jsons_folder/2014MNRAS.438.1038R.json

{
  "Bruno Rodríguez Del Pino, 1⋆Steven P. Bamford, 1 Alfonso Aragón-Salamanca, 1 Bo Milvang-Jensen2 , Michael R. Merrifield1 and Marc Balcells3,4,5 ": [],
  "(<http://arxiv.org/abs/1311.2842v1>)arXiv:1311.2842v1  [astro-ph.GA]  12 Nov 2013": [],
  "ABSTRACT ": [
    "We use integral field spectroscopy of 13 disk galaxies in the cluster AC114 at z˘ 0.31 in an attempt to disentangle the physical processes responsible for the transformation of spiral galaxies in clusters. Our sample is selected to display a dominant young stellar population, as indicated by strong Hδabsorption lines in their integrated spectra. Most of our galaxies lack the [OII]λ3727 emission line, and hence ongoing star formation. They therefore possess ‘k+a’ spectra, indicative of a recent truncation of star formation, possibly preceded by a starburst. Disky ‘k+a’ galaxies are a promising candidate for the intermediate stage of the transformation from star-forming spiral galaxies to passive S0s. ",
    "Our observations allow us to study the spatial distributions and the kinematics of the di˙erent stellar populations within the galaxies. We used three di˙erent indicators to evaluate the presence of a young population: the equivalent width of Hδ, the luminosity-weighted fraction of A stars, and the fraction of the galaxy light attributable to simple stellar populations with ages between 0.5 and 1.5 Gyr. We find a mixture of behaviours, but are able to show that in most of galaxies the last episode of star-formation occured in an extended disk, similar to preceeding generations of stars, though somewhat more centrally concentrated. We thus exclude nuclear starbursts and violent gravitational interactions as causes of the star formation truncation. Gentler mechanisms, such as ram-pressure stripping or weak galaxy-galaxy interactions, appear to be responsible for ending star-formation in these intermediate-redshift cluster disk galaxies. ",
    "Key words: galaxies:clusters:individual: AC114-galaxies:evolution-galaxies: interactions. "
  ],
  "1 INTRODUCTION ": [
    "Thepropertiesofgalaxies–suchasmorphology,colour,sizeandmass–varyaccordingtotheenvironmentwheretheyreside.Inparticular,galaxymorphologieshavebeenshowntochangewithlocalprojecteddensity(e.g.,(<>)Dressler(<>)1980;(<>)Bamfordetal.(<>)2009),withlate-typespiralandirregulargalaxiesshowingmorepreferenceforregionswithlowerdensities,whileearly-typeS0andellipticalgalaxiesaremoreabundantindenserregions.Althoughinadiffer-enttimescale,specificstarformationrates,SSFR,arealsoaffectedbytheenvironment((<>)Baloghetal.(<>)2004a;(<>)Vogtetal.",
    "(<>)2004a)andhasbeenshowntobethegalaxypropertymostaffectedbythedensityoftheenvironment((<>)Kauffmannetal.(<>)2004;(<>)Wolfetal.(<>)2009).Theconcentrationofthestarformationinclusterdiskgalaxiesisalsofoundtobe˘ 25percentsmallerthancomparablegalaxiesinthefield((<>)Bamford,Milvang-Jensen&Aragón-Salamanca(<>)2007).",
    "Thereisalsoachangeinthemorphologicalmake-upofthegalaxypopulationwithredshift,particularlyinclusters.Spiralgalaxiesshowhighfractionsinclustersatintermediateredshift(z˘ 0.5),wherethefractionofS0sislow,butwhilethefractionofspiralsdecreasesforlocalclusters,S0sbecomemoredominant,being2–3timesmoreabundanttodaythanatintermediateredshift((<>)Dressleretal.(<>)1997).Ontheotherhand,ellipticalsdonotshowasub-",
    "stantialvariation,comprisingasignificantfractionofclustergalaxiessinceatleastz ˘ 1 ((<>)Dressleretal.(<>)1997;(<>)Fasanoetal.(<>)2000;(<>)Desaietal.(<>)2007).Correspondingly,thefractionofstar-formingbluegalaxiesinclustershasbeenshowntoincreasewithredshift((<>)Butcher&Oemler(<>)1978,(<>)1984;(<>)Margonineretal.(<>)2001),knownastheButcher-Oemlereffect,andthesehavebeenfoundtocomprisenormallate-typespirals((<>)Dressleretal.(<>)1994;(<>)Couchetal.(<>)1994).",
    "AllthesedifferentfindingspointtoatransformationofgalaxiesfromspiralintoS0withintheclusterenvironment,assuggestedinmanystudies(e.g.,(<>)Larson,Tinsley&Caldwell(<>)1980;(<>)Shioyaetal.(<>)2002;(<>)Bekki,Couch&Shioya(<>)2002;(<>)Aragón-Salamanca,Bedregal&Merrifield(<>)2006).Thistransformationwouldstartwithblue,star-forming,spiralgalaxiesatintermediateredshiftfallingintoregionsofhigherdensitysuchasgroupsandclusters,experiencingthelossoftheirgasandsubsequentsuppressionofstarformation,butretainingtheirdisks,resultinginred,passive,S0galaxies.",
    "Avarietyofmechanismshavebeensuggestedtoberesponsibleforsuchtransformations:interactionwiththehotintraclustermedium(ICM)viathermalevaporation((<>)Nipoti&Binney(<>)2007)andram-pressurestrip-ping((<>)Gunn&Gott(<>)1972;(<>)Abadi,Moore&Bower(<>)1999;(<>)Bekki,Couch&Shioya(<>)2002),interactionswiththeclustertidalfield((<>)Larson,Tinsley&Caldwell(<>)1980),galaxyharassment((<>)Mooreetal.(<>)1996)andminormergers((<>)Bekkietal.(<>)2005;(<>)Eliche-Moraletal.(<>)2012,(<>)2013).Alltheseprocessesmaybeexpectedtoremoveordisturbthegascontentsofgalaxies,whileleavingthestellardistributionsrelativelyunscathed.Majormergerscanalsotriggerstarburstswhichmayconsumegasreservoirsandultimatelysupressstar-formation((<>)Mihos&Hernquist(<>)1996),althoughunlesstheyaregas-rich((<>)Hopkinsetal.(<>)2009),theirstellardisksmaybedisrupted.Importantly,noneofthesemechanismsisthoughttooperateequallyfromlow-massgroupstorichclusters.ThehighfractionofS0spresentinallthesedenseenvironmentsthereforesuggeststhatacombinationofthesemechanismsmaybeinvolved,withvaryingdegreesofimportance.",
    "Galaxiesinwhichstarformationhasbeenrecentlysuppressed,˘ 0.5–1.5 Gyrago,shouldbewelldescribedbythecompositeofanA-typestellarpopulation(whosemain-sequencelifetimerangesfrom0.5–1.5 Gyr)andanoldpopulation,whichwaspresentinthegalaxybeforethelastepisodeofstarformation.Thistypeofgalaxieswasfoundforthefirsttimeby(<>)Dressler&Gunn((<>)1983),andtheyareconspicuousbythepresenceofstrongBalmerabsorptionlinesintheirspectra,characteristicoftheAstars,superimposedontoaspectrumofanolder(severalGyr)stellarpopulation,andwithnoemissionlines(indicatingnoongoingstarformation).Thesegalaxiesarecalledeither‘k+a’,aftertheirdominantstellartypes(old‘k’andyoung‘a’),or‘E+A’,indicatingtheirspectracorrespondtothatofatypicalearly-type(‘E’)galaxywithadditionalA-stars.Wewillrefertothemas‘k+a’galaxieshereafter.",
    "Duetotheimportanceof‘k+a’galaxiesasobservableinstancesofrapidevolution,theyhavebeenthesubjectofmanystudies((<>)Dressler&Gunn(<>)1983;(<>)Zabludoffetal.(<>)1996;(<>)Nortonetal.(<>)2001;(<>)Pracyetal.(<>)2009;(<>)Poggiantietal.(<>)2009;(<>)Pracyetal.(<>)2012;(<>)Pracyetal.(<>)2013).Althoughfirstdiscoveredintheclusterenvironment((<>)Dressler&Gunn(<>)1983),",
    "theyhavealsobeenfoundinthefield((<>)Zabludoffetal.(<>)1996;(<>)Blakeetal.(<>)2004)andingroups((<>)Poggiantietal.(<>)2009).Few‘k+a’galaxiesarefoundinthelocaluniverse,buttheirprevalenceincreasessignificantlywithredshift,suchthatinintermediate-redshiftclusterstheycanrepresentupto10percentofthetotalgalaxypopulation((<>)Poggiantietal.(<>)2009).Inthoseintermediate-redshiftclusters,‘k+a’galaxiestendtoavoidthecentralregions,implyingthatthesuppressionofstarformationdoesnotrequiretheextremeconditionsofclustercores,andmaybegininlessdenseenvironmentssuchasgroups((<>)Dressleretal.(<>)1999).While‘k+a’galaxiesingeneraloftenshowearly-typemorphologies(sometimesdisturbed;(<>)Yangetal.(<>)2008),inclusterstheyaregenerallyassociatedwithdisk-likesystems((<>)Caldwell,Rose&Dendy(<>)1999;(<>)Tranetal.(<>)2003),andinmanycasestheyalsoshowspiralsignatures,implyingthatthetimescaleforthespectralevolutionisshorterthanthatforanymorphologicaltransformation((<>)Poggiantietal.(<>)1999).",
