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

14O.Czoskeetal.:SpectroscopicsurveyofCl0024:I.The

catalogue

Fig.8.Three-dimensionaldistributionoftheobjectsinourredshiftcatalogue.Inthetwoupperpanelstheobjectsareprojectedontotherightascensionaxis,inthelowertwoontothedeclinationaxis.Theupperpanelofeachpairshowsthelarge-scaledistributionfromz=0toz=1,thelowerpanelanexpandedviewoftheenvironmentoftheclusterCl0024itself.ThedashedlinemarksthedirectiontowardsthepotentialperturbationdetectedbyBonnetetal.(1994).Twogroupsatz～0.495aremarkedbyrectangles.TheconversionfromangularpositionontheskytopropertransversedistancewasdoneassuminganEinstein-deSitterUniversewithH0=100kms 1Mpc 1.Fig.4bandbythedashedlineinFig.8.Thereisnosig-ni cantover-densityalongthislinewhichcouldexplainthespatialtightnessofthesignalobservedbyBonnetetal.

5.Conclusions

Inthispaperwepresentacatalogueofspectroscopicdatafor687objectsina eldof21×25arcmin2aroundthecen-treoftheclusterofgalaxiesCl0024+1654atz～0.395,295ofwhichlieinthevicinityoftheclusteritself,intherange0.37<z<0.41.Thecompletenessofthesampleexceeds80%aroundtheclustercentreforV<22,droppingto～70%forV<!23.Themeancompletenessoverthewhole

Fig.9.“True”colourimagesoftheapparentcentresofthenorthern(top)andsouthern(bottom)groupsofgalaxiesatz 0.49.TheimageswerecreatedfromtheVandIbandimages,thegreenchannelisanaverageofthesetwoimages.Supposedmembergalaxiesofthegroupareconspicuousbytheiryellowcolour,correspondingtoV I～2.4.Notethebluearc-likestructurearoundthegalaxyatz=0.4907.

eldisabout45%downtoV=22.Thisisthereforeoneofthelargestspectroscopicsurveysavailableforaclusteratz>～0.2andthelargestatz～0.4.Apartfromredshiftsthecataloguelistsphotometricdataandequivalent widthsfor velinesaswellasthestrengthofthe4000Abreak,importantfordeterminingpresentandrecentstarform-ingactivity.Thecataloguerepresentsauniquedatabaseforinvestigationsintothestructureofamedium-redshiftclusterofgalaxiesanditsenvironment.

Wereportthediscoveryofabinarygroupofgalaxiesatz 0.495,whichincludesagravitationalarccandidate.Afurtheroverdensityintheredshiftdistributionatz～0.18canatpresentnotbeattributedtoacollapsedstructure,althoughwecannotexcludethepossibilitythatacentreforsuchastructureexistsoutsidethe eldcoveredbyoursurvey.Noneofthestructuresseenintheredshiftdistributionseemstobeabletoexplainthecoherentshearsignalat～6′tothenorth-eastoftheprojectedcentreofCl0024+1654,detectedbyBonnetetal.(1994).

ThemainresultofthisprojectisthatCl0024+1654isnotasimpleisolatedcluster,ashashithertobeenassumedininterpretingkinematical,lensingandX-raydata.Insteadthereisasecond,lessmassiveclusterpro-jectedontothecentreofthemaincluster.TheseparationofCl0024+1654intotwocomponentshasstrongconse-quencesfortheinterpretationofobservationaldataandshouldinparticularhelptoresolvethewell-knowndis-crepancybetweenmassesdeterminedusingdi erentmeth-odsforthiscluster(Soucailetal.2000).ThestructureofCl0024+1654,ascenarioforpossibleinteractionbetweenthemainandforegroundclustersandthee ectsofthisinteractiononthepropertiesoftheclustergalaxypopula-tionswillbediscussedinmoredetailinthesecondpaperofthisseries(Czoskeetal.2001).ThephotometricdataobtainedinconjunctionwiththisprojectwillbeusedbyMayenetal.(2001)toinvestigatethedepletionofback-groundobjectsduetothegravitationallensinge ectofCl0024+1654.

Acknowledgements.OCthankstheEuropeanCommissionforgeneroussupportundergrantnumberER-BFM-BI-CT97-2471.JPKthanksCNRSforsupportandacknowledgesaEuropeanLargeScaleFacilitygrantfortheWHTobservation.ThisworkwassupportedbytheTMRNetwork“GravitationalLensing:NewConstraintsonCosmologyandtheDistributionofDarkMatter”6oftheEuropeanCommissionundercon-