A&A486,85–97(2008) Astronomy DOI:10.1051/0004-6361:20078709 & (cid:2)c ESO2008 Astrophysics Kinematics and stellar populations of low-luminosity early-type (cid:2),(cid:2)(cid:2),(cid:2)(cid:2)(cid:2) galaxies in the Abell 496 cluster I.V.Chilingarian1,2,V.Cayatte3,F.Durret4,C.Adami5,C.Balkowski6,L.Chemin6,T.F.Laganá7,andP.Prugniel8,6 1 ObservatoiredeParis-Meudon,LERMA,UMR8112,61Av.del’Observatoire,75014Paris,France e-mail:igor.chilingarian@obspm.fr 2 SternbergAstronomicalInstitute,MoscowStateUniversity,13Universitetskiprospect,119992Moscow,Russia 3 ObservatoiredeParis-Meudon,LUTH,UMR8102,5pl.JulesJanssen,92195Meudon,France 4 Institutd’AstrophysiquedeParis,CNRS,UMR7095,UniversitéPierreetMarieCurie,98bisBdArago,75014Paris,France 5 LAM,Pôledel’EtoileSitedeChâteau-Gombert,38rueFrédéricJoliot-Curie,13388MarseilleCedex13,France 6 ObservatoiredeParis-Meudon,GEPI,UMR8111,5pl.JulesJanssen,92195Meudon,France 7 InstitutodeAstronomia,GeofísicaeC.Atmosf./USP,R.doMatão1226,05508-090SãoPaulo/SP,Brazil 8 Université de Lyon, 69000 Lyon; Université Lyon 1, 69622 Villeurbanne; Centre de Recherche Astronomique de Lyon, ObservatoiredeLyon,9Av.CharlesAndré,69561St.GenisLaval;CNRS,UMR5574,ÉcoleNormaleSupérieuredeLyon,Lyon, France Received19September2007/Accepted12March2008 ABSTRACT Context.Themorphology and stellar populations of low-luminosity early-typegalaxies inclustershaveuntil now been limitedto afewrelativelynearbyclusterssuchasVirgoorFornax.Scenariifortheformationandevolutionofdwarfgalaxiesinclustersare thereforenotwellconstrained. Aims.Weinvestigateherethemorphologyandstellarpopulationsoflow-luminositygalaxiesintherelaxedrichnessclass1cluster Abell496(z=0.0330). Methods.DeepmultibandimagingobtainedwiththeCFHTMegacamallowedustoselectasampleoffaintgalaxies,definedhere as objects with magnitudes 18 < r(cid:3) < 22 mag withina 1.2 arcsec fibre (−18.8 < M < −15.1mag). We observed 118 galaxies B spectroscopicallywiththeESOVLTFLAMES/GiraffespectrographwitharesolvingpowerR=6300.Wepresentstructuralanalysis andcolourmapsforthe48galaxiesbelongingtothecluster.Wefitthespectraof46objectswithPEGASE.HRsyntheticspectrato estimatetheages,metallicities,andvelocitydispersions.Weestimatedpossiblebiasesbysimilarlyanalysingspectraof∼1200early- typegalaxiesfromtheSloanDigitalSkySurveyDataRelease6(SDSSDR6).Wecomputedvaluesofα/Feabundanceratiosfrom themeasurementsofLickindices.Webrieflydiscusseffectsofthefixedaperturesizeonthemeasurements. Results.Forthefirsttime,high-precision estimatesofstellarpopulation properties havebeen obtained foralargesampleof faint galaxiesinacluster,allowingfortheextensionofrelationsbetweenstellarpopulationsandinternalkinematicstothelow-velocity dispersionregime.Wehaverevealedapeculiarpopulationofellipticalgalaxiesinthecoreofthecluster,resemblingmassiveearly- typegalaxiesbytheirstellarpopulationpropertiesandvelocitydispersions,buthavingluminositiesofabout2magfainter. Conclusions. External mechanisms of gasremoval (rampressure stripping and gravitational harassment) aremore likelytohave occurredthaninternalmechanismssuchassupernova-drivenwinds.Theviolenttidalstrippingofintermediate-luminosity,early-type galaxiesintheclustercorecanexplainthepropertiesofthepeculiarellipticalgalaxiessurroundingthecDgalaxy. Keywords.galaxies:ellipticalandlenticular,cD–galaxies:dwarf–galaxies:clusters:individual:Abell496– galaxies:kinematicsanddynamics–galaxies:stellarcontent–galaxies:fundamentalparameters 1. Introduction Understandingtheformationandevolutionofgalaxiesisoneof themostchallengingtasksinmodernastrophysics,andsubstan- (cid:3) Based on observations obtained with ESO VLT (program 074.A- tial progresshasbeen achievedin characterisingthe evolution- 0533) and MegaPrime/MegaCam, a joint project of Canada France arypatternofearly-typegalaxies.Massiveellipticals,whichare Hawaii Telescope(CFHT)and CEA/DAPNIA,at theCanada-France- found principally in clusters, are known to be already present