    "Analysingtheinternalspatialdistributions,andideallykinematics,ofthedifferentstellarpopulationsinhabitingthesegalaxiesiscrucialtounderstandingthemechanismsresponsibleforthesuppressionofstarformation.Ifthelastepisodeofstarformationtookplaceinthecentralregions,itwouldbeconsistentwithprocessessuchasgalaxy-galaxyinteractionsandminormergers((<>)Mihos&Hernquist(<>)1996;(<>)Bekkietal.(<>)2005,butsee,e.g.,(<>)Teyssier,Chapon&Bournaud(<>)2010,andthediscussioninSection(<>)4inthispaper).Incontrast,amoreextendedyoungpopulationcouldimplydepletionofagalaxy’sgasreservoirthroughinteractionwiththehotICM((<>)Roseetal.(<>)2001;(<>)Bekkietal.(<>)2005;(<>)Bekki(<>)2009).",
    "Toperformsuchananalysis,wehaveusedintegralfieldspectroscopy,obtainedusingtheFLAMES-GIRAFFEmulti-objectspectrographattheVLT((<>)Pasquinietal.(<>)2002),toanalyse13galaxieswithdiskmorphologiesandstrongH absorptionintheclusterAC114(alsoknownasAbellS1077;(<>)Abell,Corwin&Olowin((<>)1989))atz ˘ 0.3.AC114hasbeenshowntocontainasignificantpopulationofbluestar-forminggalaxiesby(<>)Couch&Sharples((<>)1987,hereafterCS87),butalsotohaveasubstantialgeneralsuppressionofthestarformation(asinferredfromHemission;(<>)Couchetal.(<>)2001),whichmakesitanideallaboratoryforstudyinghowclustergalaxiesaretransformed.Apreviousstudyof‘k+a’galaxiesinthisclusterhasbeencarriedoutby(<>)Pracyetal.((<>)2005,hereafterP05).TheyobtainedobservationsusingFLAMESwithaverysimilarconfiguration,althoughtheydidnotfocusspecificallyongalaxieswithdiskmorphology.Wewerenotawarethattheirobservationsexistedwhenourswerescheduled,butsuchrepeatedobservationsenableustocheckthereproducibilityofourmeasurements.CombiningtheP05datasetwithourownalsoaddssomeadditionalgalaxiestothesampleweconsiderinthispaper.Intheirstudy,P05onlyconsiderthespatialdistributionoftheH equivalentwidth.Weexpandonthis,measuringthestellarpopulationsinmoredetailandconsideringtheresolvedgalaxykinematics.",
    "Throughoutthispaperweassumeacosmologywithm = 0.3, = 0.7,H0 = 70 km s−1Mpc−1 ."
  ],
  "2 DATA ": [],
  "2.1 Sample ": [
    "Thecurrentsampleconsistsof13galaxies,observedandidentifiedbyCS87asmembersoftheclusterAC114atz ˘ 0.31.TheCS87catalogueprovidesredshiftsandspectrallinemeasurements,asmeasuredon8hourintegrationswiththe3.9 mAAT,usingaspectrographwithaspectralresolutionR ˘ 1400 andfedby2.6 ′′diameterfibres.AC114alsohaswide-fieldarchiveHST WFPC2imaging,whichisusedtocataloguethemorphologicalmake-upofAC114in(<>)Couchetal.((<>)1998,hereafterC98).BasedonthecombinedCS87/C98catalogue,thesamplegalaxieswereselectedtohaveH rest-frameequivalentwidthEW(H) > 3Å(thesignconventionhereisthatapositiveEWforH meansabsorption),whichisthecommoncriteriontobeconsidered‘k+a’((<>)Poggiantietal.(<>)1999),diskmorphologyandmagnitudeRF 6 20.5.Somefurthersamplelimitationswereimposedbythespectrograph’sfield-of-viewandrestrictionsontheplacementofeachintegralfieldunit(IFU)inordertoavoidbuttoncollisionsandcrossedfibres.",
    "Nineoftheobjectsselectedtobeobservedshowno[OII]3727 emissionintheCS87catalogue,andhencecorrespondtoatrue‘k+a’selection.Theremainingfourshowsome[OII]3727 emission,indicatingthattheyhaveongoingstarformation,thoughpossiblydeclininggiventheirH EW,orhostanAGN.",
    "Inadditiontothesegalaxies,weincludeinouranalysistheobjectsobservedbyP05.Thissamplewasselectedfromthesameclusterinasimilarmannertothatdescribedabove,exceptthatnorestrictionwasplacedonmorphologyandnoneoftheirgalaxieshaddetected[OII]3727 emission.SixgalaxiesfromourselectionwerealsoobservedbyP05,aswellastwoadditionaldiskgalaxies,fourellipticalsandonepeculiargalaxy.Thecombinedsamplethereforecomprisestwentygalaxies,ofwhichfifteenpossessdiskmorphology.",
    "Notethat,forgalaxyCN849,thefluxpresentinourobservationswasverylow.Thiswasfoundtobeduetoanincorrecttargetposition.Fortunately,thisgalaxywasalsoincludedintheP05datasampleandcouldbeanalysedusingthatdata.Also,whencomparingtheredshiftsmeasuredforthegalaxiesobservedbybothP05andourselveswediscoveredaninconsistencyforCN254.InspectingthecoordinateswediscoveredthatthegalaxylabelledCN254inP05isactuallyCN229,anotherdiskgalaxyatz = 0.319.Thecross-comparisonsub-samplewithmultipleobservationsthereforecomprisesfourobjects.",
    "InTable(<>)1welistalloftheobjectsconsideredinthispaper,withtheircoordinates,morphologies,colour(BJ-RF,correctedforGalacticreddening)andprojecteddistancetotheclustercentre."
  ],
  "2.2 Observations ": [
    "TheobservationswereobtainedattheVLT-UT2usingtheFibreLargeArrayMultiElementSpectrograph(FLAMES)inGIRAFFEmodeataresolutionofR˘ 9600.Withthissetup,15individualIFUsweredeployedoverthewholefieldofview,withtwoofthembeingdedicatedtotheskytoensureareliableskysubtraction.EachIFUconsistsof20squaredmicrolensesof0.52 arcseconaside,makingupa",
    "surfaceof3 × 2 arcsec2 perIFU,whichcorrespondsto˘ 14.0 × 9.3 kpc2 atthedistanceofAC114(˘ 2.3 × 2.3 kpc2 perspaxel).",
    "Thetotalexposuretimewas˘ 13 hours,distributedin14exposuresindifferentnightsofJune,AugustandDecemberof2004.Observationsweretakenwithseeingconditionswithintherequestedservicemodeconstrain(6 0.8arcsec),andDIMMseeingrangedfrom0.49to1.06arcsec.Theobservedwavelengthrangewas5015–5831Å,whichataredshiftofz ˘ 0.3 correspondsto3850–4394Åinrestframe,coveringtheKandHcalciumfeatures(3934Åand3969Å),theBalmerlinesH (4102Å)andH(4341Å)andtheG-band(4305Å).Atthatwavelengthrange,theinstrumentalresolutionis0.57Åsampledwith0.2Åpixels,yieldingavelocityresolutionof˙ = 10 km s−1 atz ˘ 0.3.Sinceweexpect˙ > 50 km s−1 ,thisresolutionisenoughtocomfortablyresolvethelines.",
    "Inordertoensureanaccuratecalibrationofthedataset,weobtainedarclampandNasmythflatfieldimagesimmediatelyaftereachscienceexposure.",
    "TheobservationsbyP05wereobtainedwithanidenticalsetup,thoughwithslightlylowerintegrationtimes.Theseeingvaluesfortheseobservationsrangedfrom0.54to0.84arcsec.Theirindependentspectraforthefourgalaxieswehaveincommonprovideausefulcheckoftherobustnessofourresults."
  ],
  "2.3 Data reduction ": [
    "ThedatawerereducedusingtheGIRAFFEpipelineprovidedbyESO((<>)Izzoetal.(<>)2004).Thepipelinefirstsubtractsthebiasandtheoverscanregions.Then,usingthecorrespondingNasmythflatfieldimage,itdeterminesthepositionandwidthofthespectraontheCCDandsimultaneouslyproducesanormalisedflatfieldtoaccountforthevariationsintransmissionfromfibretofibre.BecausetheobservationsweretakenwiththeoriginalCCD,whichwasonlychangedinMay2008,removalofthedarkwasnecessaryduetothepresenceofaprominentglowintheCCD.AdispersionsolutionwascreatedusingthecorrespondingThArarclampframe,andthespectrarebinnedtoaconstantdispersion.Nofluxcalibrationwasrequiredfortheanalysisofthedata.",
    "Thepipelinedidnotincludearecipeforthesubtractionofthesky.Therefore,thesubtractionwasdonecombiningallfibresfromthetwoIFUsdedicatedtothesky,togetherwiththesingleskyfibreassociatedwitheachIFU,givingatotalof52fibres.WenoticedthatoneoftheskyIFUswassystematicallytoobright,perhapsduetocontaminationbyalowsurfacebrightnessobject,resultinginanoversubtrac-tionoftheskyinourobjectfibres.WedecidedtoexcludethisIFUandusetheremaining32skyfibresfortheskysubtraction.",
    "Forconsistency,weobtainedtherawdatafortheP05observationsfromtheESOarchiveandreducedtheminthesamemannerasourownobservations."
