawide-angletailradiogalaxy.

SXDF850.03:Theradioemissionisapparentlyassociatedwithabright,nearbygalaxy,thoughthealignmentispoorandlensingofabackgroundSMGmustbeapossibility.

SXDF850.05:Seeminglyamulti-componentmerger;suf cientlybrightat24µmand1.4GHztosuggestitliesatrelativelylowred-shift.

SXDF850.06:Animmenselycomplexregionwithatleastthreeradio-detectedcomponents.Thebrightest24-µmidenti cationiscoincidentwiththeradiosourcemostdistantfromtheSMGcen-troid.

SXDF850.07:AnopticallyfaintSMGinacomplexregion,be-trayedbyits24-µmand1.4-GHzemission.

SXDF850.08:Arobustradioidenti cation,offsetbyseveralarcsecfromaplausible24-µmcounterpart.

SXDF850.10:Itisplausiblethatthesubmmemissionemanatesfrombetweenthehotspotsofalobe-dominatedradiogalaxy.

SXDF850.11:Anexcellent,clearlyidenti edtargetforAO-assistedstudy,exploitingthenearbystar.

SXDF850.12:AndistortedopticalcounterpartliesbeneathveryfaintradioemissionclosetotheSMGcentroid.

SXDF850.14:Near-coincident,faint24-µmand1.4-GHzemis-sion,thoughitwouldbetemptingtotargetthedistortedopticalgalaxynorthoftheSpitzeremissionforspectroscopy.

18Ivisonetal.

SXDF850.16:Faintradioemissionisoffsetfromaseeminglydis-tortedopticalcounterpartby～1arcsec.

SXDF850.21:AlocalgalaxyliesclosetothisSMG–VLA0077inthecatalogueofSimpsonetal.(2006),atz=0.044;thismustbeviewedasthemostlikelyidenti cation–arareexampleofanearbygalaxyinablank-skysubmmsurvey.SXDF850.23:AsSXDF850.16.

SXDF850.24:Tworobustradioidenti cations,onenear-coincidentwithfaint24-µmemission.

SXDF850.28:Animmenselycomplexregionwithatleastthreeradio-detectedcomponents,eachwithdifferent24-µmproperties.SXDF850.29:Abrightradioidenti cation–VLA0225inthecat-alogueofSimpsonetal.(2006)–offsetsigni cantlyfromthecen-troidofabrightz=0.264opticalgalaxy.Thecorrectidenti ca-tionbecomesobviousinthenear-IR(Clementsetal.,inprepara-tion).

SXDF850.30:ThisSMGisbetrayedby24-µmand1.4-GHzemis-sion;anearbyopticalgalaxymaybetheunobscuredcomponentofalargersystem.

SXDF850.31:Tworobust24-µmidenti cations,onecoincidentwithradioemission,bothwithbrightopticalcounterparts.

SXDF850.37:OpticallyfaintSMGwithnear-coincident24-µmand1.4-GHzemission.

SXDF850.47:Acomplexregionwiththreeradio-detectedcompo-nents,eachwithnear-coincident24-µmemission.

SXDF850.52:Tworobustradioidenti cationswithverydifferentopticalproperties,onebright,oneinvisible;thebrightestoftheop-ticalgalaxiesisnotwellalignedwithitsradioemission.

SXDF850.77:AcomplexSMGwithtworadioemitters,neitherofwhichisalignedwellwiththetwo24-µmemittersintheregion.SXDF850.119:Twoplausibleidenti cations,eachwithverydif-ferentopticalproperties–onebrightandpresumablyrelativelylo-cal;theotheropticallyinvisible,likelyathighredshift.

APPENDIXB:POSITIONANDFLUXERRORSUncorrelatednoise

MuchofthetheoryneededforanunderstandingofSCUBAposi-tionerrorscanbefoundinCondon(1997),whichtreatsthegeneralcaseof ttingaGaussianellipsoidtomapdata,forwhichtherearesixfreeparameters:sourcecoordinates,total ux,twoprincipalaxesandapositionangle.Forthepresentapplication,wegenerallyprefertoassumethatSCUBAsourceswillnotberesolvedbythebeam,althoughresolvedorblendedsourcesarecertainlyknown(Ivisonetal.2000;Stevensetal.2003;Popeetal.2005).Themapshouldthereforeconsistofascaledandshiftedreplicaofthebeam,plusnoise.Thisleavesjustthreefreeparameters.

WefollowCondonandassumethatthebeamisasingle2DGaussianwithanr.m.s.‘width’σ( FWHM/2.354)ineachco-ordinate.Letthecoordinatesofthecentroidbe(α,δ)andassumethatthemapisdigitisedona( ne)gridwherethepixelspacingishandthenoisevalueateachpixelisanindependentzero-meanGaussiandeviatewithr.m.s.value,µ;theunitsofµarethoseofsurfacebrightness.Thepeakvalueofthe ttedpro leisA;strictly,thisisasurfacebrightnessvalueandthetotalintegrated uxden-sitywillbeS=2πσ2A.However,normallythefactor2πσ2willbeabsorbedintomapunitsofmJybeam 1orequivalent,sothatAhasthenumericalvalueofthe uxdensityofa ttedunresolvedsource.Withthisnotation,Condon’ssolutionforther.m.s.errors( )onthethree-parameter tis

hπ(B1)

2Ah.

Forapracticalformula,itmakessensetocombinethesebyde ningthe uxsignal-to-noiseratio:SNR=A/ A:

α= δ=

√

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

FigureA1.25×25-arcsecpostagestampimagesofeachSMGintheLHSHADESSourceCatalogue.GreyscaleR-bandand24-µmdataareshownintheleft-andright-handpanels,respectively,superimposedwithradiocontours.Circlesindicate2σpositionaluncertainties.Solidboxesindicaterobustidenti cations,whereP 0.05basedontheradioor24-µmcounts,oracombinationofthetwo.Dashedboxesindicatetentativeassociations.c0000RAS,MNRAS000,000–000

20Ivisonet

al.

FigureA1.Cont...

c0000RAS,MNRAS000,000–000

Identi cationofsubmillimetregalaxiesintheSHADESSourceCatalogue

FigureA1.Cont...

c0000RAS,MNRAS000,000–000

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