HawaiiTelescope(program03BF12),whichisoperatedbytheNational at z (cid:5) 1 (Ziegler 2000, and references therein). Recent large ResearchCouncil(NRC)ofCanada,theInstitutNationaldesSciences surveys give strong support to: (1) the downsizing star forma- de l’Univers of the Centre National de la Recherche Scientifique tionconcept(thestarformationactivityisseentoprogresswith (CNRS)ofFrance,andtheUniversityofHawaii. (cid:3)(cid:3) All the spectral and imaging data used in this paper are avail- time, from high mass galaxies to smaller ones (as first sug- ablethroughtheVO-ParisDataCentre:http://voplus.obspm.fr/ gested by Matteucci 1994 from variationsof the [Mg/Fe] ratio DataCollections/Abell0496/ withgalaxyluminosityinellipticals,thenbyCowieetal.1996); (cid:3)(cid:3)(cid:3) Appendicesareonlyavailableinelectronicformat and (2) the top-down formation where mass assembly occurs http://www.aanda.org at lower redshifts for lower galaxy masses (Bundy et al. 2006; ArticlepublishedbyEDPSciences 86 I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations Cimattietal.2006).However,ifaconsensusexistsforstarfor- galaxy of an intermediate mass lenticular (Chilingarian et al. mation timescales and chemical evolution, it is not clear even 2007c). formassiveellipticalsifthemassassemblytimecorrespondsto Abell 496 is a richness class 1 cluster (Abell 1958) of the star formation episode duration (Bell et al. 2006; Pozzetti cD type (Struble & Rood 1987) at a heliocentric velocity of et al. 2007; Scarlata et al. 2007). In addition, multiple merg- 9885 km s−1 (z = 0.0330, Durret et al. 2000, and references ers of smaller galaxies are not the only formation mechanism, therein), or 9707 km s−1 (z = 0.0324) after correcting for corr alsoleavingroomforarapidcollapseofgasintheremotepast. infall of the Local Group toward Virgo. For a Hubble constant Moreover,galaxyevolutioninclustersisexpectedtobedifferent H = 73 km s−1Mpc−1, the distance modulus is 35.70 (d = 0 fromthatinlowerdensityenvironments.Thelocalmorphology- 133 Mpc) and the corresponding scale is 0.627 kpc arcsec−1. density relation revealed by Dressler (1980), with a very high Abell 496 is a cluster with several hundred measured galaxy fraction of early-type galaxies found in cluster cores, has been redshifts (Durret et al. 1999). The analysis of the distribution extended to a larger range of galaxy density from the SDSS of 466 redshifts in the direction of this cluster has revealed (Goto et al. 2003) and to higher redshift (Capak et al. 2007). the existenceof severalstructuresalong the line of sight;how- Boththelatterstudyandthedisc-fadingmodeldiscussionforS0 ever, the redshift distribution of the 274 galaxies found to be- galaxyformationbyChristlein&Zabludoff(2004)indicatethat long to the cluster itself implied that Abell 496 has a regular the morphologyand star formationcould be affected by differ- morphology and a well relaxed structure (Durret et al. 2000). entprocesses:theincreaseinearly-typefractionismostlydriven This is confirmedby X-ray data:the X-ray map obtainedfrom by galaxy interactions and harassment (Moore et al. 1998) or XMM-Newtonobservationsisindeedquiteregular,contraryto by tidal effects induced by the crossing of the cluster potential most clusters where even if the X-ray emissivity map appears well (including dynamical friction); the reduction or suppres- regular,thetemperaturemapofthehotgasdoesnot(e.g.,Durret sion of the star formation is caused by ram pressure gas strip- etal.2005). ping(Gunn&Gott1972;Abadietal.1999)orbystrangulation, We will presenthere resultsfor 48low-luminositygalaxies whichpreventsfurthergasaccretionbycuttingofftheouterneu- inAbell496.Theobservationsanddatareductionaredescribed tralgasreservoir(Larsonetal.1980).Inconclusion,thebulges in Sect.2.We givethephotometricpropertiesoftheAbell496 of lenticularsin clustersare suspected to be the results of tidal sample in Sect. 3 and a description of the spectral fitting