  ],
  "2.4 Stellar population and kinematic analysis ": [
    "Toextractinformationaboutthekinematicsandstellarpopulationsofthegalaxies,weusedthepenalizedpixelfittingpPXF softwaredescribedin(<>)Cappellari&Emsellem((<>)2004).",
    "Table 1. Identity number of the galaxy from (<>)Couch & Newell ((<>)1984), coordinates, morphologies, Galactic reddening corrected colours and distance to the cluster centre for the galaxies in our sample (top) and the P05 sample (bottom). At z = 0.3 one arc second corresponds to 4.454 kpc. Galaxies labeled with * have detected emission in [OII]3727. Galaxies labeled with \"P05\" are present in both samples. ",
    "Thisalgorithmusesamaximum-likelihoodapproachtofitthespectrainpixelspace,simultaneouslydeterminingboththestellarkinematicsandtheoptimallinearcombinationofspectraltemplatesrequiredtomatchtheinputspectrum.Weemployedtwoseparatecollectionsoftemplates,onedrawnfromtheELODIE3.1stellarlibrary((<>)Prugnieletal.(<>)2007)andtheothercontainingPEGASE-HRsimplestellarpopulation(SSP)models((<>)LeBorgneetal.(<>)2004).ThelatterspectraareconstructedusingtheELODIElibrary,andhencebothhavethehighresolutionandwavelengthcoveragerequiredtofitourspectra(0.5ÅFWHMand4000–6000Å,respectively).Internally,pPXF convolvedthetemplatespectrawithaGaussianinordertomatchthespectralresolutionofourobservations.Werestrictedthetemplatestotwoclasses(II-IIIandV)foreachstellartypeOBAFGKM,andtoSSPswith12differentageslogarithmicallydistributedbetween1Myrand15Gyrand5differentmetallicities[Fe/H]rangingfrom-1.7to0.4.",
    "Foreachspectrum,theprogramoutputsthevelocity,V ,andvelocitydispersion,˙,togetherwitharefinedestimateoftheredshift.ThevaluesobtainedforthekinematicswhenusingthestellarlibrarytemplatesandthoseobtainedusingtheSSPmodelswere,ingeneral,verysimilar.However,forsomeofthegalaxyspectra,occasionalnoisefeaturespresentinthestellarlibrarytemplatesspectraresultedinobviouslydiscrepantfitsandwrongvaluesforthekinematics.Inthesecases,weonlyusetheresultsobtainedusingtheSSPmodels.ErrorsinthekinematicparametersobtainedwithpPXF wereestimatedintherecommendedmanner,byperformingMonteCarlosimulationsontheoriginalspectrawithaddednoise.",
    "Inadditiontothekinematics,pPXF alsoprovidestheweightsofthetemplateswhichprovidethebestfittotheobservedspectrum.Theseweights,afternormalisation,representthefractionalcontributionofeachtemplatetothetotalluminosity.Belowweusetheweightsobtainedusing",
    "thestellarlibraryandtheSSPmodelsseparately,inordertostudythedistributionofdifferentstellartypesandstellarpopulationsthroughoutoursamplegalaxies."
  ],
  "3 ANALYSIS ": [
    "Webeginbystudyingtheglobalpropertiesofthesample,byintegratingthefibresfromeachIFUtoproduceasinglespectrumpergalaxy.Formostofthesamplewecombinedallthefibres.However,inafewcasesthisresultedinanexcessively-noisyspectrum,andthereforeonlyfibreswithsignal-to-noiseratio(S/N)& 5Å−1 (definedinselectedregionsofthecontinuum)werethencombined.Therejectedfibreswerealwaysfarfromthebrightestpixel,intheoutskirtsofthetargetgalaxy.",
    "InFigure(<>)1weplottheintegratedspectraforarepresentativesubsampleofthegalaxies:CN191,CN232,CN143andCN74.Thesignal-to-noiseratios(S/N)oftheintegratedspectrawererelativelyhigh,reachingvaluesof˘ 22Å−1 (CN146).AllofthespectradisplaytheKandHCalciumlinesandtheG-band,whicharecharacteristicofanoldpopulation.However,theH andHabsorptionlines,producedbytheyoung,A-starpopulationareonlystrongintwoofthespectra.ThelackofstrongBalmerabsorptionintheremaindercontrastswiththeirselectionas‘k+a’galaxies.BelowwemeasuretheH indexofthegalaxiesinordertoquantifythestrengthofthisfeature.",
    "Wearealsointerestedinconsideringspatially-resolvedinformationfromthedifferentregionsofthegalaxiescoveredbytheIFUs.InthemajorityofthegalaxiesatleastsomeofthecentralfibreshadsufficientS/N(reachingvaluesof˘ 15Å−1)tobeanalysedindividually,althoughthedegreetowhichthisispossiblevariesbetweengalaxies.Forthisreason,inadditiontoperformingtheanalysisfortheindividualfibres,ineachgalaxywecombinedallthepixelsimmediatelyadjacenttothebrightestone,whichwerefertoasthe‘surroundings’(coveringfrom˘1.6to3.2kpc),andthoseplacedfurtheraway,whichwedefineasthe‘outskirts’(˘3.2to4.8kpc).Insomecases,duetothelowS/Ninthepixelsfarawayfromthecentre,wecouldnotobtainreasonablequalityspectraforthe‘outskirts’.",
    "TofindthecentreofeachgalaxywebuiltimagesofthelightdistributioninthecontinuumregionbetweentheH featureandtheskylineat5577foreachIFU.Thecentreofthegalaxywasassociatedwiththebrightest(andhenceusuallyhighestS/N)pixel.SomelargeandinclinedgalaxieswerepurposefullyoffsettoincludetheirouterregionsintheIFU.However,manyoftheothergalaxiesalsodisplayoffsetsfromtheIFUcentre.Theseoffsets,whicharealsopresentintheobservationscarriedoutbyP05,arelikelyaresultofinaccuraciesintheastrometryandIFUpositioningerrors.Theyare,however,significantlysmallerthanthefieldofviewsodonotcompromisetheanalysis."
