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

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

P

Notes

5

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