ap- interactions. pliedtorecoverthestellarpopulationandkinematicalproperties Suchscenariiof morphologicaltransformationsofinfalling inSect.4.Wepresentresultsfromthephotometricandspectro- galaxiesarealsoinvokedforthetransformationofdwarfirregu- scopicanalysesinSect.5,anddiscussourconclusionsinSect.6. larsorfaintlate-typespiralsintoearly-typedwarfgalaxiescon- stitutingthemostnumerousclassofgalaxiesinnearbyclusters. However, until now both the harassment model and the classi- 2. Observationsanddatareduction calwindmodelinavirializedprotogalaxyfailtoreproducethe observedpositionsofdwarfellipticalsinthefundamentalplane 2.1.Imagingobservationsandsampleselection (Djorgovski & Davis 1987) which links the internal kinematics and the structural properties (De Rijcke et al. 2005). Geha We obtainedimageswiththe CanadaFranceHawaiiTelescope et al. (2003) and Van Zee et al. (2004) faced similar difficul- withtheMegacamcamerainthefallof2003(program03BF12, ties trying to explain the intermediate ages and slightly subso- P.I. V. Cayatte). Megacamcoversa field of 1◦ ×1◦ on the sky, lar to solar metallicities found for dwarf ellipticals (dEs) from withapixelsizeof0.187×0.187arcsec2.Weobtaineddeepim- the analysis of Lick indices:no evolutionaryscenario couldbe agesin the u∗, g(cid:3), r(cid:3), andi(cid:3) filters. We reducedthese imagesin ruledoutorconfirmed.Tocomplicatethesituation,Liskeretal. the usual way (bias and flat field corrections, photometric,and (2007) have shown that early-type dwarfs including ellipticals astrometriccalibrations)bythestaffoftheTerapixdatacenterat and dwarf lenticulars(dS0s) do not forma homogeneousclass Institutd’AstrophysiquedeParis(IAP),France.TheSExtractor of galaxy. At least five subclasses with different morphologi- software was runonthe r(cid:3) image(the imagewith the bestsee- cal andclusteringpropertiesarefoundin the Virgocluster:the ing) to detect objects and measure their positions and magni- dE(di)s displaying disc-like features (Lisker et al. 2006a); the tudes.Inparticular,wemeasuredmagnitudeswithina1.2arcsec dE(bc)s showing a blue centre with recentor ongoingstar for- diameter,toprepareFLAMES/Giraffeobservations(seebelow). mation(Liskeretal.2006b);thebrightnucleateddwarfsdE(N)s Details on the data reduction of these images can be found in form an unrelaxed population of disc-shaped dwarfs, which is Bouéetal.(2008). suspectedtobetheresultoftransformationsofinfallingprogen- We thendiscardedstarsbasedonadiagramofaperturemi- itors; faint nucleateddwarfs; and all non-nucleateddEs form a nus totalmagnitudeversustotalmagnitudefor r(cid:3) < 21.Above relaxedpopulationofclassicalspheroidalobjectsthatformedor thismagnitude,wekeptallobjectsinourgalaxysample.Apho- arrivedintheclusteralongtimeago. tometric redshift code was kindly applied by Ilbert to our cat- Inordertoclarifytheevolutionarypathofintermediateand aloguein anattempttoeliminate backgroundgalaxies.Finally, low-mass, early-type galaxies in clusters we have studied the the galaxies observed with FLAMES/Giraffe were taken from photometric, stellar population and kinematical properties of a this imaging catalogue, with a magnitude within a diameter of sample of dwarf galaxies in the nearby cluster Abell 496. Our 1.2arcsecinther(cid:3)band(r(cid:3) )inthe[18−22]interval. 1.2 goalistopointoutobjectsthatareclearlytheresultsofthedif- Wetooktheobservedgalaxiesfromthecataloguedescribed ferentproposedscenariitodefinewhichpropertiesarediscrimi- above, with the following priorities: top priority, objects with nantfortheformationandevolutionoffaintearly-typegalaxies. 