  ],
  "3.1 Indicators of a young population in the ‘k+a’ galaxies ": [],
  "3.1.1 Line index measurements ": [
    "Asexplainedabove,‘k+a’spectralfeaturesarisefromthetruncationofstarformationinagalaxy,whichmaybepre-",
    "Figure 1. Integrated spectra for a representative sample of our galaxies, from left to right and top to bottom: CN191, CN232, CN143 and CN74. We provide two examples of targets with prominent Balmer absorption (left) and two targets without (right). The spectra have been smoothed with a Gaussian of FWHM 1Å to improve their presentation. Note that the spectra have not been flux calibrated. ",
    "cededbyastarburst,andreflectthecompositeofayoungandanoldstellarpopulation.ThesegalaxiesareusuallyidentifiedbythestrongBalmerabsorptionlinesintheirspectra.Sincethehigher-orderBalmerlinesarelessaffectedbyemissionfromionizedgas((<>)Osterbrock(<>)1989),themostcommonlyusedindicatoristheH lineat4102Å,whichisalsoconvenientlylocatedintheopticalpartofthespectrumatlowandintermediateredshift.Althoughtheselectioncriteriadoesvarydependingonthestudy,‘k+a’galaxiesaregenerallyselectedtohaveEW(H) > 3Åandnodetectedemissionlines.",
    "ThestrengthoftheH absorptionlineisrelatedtothemechanismresponsibleforthe‘k+a’feature.(<>)Poggiantietal.((<>)1999)showedthatstrongH absorptionlines(EW(H) > 4–5Å)canonlybecausedbytheabrupttruncationofstarformationafterastarburst.Lowervaluesof[EW(H)]canalsobeachievedbyasimpletruncationofacontinuousandregularstarformationinthegalaxy.However,thestrengthoftheH linesubsideswithtime,soitisdifficulttodistinguishbetweenrecenttruncationandanolderonethatwasprecededbyastarburst.",
    "Althoughweconsidermoresophisticatedindicatorsofthestellarpopulationlaterinthispaper,giventheimportanceandsimplicityoftheH absorptionfeature,wefirstmeasuretheequivalentwidthofthislineforthesamplegalaxies.WeutilisedtheredshiftsobtainedfromthetemplatefitswithpPXF (Section(<>)2.4),aslistedinTable(<>)2.EquivalentwidthsweremeasuredusingthesoftwareINDEXF ",
    "((<>)Cardiel(<>)2010),whichusestheLick/IDSindexdefinitionsof(<>)Worthey&Ottaviani((<>)1997)tomeasurethesignalwithinthelinewithrespecttotheneighbouringcontinuum.TomakeourresultscomparablewiththoseobtainedbyP05,weusetheindexHF,whichtakesthecontinuumintervals4057.25–4088.5Åand4114.75–4137.25Åaroundthecentral4091.00–4112.25Åbandpass.Errorsareestimatedfromthepropagationofuncertaintiesinthespectraandthemeasuredradialvelocities.",
    "Fourofthegalaxiesinoursampledisplayemissionlines,whichwouldaffectthelineindexmeasurementduetothefillingoftheabsorptionlines.Toavoidthis,forthesefourgalaxiesinsteadofusingtheoriginalspectrumwemeasuredthelineindexonthebestfitspectrumconstructedbypPXF.Thisprocedurehasbeenshowntoproduceverygoodresultsby(<>)Johnstonetal.((<>)2013b).",
    "ThevaluesofHF forallthegalaxiesinoursamplearelistedinTable(<>)2.WealsolistthevaluesobtainedforthegalaxiesfromtheP05sample.Forgalaxiesthatarepresentinbothsamplesweobtainedverysimilarvalues,consistentwithinthegivenuncertainties.Hereafterweusedthevaluesmeasuredinourdata,becausetheypossesshigherS/Nratios.",
    "ThefirstsurprisingfindingisthenumberofgalaxiesforwhichwemeasureHF lowerthan3Å.ThiswasalreadysuggestedfromtheweakBalmerabsorptionlinesapparentinsomeofthespectrauponvisualinspection,see(Figure(<>)1).Theselowvaluescontrastwiththoseexpectedfromthespec-",
    "troscopicstudybyCS87,inwhichallofoursampleshowedEW(H) higherthan3Å.ThisdiscrepancywasalsofoundbyP05.ItappearsthattheuncertaintiesintheCS87H EWsareratherlarge,andhencetheirspectralclassificationsareonlyreliableforthemostextreme‘k+a’cases.",
    "Fromouranalysis,onlysevenofthetwentygalaxiesdisplayEW(H) > 3Å,withthreeofthemalsohavingdetected[OII]3727emission.IfweconsideralsothosewithEW(H) > 2 Å,threemoregalaxiesareincluded,givingatotaloften.ThevaluesobtainedinouranalysisoftheP05sampleareinreasonablegoodagreementwithwhattheyfound,consideringthateachstudyappliedadiffer-entmethod.Weonlyfoundonegalaxy,CN849,wherewemeasuredalowervalueofHF (2.3± 0.4)thanwhattheyobtained(3.6± 0.3),whichinthiscaseissignificantbecauseitchangesthegalaxy’s‘k+a’classification.",
    "Asmentionedabove,fourofthegalaxiesweobservedarelistedashaving[OII]3727emissioninCS87andthereforedonotmeetthestandard‘k+a’criteria.TheirEW([OII])valuesrangefrom7.6Åto39.6Å.Thesearelikelyreliableemissionlineidentifications.ThreeofthemarefoundtohaveHF > 3 Å(CN146,CN155andCN243)andtheyalsoshowsignsofemissioninHandH inourdata.However,itisnotclearwhethertheseemissionlinesresultfromresidualstarformationorAGNactivity.",
    "2",
    "OnewouldexpectthatifstarformationhasbeenrecentlytruncatedinthosegalaxieswithstrongH absorption,theyshouldhavebluercolorsduetothepresenceoftheyoungpopulation.Totestthis,inFig.weplotHF versusBJ− RFforallthesamplegalaxies.ObjectswithstrongH absorptionareconspicuouslybluerthanthosewithweakerH absorption.CS87alsopresentthisplot,findingaconsistenttrend,thoughsomewhatweaker,presumablyduetothelargeruncertaintiesontheirEW(H) estimates.ThistrendgivescompellingsupportthatthegalaxiesinoursamplewithstrongerH absorption,andparticularlyEW(H) & 2,containyoungerstellarpopulations.",
    "InFig.(<>)2wealsoindicatethegalaxieswhichhaveobserved[OII]emission.Recallthat,forthesegalaxies,EW(H) wasmeasuredonthetemplatefitsproducedbypPXF,ratherthanthedataitself,toavoidtheeffectofline-filling.Itispossiblethatinthecaseofthebluestgalaxytheline-fillinghasaffectedthepPXF fititself,resultinginanunderestimateofEW(H).Thesefourgalaxiesarenotstrictly‘k+a’systems,theymaysimplybenormalstar-forminggalaxies,althoughtheirhighEW(H) mightindicatesomerecentsuppressionoftheirstar-formation.Nevertheless,weretainthemintheanalysisbecausetheyprobablyliejustoutsidetheboundariesofthe‘k+a’class,andmayprovideusefulcluesontheprocessbywhichgalaxiesbecome‘k+a’systems."
  ],
  "3.1.2 A/(AFGKM) and fyoung measurement ": [
    "AlthoughstrongH absorptionisthestandardindicatorofayoungpopulationin‘k+a’galaxies,thissimplyreflectsthepresenceofasubstantialstellarpopulationwithagesbetween0.5 and1.5 Gyr,whoselightisdominatedbyAstars,butanabsenceofyoungerpopulationscontainingOBstars,poweringnebularemissionfromHIIregions.Thepresenceofthisintemediate-agestellarpopulationmayalsobeinferredusingother,morequantitative,methods.Oneap-",
    "Figure 2. HF versus BJ− RFcolour for our entire galaxy sample. In the case of objects that were observed by both P05 and ourselves, we only plot our values. Galaxies with detected emission in [OII] by CS87 are indicated by a red cross. ",
    "proachistemplatefitting,whichusesthefullwavelengthrangeavailableandaccountsforthefactthatpopulationsofallagescontributetoEW(H) (andotherspectralfeatures).WeusetheresultsoftemplatefitsperformedusingpPXF,asdescribedinSection(<>)2.4.",
    "Toestimatetherelativeproportionofeachstellarpopulation,weusethenormalizedlight-weightedproportionsassignedtothevarioustemplatesinthebest-fittingmodel.",
    "Fromtheweightsobtainedusingthestellarlibrarytemplateswedeterminethefractionsofeachstellartype(OBAFGKM)contributingtothegalaxyspectrum.ForthefitsusingtheSSPmodels,wegroupthetemplatesintofouragebins:‘Age < 0.5 Gyr’,‘0.5 < Age < 1.5 Gyr’,‘1.5 < Age < 7 Gyr’and‘Age > 7 Gyr’.OneexpectsanapproximatecorrespondencebetweenthestellartypesandSSPages:starsformedveryrecently(OB)willdominatetheAge < 0.5 Gyrbin,starswithlifetimes˘ 1 Gyr(mainsequenceAandFstars)willdominatethe‘0.5 < Age < 1.5 Gyr’bin,andlonger-livedstars(GKM)willcorrespondtothetwoolderagebins.However,thestellarpopulationtemplatescontaincontributionsfromstarsofalltypeswithlifetimeslongerthattheSSPage.",
    "ToevaluatethefractionofA-typestarsweusetheratioA/(AFGKM).OBstarsareexcludedfromthisfractionbecausetheirpresenceisill-constrainedbyourfits,duetotheirfeaturelessspectratogetherwiththeuncertainfluxcalibrationandlimitedwavelengthrangeofourdata.Also,OBstarsdonotcontributesignificantlytothestellarmassofagalaxy.Forthestellarpopulations,ourprimaryquantityisthefractionalcontributionofSSPswith0.5 < Age < 1.5 Gyroverthetotal,hereafterfyoung.ThevaluesofA/(AFGKM)andfyoung,determinedwiththeintegratedspectraforeachgalaxy,arelistedinTable(<>)2.",
    "3",
    "Thusnowwehavethreedifferentindicatorsofthepresenceofayoungpopulationsinthesegalaxies,HF,A/(AFGKM)andfyoung.Comparingtheseparametersprovidesausefulindicationoftheirrobustness,andthereforethereliabilityofusingonlyoneofthemincaseswhentheotheronescannotbeobtained.ThiscomparisonisdoneinFig.,whereweplotA/(AFGKM)andfyoung againstHF forourentiregalaxysample.UncertaintiesontheHF mea-",
    "Figure 3. Values of A/(AFGKM), fyoung and HF plotted against each other for our sample (green circles) and measured by us on spectra obtained by P05 (blue squares). For galaxies that are present in both samples we only plot values measured in our data because they have higher S/N. We only plot error bars in HF, as pPXF does not provide uncertainties on the weights in the best-fitting combination of templates. Galaxies with detected emission in [OII] by CS87 are indicated by a red cross. In the plot of fyoung vs A/(AFGKM) there are fewer visible points because they are superimposed onto each other. ",
    "surementsareindicated,butpPXF doesnotprovideerrorestimatesforthetemplateweights.WethereforeestimateaverageuncertaintiesforA/(AFGKM)andfyoung fromthestandarddeviationofthescatterfromalinearcorrelationwithrespecttoHF aftersubtractingthecontributiontotheirerrorbyHF.Weobtainanuncertaintyinbothquantitiesof0.2.whichisalsonotedinTable(<>)2.",
    "Asexpected,thereisagoodcorrelationbetweenthese",
    "Figure 4. Maps of the individual fibre values of A/(AFGKM), fyoung and HF index (top) and the corresponding values for the integrated regions ‘centre’ and ‘surroundings’ (bottom) in CN228. Errors of the HF index are printed over the regions. Each spatial pixel (or spaxel) has a size of 0.52 x 0.52 arcsec2 which corresponds to ∼ 2.3 x 2.3 kpc2 at the redshift of AC114. ",
    "quantities.GalaxieswithstrongBalmerabsorptionlinesalsoshowhighfractionsofA/(AFGKM)andfyoung,whilethosewithweakHF showverylowvaluesofA/(AFGKM)andfyoung.ThefractionsofA/(AFGKM)andfyoung alsopresentgoodcorrelationbetweenthem.Morequantitatively,forA/(AFGKM)andHF weobtainaSpearman’scorrelationcoefficientˆ = 0.65,whileforfyoung andHF,ˆ = 0.56.InthecaseofA/(AFGKM)andfyoung,ˆ = 0.76.Thechanceofanyofthesecorrelationsbeingspuriousis. 1 percent."
