The SCUBA HAlf Degree Extragalactic Survey (SHADES) - III. I(2)

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

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).

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c0000RAS,MNRAS000,000–000