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.Thecatalogue3

environmentaswellasthespectralpropertiesofitsmem-bergalaxieswillbethe

subject

of

a

forthcoming

paper

(Czoskeetal.2001).

Throughoutthis paper1weuseaHubbleconstantH10=100h kms 1Mpc, M=1and 1Λ=0,whichgivesaphysicalscaleof3.195hkpcarcsec 1attheclusterredshift.

2.Observations2.1.Imaging

Wewillusephotometricresultsfromtwobroad-bandim-agesintheanalysisofthespectroscopicsurveypresentedinthispaper.

On26September1995,weobtainedanI-bandimageusingtheUH8kcamera(Luppino,Metzger&Miyazaki1995)onCFHT.TheI-bandimagewasreducedchipbychip,i.e.the nalresultisintheformofeightindividualimages,oneforeachchipofthecamera.The nalimageswereobtainedasthemeanof10exposuresof1200seach,usingsigmaclipping(2.5σabovethemeanleveland4σbelow)torejecthotpixelsandcosmicrayhits,andhaveverygoodseeingof0′′.7FWHM;however,thebackgroundismarredbystraylight,presumablyduetothebrightstar47Psc(V=5.1,spectraltypeM3)at～50′distancefromthecentreofCl0024+1654,althoughoutsidethe eldofviewoftheUH8k.TheI-bandphotometriccataloguewasobtainedfromtheindividuallystackedchipsusingtheSExtractorpackage(Bertin&Arnouts1996)withathresholdof1.5σandaminimumdetectionareaof5pix-els.Thecataloguecontainsmorethan4×1042objectsovera eldofabout28×28arcmin,thelimitingmagnitudeisI～24.Unlessotherwisenoted,weusetotalmagnitudesasgivenbySExtractor’sMAG

2

IRAFisdistributedbytheNationalOpticalAstronomyObservatories,whichareoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,undercooper-ativeagreementwiththeNationalScienceFoundation.3

http://archive.stsci.edu/dss/

V<23.3(0.01forV<21.1).ThelimitingmagnitudeisV～25.

Inordertoobtaincolourinformationaperturemagni-tudesweremeasuredin14pixel(2′′.8)diameteraperturesfortheV-andI-bandimages.Thisdiameterissu cientlylargecomparedtotheseeingFWHMtoenclosemostofthelightfromtheobjectunderconsiderationandsmallenoughtoavoidcontaminationbyneighbouringobjectsincrowdedregionsliketheclustercentre.TheobjectswerethenmatchedupusingapolynomialtransformationoftheI-bandimagecoordinatesontothesystemoftheV-bandimage.Thecentresofthetwoimagescoincidetowithin40arcsec,sothattheoverlappingregioncoversvirtuallythewholeUH8k eld.NotethatthegapsfromtheIbandimageshowupinthecolour-magnitudecat-alogueaswell.Theresultingcolour-magnitudecataloguecontains～2.1×104objectswitherrorsonthecoloursofsmallerthan0.05forV<23.3(0.01forV<21.1).Star-galaxyclassi cationoverasub-regionofthecolour-magnitudeplane(asrelevantforthepresentpaper)willbedescribedinSect.3.5.

Amoredetaileddescriptionofthephotometriccat-alogue(includingstar-galaxyseparationoverthewholecolour-magnitudeplane)isgiveninMayenetal.(2001)whousethiscataloguetoinvestigatethedepletionofbackgroundgalaxiesduetothegravitationallense ectofCl0024.

2.2.Spectroscopy

Spectrawereobtainedusingmulti-slitspectroscopyduringthreeobservingrunsatCFHTandoneatWHT.Table1showstheobservinglog.

Candidatesforalltheruns(exceptforrun1)wereselectedfromaV-bandmosaicobtainedattheESONewTechnologyTelescope(NTT)on17/18October1993,withtheprimaryselectioncriterionbeingVNTT<23.Thesee-ingonthisimagewasratherpoor,～1′′.7FWHM.Inordertoreducecontaminationbystars,thepreparatoryshal-lowR-bandimageswerecarefullyexaminedduringalltheCFHTruns;forruns3and4,wetookadditionaladvan-tageoftheexcellentseeingofthedeepUH8kI-bandim-age.

AlltheCFHTobservationsweredonewiththeMulti-ObjectSpectrograph(MOS,LeF`evreetal.1994)withtheO300grism.ThecamerasusedduringeachrunandthecorrespondingpixelscalesanddispersionsarelistedinTable1.Twoto veexposurespermaskwereobtained,dependingonthemagnitudesoftheselectedobjectsineachmask.

Band-limiting lters,chosensuchthatprominentspec-tralfeatures(forinstancetheCaiH/Klinesbluewardofthe4000 Abreak)fallintothebandattheredshiftofinterest,allowstackingofseveralrowsofspectraononemask,thusincreasingthenumberofspectraobservableinagiventime.Thisstrategyhasbeensuccessfullyemployedbye.g.Yeeetal.(1996,2000).However,spectracovering