  ],
  "3.2 Spatial distributions ": [
    "Asmentionedpreviously,inadditiontoprovidingglobalinformationaboutagalaxy,integralfieldspectroscopyallowsustostudypropertiesatsmallerspatialscalesandhenceconsiderdifferentregionswithinagalaxy.Weexploitthispossibilitybyperformingthesameanalysisdescribedabove,butnowappliedbothtothespectrafromindividualIFUelementsandtocombinedspectrafromthe‘centre’,‘surroundings’and‘outskirts’regionsofeachgalaxy.",
    "Wehaveusedtheseresultstoconstructmapsofthethreedifferentageindicators,HF,A/(AFGKM)andfyoung,foreachgalaxy.Inmanygalaxies,duetotheS/Nbeingtoolowinthe‘outskirts’,onlythe‘centre’and‘surroundings’couldbeanalysed.CaseswherethethreeintegratedregionscouldbeanalysedcanbefoundintheAppendix(CN74andCN849).AnexampleofthisanalysisisshowninFig.(<>)4forthegalaxyCN228,wherethethreeindicatorsshowahighconcentrationoftheyoungpopulationinthecentreofthegalaxy.Thisisparticularlyclearwhenconsideringthe‘centre‘versus‘surroundings‘regions.ThevaluesoftheindividualfibresforA/(AFGKM)andfyoung alsoshowahighconcentrationtowardsthecentre,whiletheindividualHF arelessconclusive.",
    "Toexaminethestellarpopulationinmoredetail,inFig.(<>)5weshowthenormalizeddistributionsofthedifferentspectraltypesandSSPagesobtainedfordifferentregionsofthesamegalaxy.Thetwoapproachesarebroadlyconsistent:aprominentfractionofA-starsisassociatedwithasignificantyoung-agepopulation.",
    "ThemapsofHF,A/(AFGKM)andfyoung foreachgalaxyareourprimarysourceofinformationregardingthespatialdistributionsoftheyoungandoldstellarpopulations.However,themapsaredifficulttodealwithquantitatively,andthereissomesubjectivityinidentifyingthe",
    "Figure 5. Histograms of stellar type and stellar population age obtained with pPXF for the integrated spectra, ‘centre’ and ‘surroundings’ of CN228. ",
    "trendstheyreveal.Wehaveexaminedthesemapsindetail,andintheAppendixwepresentqualitativedescriptionsofeachgalaxy,inadditiontothemapsthemselves.",
    "Inanattempttoquantifythedifferencesinthespatialdistributionsoftheyoungandoldstellarpopulationswehaveusedthesemapstoestimatetheluminosity-weightedfractionoftheyoungstellarpopulationcontainedwithinthehalf-lightradiusoftheoldpopulation.Wehaveassumedexponentialintensityprofilesforbothpopulations.Avalueofthisfractionlargerthan0.5 indicatesthattheyoungpopulationismoreconcentratedthantheoldone.Figure(<>)6showsthisfractionplottedagainsttheglobalHF values.Thereisalargescatter,indicatingsignificantdifferencesinthecurrentpropertiesandformationhistoriesofthegalaxies.Nevertheless,galaxieswiththestrongestHF seemtoshowsometendencytohavemorecentrally-concentratedyoungpopulations.Thissuggeststhatthelastepisodeofstarformationoftentookplaceinthecentralregionsofthesegalaxies.Thesamplesize,spatialresolutionanduncertaintiesofthisstudypreventusfromreachingaveryrobustconclusioninthisrespect,butitisreassuringthatourfindingsareconsistentwithindependentevidencefromrecentstudiesoflocalS0galaxies((<>)Bedregaletal.(<>)2011;(<>)Johnstonetal.(<>)2012;(<>)Johnstonetal.(<>)2013a)."
  ],
  "3.3 Kinematics ": [
    "IfspiralgalaxiesarebeingtransformedintoS0sbyanyoftheprocessesdiscussedintheintroduction,inadditiontothechangesinstellarpopulationsconsideredabove,theirkinematicsmayalsobeaffected.Thekinematicsofthe‘k+a’galaxiesinoursamplecanthereforeindicatewhatmechanismsareresponsibleforthetruncationoftheirstarformation.Iftheprocessactsprimarilytostarveaspiralgalaxyofitsgassupply,thediskrotationshouldbepreservedintheresultinggalaxy.However,ifamergerisinvolved,theremnantwouldbeexpectedtoshowmorerandommotions.",
    "ThekinematicsofthegalaxiesanalysedherewereextractedusingthesoftwarepPXF,asexplainedinSection(<>)2.4.Firstofall,weobtainedavalueoftheoverallvelocitydispersion˙int fortheintegratedspectrumofeachgalaxy,which",
    "Figure 6. The luminosity-weighted fraction of the young stellar population contained within the half-light radius of the old population plotted against the global HF. A value of this fraction larger than 0.5 indicates that the young population is more concentrated than the old one (see text for details). ",
    "arelistedin(<>)2.ItshouldbeborneinmindthatforthegalaxieswithdetectedemissionbyCS87,themeasurementof˙int mightbeaffectedbythefillingoftheabsorptionlinesduetoemission.Awiderangeofvaluesof˙int arefound,from˘ 60 to˘ 280 km s−1 .Theseoverall˙int includecontributionsfrombothrotationalandrandommotions,whichwewillattempttoseparatebelow.",
    "Ifthegasandthekinematicsofthegalaxiesarebeingaffectedbytheclusterenvironment,onewouldexpectgalaxiesclosertotheclustercentretoshowdifferentbehaviourtothosethatarefurtherout,asfoundby(<>)Jafféetal.((<>)2011).Totestthis,weconsider˙int asafunctionoftheprojecteddistancefromtheclustercentre(Table(<>)1),whichisplottedinFigure(<>)7.Forthefullsampletheredoesnotappeartobeanycleartrend.However,ifweseparategalaxieswithhigh(> 3Å)andlow(< 3Å)HF,weseethatthosewithhighHF (bluesquares)presentastrongtrend.HighHF galaxieshavehigher˙int thefurthertheyarefromtheclustercentre,whilethosewithlowHF (greencircles)showlittlechangewithposition.Someofthehigh-HF andhigh-˙int galaxiesdisplayemission-lines(redcrosses),whichmaymaketheestimationof˙int unreliable.However,ifweremovethemfromtheplot,weseethatthetrendremains.",
    "Wenowturnourattentiontothekinematicsofthegalaxiesonsmallerscales,whichcanbestudiedusingtheoutputsoffitsperformedtotheindividualIFUfibres.Weconstructline-of-sightvelocity,Vobs,andvelocitydispersion,˙,mapsofthegalaxies,inasimilarmannertothosefortheyoungpopulationindicators.AnexampleisshowninFigure(<>)8forthegalaxyCN228.",
    "Wehavestudiedthesemapsforsignsofrotationanddifferencesinvelocitydispersionbetweenthecentralandsurroundingpixels.Oneproblemwehadtofaceherewasthatthe‘good’fibreswerenotalwaysdistributedaroundthebrightestpixelintheIFU,anditwassometimesdiffi-culttoidentifypatternsofrotationorvelocitydispersion.SincetheobservedvelocityisVobs = Vrotsini,whereVrot istherotationalvelocityandi theinclinationofthegalaxy,weneedtoknowthegalaxyinclinationinordertoobtaintheactualrotationalvelocity.Theinclinationwasthereforedeterminedbytheapparentellipticityobtainedbyfitting",
    "Table 2. Galaxy ID, redshifts, young population indicators, velocity dispersions, Vrot/˙ and state of interaction for the galaxies in our sample (top) and the P05 sample (bottom). Note that morphology and colour are listed in Table (<>)1. Galaxies labeled with * have [OII]3727 detected emission by CS87. ",
    "Figure 7. Velocity dispersion ˙ vs projected distance to the cen-tre of the cluster for galaxies with HF > 3Å (blue squares) and HF < 3Å (green circles). For those galaxies that are observed by both P05 and ourselves we plot the mean value. Galaxies with detected emission in [OII] by CS87 have a red cross overplotted. ",
    "anellipsetotheHST/WFPC2imagesusingtheIRAF taskELLIPSE.InthecasepresentedinFig.(<>)8,wecanseeaclearpatterntypicalofrotation,withVrot = 177±38 km s−1 .Thedistributionof˙ isroughlyflat.",
    "Previousstudiesofthekinematicsof‘k+a’galaxieshavefoundsignificantrotationinmanyofthem((<>)Franx(<>)1993;(<>)Caldwelletal.(<>)1996;(<>)Pracyetal.(<>)2009;(<>)Swinbanketal.(<>)2012;(<>)Pracyetal.(<>)2013),althoughsomearefoundtobemainlypressure-supported((<>)Nortonetal.(<>)2001).Weattemptedourkinematicanalysisinallthegalaxies,includingtheobservationsofP05,andfoundthatatleast8galaxiesdisplayrotation,withvaluesofVrot ˘ 85–180 km s−1 .",
    "2",
    "Themeasuredvaluesof˙int andVrot arelistedinTable,alongwiththeirratio(Vrot/˙int),whichindicateswhetheragalaxyisarotationally(> 1)orpressure(< 1)supported",
    "Figure 8. Example of the radial velocity and velocity dispersion maps, with errors plotted below. The plus (‘+’) and minus (‘-’) symbols indicate the direction of rotation. In this image we show the example of CN228, showing a clear pattern of rotation and with similar values of ˙ along the galaxy. ",
    "system.Usingthislastparameter,weseethat2ofthesystemsdisplayingrotationareclearlyrotationallysupported,typicalofdisk-likesystems,while5showVrot/˙int < 1 indicatingtheyaredominatedbyrandommotions.ComingbacktoFigure(<>)7,nowweareabletoestablishifthehighvaluesof˙int foundforsomegalaxiesareduetorotationortorandommotions.Fromthe10galaxieswithHF > 2Å,rotationisdetectedin6ofthemanddominantin2ofthese.However,theamountofrotationingalaxiesfarfromthecen-tre,inparticularCN254andCN228isconspicuouslyhigher(> 160 km s−1)thaninthoseclosertothecentresuchasCN143andCN191(< 140 km s−1).Theobservedtrendtolowerinternalvelocitieswithdecreasingdistancefromtheclustercoremaythereforeindicateatrendtolessregularkinematics,andhenceenvironmentallyinduceddisturbancesinthecentreofthecluster."
