We present the catalogue of a wide-field CFHT/WHT spectroscopic survey of the lensing cluster Cl0024+1654 at z=0.395. This catalogue contains 618 new spectra, of which 581 have identified redshifts. Adding redshifts available from the literature, the final

O.Czoskeetal.:SpectroscopicsurveyofCl0024:I.Thecatalogue

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Fig.5.V-Icolour-magnitudediagrams.Thetop-leftdia-gramshowsthefullphotometriccatalogue,thetop-rightdiagramthefullspectroscopiccatalogue.Thenextthreediagramssplitthespectroscopiccatalogueaccordingtoredshift,showingforegroundandbackgroundgalaxiesaswellasgalaxiesaroundtheclusterredshiftz～0.39.NotetheclearlyvisibleclustersequenceatV I 2inthelatterdiagram.Thebottom-rightdiagramshowsthemembersofthe

newlydiscoveredgroupofgalaxiesatz～0.495(seeSection4).Theparallelogrammarksthesubsampleusedinthecompletenessmap(Fig.7),20<V<23,0.6<V I<2.4.

pletenessofthespectroscopicsurveyinVmagnitude.Foreachgalaxyfromthecataloguethisisgivenastheratioofthenumbersofgalaxiesinthespec-troscopicandphotometriccataloguesinagivenbinwidthcenteredonthemagnitudeofthegalaxy.Plusesmarkgalaxiestakenfromthewholesurveyarea(asoutlinedinFig.7),crossesgalaxieswithin3′oftheclustercentre.Intheformercase,abinwidthof0.5magwasused,inthelatterabinwidthof1mag.

Fig.7.Mapofthecompletenessvariationofthespectro-scopiccatalogueasgray-scalewithoverlaidcontours.Thecompletenessatanypointisdeterminedinacirculartop-hatencompassingthe10nearestneighboursinthespec-troscopicsurvey;themapissmoothedwithaGaussianofwidth30′′.Contourlinesarespacedin10%steps.The50%contourismarkedbyaboldline,contoursatlessthan50%aredrawninblack,highercontoursinwhite.

persionsarepresumablyenhancedbytidalinteractionbe-tweenthegroups.

Fig.9showscolourimagesofthenorthernandsouth-erngroupscreatedfromtheI-andV-bandCCDimages.Thegalaxyatz=0.4907issurroundedbythreeobjectsofsimilar,bluecolour.Itistemptingtointerpretthisgroupasmultipleimagesofthesamebackgroundob-ject.Inthiscase,usingthecurvatureradius(～5′′)asanestimatefortheEinsteinradiusandzs=1asaroughguessfortheredshiftofthebackgroundsource,weobtain5.8×1012h 1M⊙forthemasswithinthisradius.

AnotheroverdensityinFig.8occursatz～0.18.Thesegalaxiesarehoweverdistributedfairlyuniformlyacrossthe eldwithnoobviousspatialconcentrationandarethereforejustpartofthegenerallarge-scalestructureintheUniverse.

Inthe rstdetectionofacoherentshear eldaroundaclusterofgalaxies,Bonnetetal.(1994)foundasignaltothenorth-eastofthecentreofCl0024+1654,indicatingaconcentrationofmassatapointwherenooverdensityofgalaxiesisapparentinthetwo-dimensionalimages.Thedirectiontothisdark“clump”isindicatedbyacirclein