The SCUBA HAlf Degree Extragalactic Survey (SHADES) - III. I(11)

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

11

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