  ],
  "3.4 Kinematic decomposition ": [
    "Thekinematicsstudiedintheprevioussectionarederivedassumingthatallstellarpopulationscontributingtoaspectrumhavethesamekinematics.However,ourdataaffordsthepossibilityofmeasuringthekinematicsoftheyoungandoldpopulationsin‘k+a’galaxiesseparately(e.g.,(<>)Franx(<>)1993;(<>)Nortonetal.(<>)2001).Separatedkinematicsofferafurthermethodofdistinguishingbetweenthemechanismsresponsiblefor‘k+a’signature.Rotationintheyoungcomponentsimpliesitisinadiskandthatthegalaxyhasnotbeensubjecttoaviolentprocess,particularlyiftheyoungpopulationkinematicsanddistributionareconsistentwiththeolderpopulation.Ontheotherhand,apressure-supportedyoungpopulationimpliesthatasignificantinteractionhasoccurred.Thedegreeofrotationalsupportintheoldpopulationmaythenindicatethestrengthofthisinteraction.",
    "Inordertostudythekinematicsofthetwodifferentpopulations,wemodifiedthepPXF algorithminsuchawaythatitcouldfittwodifferentstellartemplatestoonespectrumsimultaneously,convolvingeachonewithdifferentradialvelocitiesandvelocitydispersions.Thesamemodifiedalgorithmhasbeenusedtostudyagalaxywithtwocounter-rotatingdisksby(<>)Johnstonetal.((<>)2013b),withgoodresults.Inourcase,weusedasetoftemplatescontainingA-starsandK-starswithdifferentmetallicitiessothatpPXF couldclearlydistinguishbetweenthetwopopulations.",
    "Decomposingthekinematicsisverychallenging,andrequireshighersignal-to-noisethanavailableinmostoftheindividualIFUelements.Thedecompositionwasthereforeattemptedoncoaddedspectracorrespondingtothreeregionsforeachgalaxy,thecentreandbothsides,wheretheorientationofeachgalaxyisjudgedfromthekinematicmapsfromSection(<>)3.3.WefoundthatthealgorithmwassometimessensitivetotheinitialvaluesofV and˙ used.Wethereforevariedtheseinputvaluesand,inordertobeconsideredrobust,theoutputsofthefitswererequiredtoremainconstantforawiderangeofinitialvalues.",
    "TheresultsarepresentedintermsofV and˙ mapsinasimilarmannertotheprevioussection.Asanexample,thekinematicdecompositionofCN228isshowninFigure(<>)9.Inthiscase,thegalaxyisacompositeoftwopopulationswithsimilarpatternsofrotation,whiletheyoungpopulationdisplayshighervaluesof˙ thantheoldpopulation,throughoutthegalaxy.AsshownpreviouslyinFigure(<>)4,CN228showscentralconcentrationoftheyoungpopulationinthedistributionofthethreeindicatorsHF,A/(AFGKM)andfyoung,impliyingaconcentrationoftheyoungpopulationinthecentreofthegalaxy.Now,addingtheinformationprovidedbythekinematicdecomposition,thefactthatthisgalaxyandothersshowsimilarrotationbetweentheyoungandoldpopulationseemstoindicatethatthesewerefairlynormaldiskgalaxieswhichhavenotexperiencedamajormergerordominantcentralstarburst.However,thehigher˙ suggeststhattheyhaveexperiencedaninteractionwhichincreasedtherandommotionsinthegasfromwhichthelastpopulationofstarswasformed.",
    "Intotal,threediskgalaxieswithHF > 3Å(CN228,CN146andCN191)andtwowithHF > 2Å(CN254,CN849)couldbekinematicallydecomposedintotwopopulations.Infourofthesecasesboththeyoungandoldpopulationswerefoundtohavesimilarpatternsofrotation,",
    "Figure 9. Kinematic decomposition of the young and old stellar populations in CN228, obtained using our two-component fitting method. Mean velocity and velocity dispersion values are presented in the IFU image. The plus (‘+’) and minus (‘-’) symbols indicate the direction of rotation. Errors in the fits are plotted over the corresponding regions. ",
    "whereasnoclearpatternwasfoundintheremainingone(CN146).",
    "The˙ valuesobtainedforthetwopopulationsdisplayavarietyofbehaviours,bothintermsoftheirrelativestrengthandtheirradialgradients.InthecaseofCN228discussedabove,the˙ oftheyoungstarsishigherthanthatoftheoldpopulation,whichsuggeststhatthisdisky‘k+a’galaxymayhaveexperiencedarecentinteraction,whichhasincreasedtherandommotionsoftheircoldgas,fromwhichthelatestgenerationofstarshaveformed,buthadlesseffectontheirpreviouslyexistingstellarpopulations.Thus,theprocesscannotbepurelygravitational,sincetheoldpopulationisnotperturbed,anditmustbeaffectingonlythegas((<>)Jafféetal.(<>)2011).AmoredetailedanalysisofthekinematicdecompositionforeachgalaxyispresentedintheAppendix."
  ],
  "3.5 Influence of interactions ": [
    "Dynamicallyinteractinggalaxiesareoftenobservedtobeexperiencingastarburst(e.g.(<>)Keeletal.(<>)1985).Simulationshavelongsuggestedthatmergersandinteractionscancausegasinagalaxydisktoloseangularmomentumandfalltowardthecentreofthegalaxy,poten-tiallyfuelingacentralstarburst((<>)Barnes&Hernquist(<>)1991;(<>)Mihos&Hernquist(<>)1996;(<>)Bekkietal.(<>)2005).However,observationsoftenfindthatinteractionspromotestarformationthroughoutthegalaxiesinvolved(e.g.(<>)Kennicuttetal.(<>)1987;(<>)Elmegreenetal.(<>)2006),notjustinthenuclearregion.Thiscannowbereproducedbymodelswhichpaycloserattentiontotheroleofshock-inducedstar-formation(e.g.(<>)Chien&Barnes(<>)2010;(<>)Teyssier,Chapon&Bournaud(<>)2010).",
    "Assumingthatthestarburstprocessoccursfasterthanthereplenishmentofthegasdiskviainfall,oralternativelythatsuchinfallissuppressed,thenfollowingthestarburstthegalaxywillceasestarformation.Theresultinggalaxywillthereforedisplayak+aspectrumforatime.",
    "Theimportanceofmergersandinteractionsastheoriginofthe‘k+a’featureissupportedbystudieswhichfindthat‘k+a’galaxies(ofallmorphologies)aremorelikelytobefoundwithacompaniongalaxy,whencomparedtonormalgalaxies((<>)Goto(<>)2003,(<>)2005;(<>)Yamauchi,Yagi&Goto",
    "(<>)2008;(<>)Pracyetal.(<>)2012).Forexample,intheircatalogueofk+aandtheircompaniongalaxies,(<>)Yamauchi,Yagi&Goto((<>)2008)foundthatk+agalaxieswere54percentmorelikelythannormalgalaxiestohaveasignificantcompanion.Similarly,thetwo‘k+a’galaxieswithlate-typemorphologyandwithacentralconcentrationoftheyoungpopulationstudiedby(<>)Pracyetal.((<>)2012)havenearbycompanionsandcouldbeexperiencingtidalinteractions.However,notethatalloftheseresultsarebasedonthegeneral‘k+a’population,andthusmaydifferfromthedisky,cluster‘k+a’populationconsideredinthispaper.Wehavethereforelookedforevidenceofinteractionsinthesample.",
    "InTable(<>)2wehaveincludedacolumnspecifyingwhethereachgalaxydisplaysindicationsofinteractingwithotherobjects.ThiswasevaluatedbyvisualinspectionoftheHST/WFPC2imagesoftheAC114cluster.Ofthetwentygalaxiesinoursample,sevenhaveaclosecompanionandshowclearsignsofamergerorinteraction.Theremainderappearfairlyisolatedandundistorted.However,thefactthatagalaxydoesnotappeartobecurrentlyinteractingdoesnotruleoutsuchaprocessasthecauseofa‘k+a’feature.Thespectral‘k+a’signaturecanlastforupto1.5 Gyr,whichisenoughtimeforaninteractinggalaxytohavemovedtoacompletelydifferentregionoftheclusterandanydistortionfeaturemighthavefaded.",
    "Totestifinteractionshaveanyinfluenceinthepropertiesofthegalaxies,welookedforanykindofcorrelationwithanyoftheresultsobtainedsofarinthisstudy.OfthetengalaxieswithH > 2Å,fiveshowsignsofinteraction.OfthesevenH > 2Ågalaxieswithdiskymorphologyandusablespatialinformation,threehavecentrallyconcentratedyoungpopulations(CN155,CN228and849)andalloftheseshowevidenceforinteractions.Incontrast,thefourdiskgalaxieswiththeiryoungpopulationextendedthroughoutthegalaxydonotshowanysignofmergersorinteractions.",
    "Thisfindingstronglysupportsalinkbetweendynamicalinteractionsandacentrallyconcentratedstarburstindisky,clusterk+agalaxies.Theremainder,withanapparentlylessconcentratedyoungstellarpopulationmaysimplybetheresultofweakerorolderinteractions,orcausedbyanalternativemechanism.However,thestrengthofHF fortheinteractingandnon-interactinggalaxiesdoesnotdiffersignificantly."
  ],
  "4 DISCUSSION ": [
    "Ouranalysisrevealsthatdisky‘k+a’galaxiesinintermediate-redshiftclustersareamixedpopulation.However,despitethesmallsamplesize,wedoseesomeconsistentbehaviourinanumberofimportantrespects.Theseresultsarerobusttochangesinthewaywequantifythepresenceandkinematicsoftheyoungandoldstellarpopulations.",
    "Theyoungstellarpopulationswithinoursamplegalaxiesarealwayseitherdistributedsimilarlyto,ormorecompactlythan,theolderpopulation.Importantly,however,theyarerarelyconsistentwithbeingpurelyconfinedtothegalaxynucleus.Furthermore,theyoungstarsoftendisplayrotationalkinematicscorrespondingtotherestofthegalaxy,implyingtheyarelocatedinthedisk.However,therearesomeindicationsthattheirvelocitydispersionsaresomewhatgreaterthaninnormalspiralgalaxies.",
    "Togethertheseresultssuggestthattheyoungstellarcomponentformedinanextendeddisk,inamannersimilartopreviousgenerationsofstarsinthesegalaxies.Itisnotassociatedwiththeaftermathofanuclearstarburst,norstarformationintidallyaccretedmaterial.Howeverthegasfromwhichthelateststarsformedwastypicallymorecentrallyconcentratedthanthatfromwhichtheirpredecessorswereborn.",
    "Thescenariopresentedbyourdatacanbebroughttogetherwithmanyotherpiecesofobservationalevidencetosupportaconsistentpicturedescribingtheevolutionofthemajorityofdiskgalaxiesinintermediate-redshiftclustersandgroups.",
    "Firstly,wenotethatanysatellitegalaxywithinalargerhalo,particularlyonemassiveenoughtohavedevelopedaquasi-statichotatmosphere((<>)Rees&Ostriker(<>)1977),isverylikelytohaveitsowngashalorapidlyremovedbyinteractionswiththehosthalo’sintergalacticmediumandtidalfield,viathemechanismsdiscussedintheintroduction.TheenvironmentalremovalofHIgasreservoirsisobservedbothlocally(e.g.,(<>)Vogtetal.(<>)2004a)andatintermediateredshift(e.g.,(<>)Jafféetal.(<>)2012).Star-forminggalaxiesenteringadenseenvironment(i.e.becomingsatellites:lowmassgalaxiesingroupsandhighermassgalaxiesinclusters),wouldthereforebeexpectedtograduallydecreasetheirstar-formationrateastheyconsumetheirremainingsupplyofdensegas.",
    "However,agradualdeclineinthestarformationratesofstar-forminggalaxiesindenseenvironmentsisatoddswithresultsfromlargesurveys.ThecoloursandHequivalentwidthsofstar-forminggalaxiesareinvariantwithenvironment(e.g.,(<>)Baloghetal.(<>)2004a,(<>)b;(<>)Baldryetal.(<>)2006;(<>)Bamfordetal.(<>)2008),althoughtherelativeproportionsofblueversusredorstar-formingversuspassivegalaxiesvarysubstantially.Thisstronglyimpliesthatgalaxiesmustrapidlytransformfromstar-formingtopassive,suchthatatransitionpopulationisnotseen.Thetransformationmechanismcannotbeparticularlyviolent,asmanygalaxiesbecomepassivewhilstmaintainingtheirdiskmorphology,firstasredspirals,andthenaslenticulars(e.g.,(<>)Laneetal.(<>)2007;(<>)Bamfordetal.(<>)2009;(<>)Maltbyetal.(<>)2012).Wemustthereforereconciletheneedforarapidtransformationintermsofobservedcolourandemission-lineproperties,withtherequirementthatthemechanismonlyactrelativelygentlyongalaxystructure.",
    "Star-forminggalaxiesareobservedinenvironmentsofalldensities,thoughtheybecomemuchrarerindenseregions.However,itisnotyetclearwhetherthosestar-forminggalaxieswhichappeartoinhabitdenseregionsaresimplytheresultofprojectioneffects,orwhethersomegalaxiesareabletomaintaintheirstarformation,atleastforawhile,insuchextremeenvironments.Theformerwouldimplythatthetransitionfromstar-formingtopassiveisdrivenbyadeterministicmechanism,specifictoparticularenvironments,whereasthelattercouldwouldpermitsomethingmorestochasticinnature,inwhichtheeffectofenvironmentissimplytoincreasethelikelihoodofsuchatransition((<>)Pengetal.(<>)2010).",
    "Astochasticmechanism,whichisnotdirectlyrelatedtoagalaxy’sbroad-scaleenvironment,issupportedbytheobservationthattheproportionsofredorpassivegalaxiesshowtrendsacrossawiderangeofenvironmentaldensity,and",
    "thatgalaxieswithtruncatedstar-formationareoftenassociatedwithgroups((<>)Moranetal.(<>)2007;(<>)Poggiantietal.(<>)2009;(<>)Wilmanetal.(<>)2009;(<>)Lackner&Gunn(<>)2013),whichalsohostnormalstar-forminggalaxies.",
    "Therealityisprobablyacombinationofthedeterministicandstochasticpictures,forexampleamechanismwhoseeffectivenessdependssensitivelyonthedetailedsmall-scalesubstructureoftheenvironmentandagalaxy’sorbitthroughit(e.g.,(<>)Fontetal.(<>)2008;(<>)Pengetal.(<>)2012).Inanycase,thedeterministicremovalofagalaxy’sgashalosoonafteritbecomesasatellitemakesthegalaxymorevulnerable,helpingtoreducethetimescaleofanytranforma-tioninstigatedbyasubsequentmechanism.",
    "Aninitialenhancementofstar-formationefficiencyearlyinthestar-forming-to-passivetransformationprocesswilleffectivelyreducetheobservabilityofthetransition.Theincreasedstarformationefficiencywouldbalancetheeffectofthedecliningfuelsupply,maintainingtheappearanceofnormality,untilthefuelsupplyisentirelydepleted.Thegalaxywouldthenimmediatelyceasestar-formationandrapidlyappearpassive.",
    "Brieflyenhancedorextendedstar-formationinthecentralregionsofclusterspiralsissupportedbyourresults,aswellastheprevelanceofclustergalaxieswith‘k+a’spectraltypes((<>)Poggiantietal.(<>)2009)andmorecentrallyconcentratedyoungpopulationsinspi-rals((<>)Caldwelletal.(<>)1996;(<>)Koopmann&Kenney(<>)2004b,(<>)a;(<>)Vogtetal.(<>)2004a,(<>)b;(<>)Crowl&Kenney(<>)2008;(<>)Roseetal.(<>)2010;(<>)Bamford,Milvang-Jensen&Aragón-Salamanca(<>)2007;(<>)Jafféetal.(<>)2011;(<>)Böschetal.(<>)2013a)andS0s((<>)Bedregaletal.(<>)2011;(<>)Johnstonetal.(<>)2012;contrarytoearlierresults,e.g.,(<>)Fisher,Franx&Illingworth(<>)1996),aswellashintsofabrightenedpopulationintheTully-Fisherrelation((<>)Bamfordetal.(<>)2005;(<>)Böschetal.(<>)2013b).Simulationsdemonstratesimilarbehaviour(e.g.,(<>)Kronbergeretal.(<>)2008).Theprocessresponsibleofamorecentrallyconcentratedyoungpopulationcouldbeeitherfadingoftheexternalpartsofthegalaxiesorpushingthegasinwards.",
    "Recentstudieshavefounddisturbedkinematicsintheemission-linegasinclusterspirals,fromwhichtheirfinalgenerationofstarswouldbeexpectedtoform((<>)Jafféetal.(<>)2011;(<>)Böschetal.(<>)2013a).Theincreasedcentralconcentrationoftheyoungpopulationinmanyofourgalaxiesiscertainlyconsistentwithadecreaseinthedegreeofrotationalsupport.Unfortunately,thequalityofourdatamakeithardtodirectlydeterminewhethertherelativevelocitydispersionoftheyoungstarsintheclusterspiralsishigherthanthatoftheoldstellarpopulations.However,togethertheseresultssuggestthatfuturestudiesofclusterS0smayexpecttofindthatthemostrecentdiskstellarpopulationhasasmallerscalelength(andpossiblygreaterscaleheight)comparedtopreviousgenerations,implyingthepresenceofayoung,small,thickdisk.Suchafeaturemayalsobeinterpretedasalenseoradditionalexponentialbulge.",
    "Dustmayalsoplayaroleinacceleratingtheprogressionoftheobservationalsignaturesthatwouldbeassociatedwithatransition.Thecentralconcentrationofstar-formation,asdescribedabove,tothedustierinnerregionsofgalaxies((<>)Driveretal.(<>)2007)resultsinagreaterfractionofthatstarformationbeingobscuredfromopticalindicators((<>)Wolfetal.(<>)2009).Thetransitionstagemaythusbehiddenfromopticalstudies,butapopulationofdusty,red",
    "galaxiesformingstarsatasignificant,thoughpossiblysuppressed,rateisrevealedbyobservationsatlongerwavelengths((<>)Gallazzietal.(<>)2009;(<>)Geachetal.(<>)2009).",
    "Ourresultsindicatethatgalaxy–galaxyinteractionsmaybeassociatedwithstrongerormorerecenttruncatedstarbursts,andhencemaybeasignificanttransitionmechanism.Wethereforesupporttheconclusionsof(<>)Moranetal.((<>)2007),thatacombinationofgalaxy–galaxyinteractions,ram-pressurestripping,andothermoreminormechanismsareresponsibleforspiraltoS0transformation.",
    "Galaxy–galaxyinteractionshavelongbeentheoreticallyassociatedwithstrongbarformationandnuclearstarbursts(e.g.,(<>)Mihos&Hernquist(<>)1996).However,duetothehighrelativevelocitiesofgalaxiesinadenseenvironment,tidalinteractionscanalsohavearelativelygentleeffect((<>)Mooreetal.(<>)1996).Thereisgrowingobservationalevidencethatevenpairinteractionsmaynotcausenuclearstarburstsasreadilyasanticipated,enhancingstar-formationinspiralarmsinstead(e.g.,(<>)Casteelsetal.(<>)2013).Furthermore,barsarefoundtobeprevalentingas-poor,redspirals(e.g.,(<>)Mastersetal.(<>)2011,(<>)2012),andsomaybemoreassociatedwiththesuppressionofstar-formation,ratherthanitsenhancement.",
    "Thefinalargumentforaspiraltolenticulartranformationisthepropertiesofthefinalgalaxies.LenticularsareconsistentwithbeingformedfromfadedspiralsintermsoftheirTully-Fisherrelation((<>)Bedregal,Aragón-Salamanca&Merrifield(<>)2006),globularclusterspecificfrequencies((<>)Aragón-Salamanca,Bedregal&Merrifield(<>)2006).However,theydotendtobemorebulgedominated((<>)Christlein&Zabludoff(<>)2004)andhavehotterdisksthanspiralgalaxies((<>)Cortesietal.(<>)2013).Thiscanbeachievedbyanenhancementofcentralstar-formationpriortotransformation,andamarginalincreaseinpressuresupport,perhapsthroughanaccumulationofgalaxy-galaxyinteractions.Bothoftheseprocessesaresuggestedbyourresultsandmanyoftheotherstudiesdiscussedabove.TheclearingofdustinthecentralregionsduringthetransitionfromspiraltoS0mayalsoenhancethebulge-to-diskratio((<>)Driveretal.(<>)2007).SeparatelymeasuringthestellarpopulationpropertiesofbulgesanddisksforlargesamplesofspiralandS0galaxies,inbothspectroscopic(e.g.,(<>)Johnstonetal.(<>)2012)andmulti-bandphotometricdata(e.g.,(<>)Simardetal.(<>)2011;(<>)Lackner&Gunn(<>)2012;(<>)Bamfordetal.(<>)2012;(<>)Häuleretal.(<>)2013),willhelptofillinmanyofthemissingdetails."
  ],
  "5 CONCLUSIONS ": [
    "ThetransformationfromspiralgalaxiesintoS0s,ifitactuallyoccurs,mustcompriseaspectraltransformation,resultingfromthesuppressionofstarformationinthediskofthegalaxy;amorphologicaltransformation,intermsoftheremovalofspiralfeaturesfromthediskandgrowthofthebulge;andamodestdynamicaltransformation,withasmallincreaseintheratioofpressureversusrotationalsupport.",
    "Wehavestudiedthesignificanceofdisky‘k+a’galaxies,indicativeofaspiralgalaxyinwhichstarformationwastruncated˘ 0.5–1.5 Gyrago,asthepossibleintermediate",
    "stepinthetransformationofstar-formingspiralsintopassiveS0sintheintermediate-redshiftclusterenvironment.",
    "ThesegalaxiesaretypicallyidentifiedbytheirstrongBalmerabsorptionlineequivalentwidths,anexpectedsignatureofadominant˘ 1 Gyroldstellarpopulation.WehaveusedspectraltemplatefittingtoshowthatgalaxiesselectedviatheHF indexdo,indeed,containsignificantfractionsofA-typestarsandstellarpopulationswithagesbetween0.5 and1.5 Gyr.Westudythespatialdistributionoftheyoungpopulationusingthesedifferentindicators,findinggenerallyconsistentresults.Whilethedisky‘k+a’galaxiesappeartobearathermixedpopulation,theirfinalepisodeofstar-formationisalwaysdistributedoveraregionofsizesimilarto,orsomewhatsmallerthan,theolderstars.",
    "Wehavecoarselymeasuredthevelocityfieldofthesegalaxies,bothintermsofthefullstellarpopulationand,inalimitednumberofcases,theseparateyoungandoldpopulations.Theresultssupportthepicturethat,inthemajorityofoursample,thelastgenerationofstarsformedinadisk,inaverysimilarmannertopreviousgenerations.",
    "Noneofthedisky‘k+a’galaxiesinthisintermediateredshiftclusterappeartohaveexperiencedaviolentevent,suchasamergerorsignificantnuclearstarburst,priortothetruncationoftheirstar-formation.Instead,theirregulardiskstar-formationhassimplyceasedwithonly,insomecases,asmallincreaseincentralconcentrationbeforehand.",
    "Arelativelygentlemechanismmustthusberesponsibleforthecessationofstar-formation.Gas-relatedmechanisms,suchasrampressurestripping,arethereforefavoured.However,thereisalsoanindicationthatmanyofourgalaxieswithmorecentrallyconcentratedyoungpopulationshaveexperiencedrecentgalaxy-galaxyinteractions.Thisraisesthepossibilitythat,thankstopriorremovalofthegashalo,stochasticgravitationalinteractionsmayprovidethenecessaryimpetustohaltstar-formation,perhapsviaabriefperiodofcentralenhancement."
  ],
  "6 ACKNOWLEDGMENTS ": [
    "WethankEvelynJohnstonforherinvaluablehelpmodifyingthecodeandforveryusefuldiscussions.Wealsothanktheanonymousrefereeforveryusefulcommentsthathavehelpedimprovingthemanuscript.BRDPacknowledgessupportfromIACandSTFC.BMJacknowledgessupportfromtheERC-StGgrantEGGS-278202.TheDarkCosmologyCentreisfundedbytheDanishNationalResearchFoundation.BasedonobservationsmadewithESOtelescopesattheLaSillaParanalObservatoryunderprogrammeID073.A-0362A."
  ]
}