18.0−17.5(28objects,i.e.,57percentofthewholesam- B ple). Among all three subsets, the galaxiesare at differentpro- jecteddistancesfromthecD(takenasthecentreofthecluster). Themorphologicalclassificationseemspertinentforthebright- est objects,butwe willnotdiscussthe separationbetweendS0 and dE, as suggested by Lisker et al. (2007) who assigned a commonabbreviation“dE”tothisratherheterogeneousclassof faintgalaxies.Theyconcludedtheirstudybyevidencingvarious subclasses of dEs, and we will examine our sample in such a context. We have applied an elliptically-smoothedunsharp masking technique(e.g.Liskeret al.2006a) to the CFHT/Megacamim- ages of our galaxies to search for embedded structures. Using differentsmoothingradii(semimajor-axesofellipses)from0.3 to4arcsec,weclassifiedalltheobjectsintothreecategories:no, weak,andstrongembeddedstructures.Theyincludedbar,disc, spiral arms, and ring types (see Table A.1). Nine and five ob- jectswerefoundtohavestrongandweakembeddedstructures, respectively. We stress that only one galaxy with M > −17.5 B showsstrongembeddedstructuresandthreeotherfaint−17.5< Fig.2.Absolute Bmagnitude(toppanel)andeffectiveradius(bottom MB <−16.5objectshaveweakones;ontheotherhand,brighter panel,cf.Kormendyrelation)asafunctionofmeansurfacebrightness objectsoftenexhibitstrongandcomplexstructures,notobserved within the effective radius. Giant and intermediate-luminosity ellipti- infainterearly-typedwarfs. cals, power-law and core galaxies are shown in blue, bulges of disc Among the nine objects with strong embedded structures galaxiesinred,dwarfellipticalsandlenticularsinblack.Wekeepthe originalmorphologicalclassification(E/dE)forthedatapointscoming twosetsofthreegalaxiesarefoundrelativelyneareachotherin projectionbutwith verydifferentradialvelocities.The first set fromtheVirgoClusterACSSurvey,andfromCaonetal.(1993),and D’Onofrioetal.(1994).Therefore,theyappearbothinblueandblack. (ACO496J043308.85-130235.6 (G-02), ACO496J043320.35- Abell496galaxiesareshownasfilledbluecircleswithcrosses. 130314.9(G-06) and ACO496J043321.37-130416.6(G-07))is located at the northern limit of the X-ray halo (see Fig. 12 of Tanaka et al. 2006); the second set (ACO496J043326.49- colourmapswithsuchaproperty;allofthemareratherflattened 131717.8 (G-13), ACO496J043331.48-131654.6 (G-15) and galaxiescomparedtotheotherdwarfsandcouldperfectlycorre- ACO496J043333.53-131852.6(G-18)) is in the south near the spondtotheflattenedspheroidinvokedbyLiskeretal.(2006b) cluster centre, in a region where the X-ray surface brightness toexplaintheirsubclassofdEswithbluecores.Alastsubclass is seen in excess compared to its azimuthally averaged value defined in the Virgo cluster is obtained by separating the “fea- and corresponding to a cold front. However, we interpret the tureless” dE class into nucleated and non-nucleated galaxies. set of three galaxies projected on the cluster core as three ob- Since Abell 496 is more distant than Virgo, the identification jects only seen close in projection (see notes for each galaxy of a nucleusis more difficultto define with the same precision in AppendixA). In bothcases, the existenceof a realgroupof in termsof flux and size. From unsharpmasks we can identify galaxieshastobetested. the galaxies where strong gradients are observed in the central Concerningthegalaxieswithweakembeddedstructures,one regions; nine dwarf galaxies with M > −18.0 mag present B of them is the brightest object found among a group of four brightcompactcomponentsintheircentres,oneofthemhaving galaxiesinourspectroscopicsample,whichareallinthesame a brightblue core. Compared to the Virgo dwarf study, the es- regionoftheskyabout400kpcnorthofthecD.Asdiscussedin timatednumberofnucleatedobjectsmissedcouldbelargerbe- thenotesofAppendixA,ACO496J043348.59-130558.3(G-32), causewecannotbesurethatsmallsizenucleiarenotsmoothed ACO496J043343.04-130514.1 (G-28), ACO496J043345.67- bytheseeingeffects.Inthefollowingsubsectionwewilldiscuss 130542.2 (G-30) and ACO496J043349.08-130520.5 (G-33) theresultsofthespectroscopicdataintermsofageofthecentral probablybelongto a realgroupsincetheir radialvelocitiesare stellarpopulation;anothersubclassificationwillbeproposedto closetoeachother. providetestsforformationandevolutionscenarii. Finally,eightdwarfgalaxieswith M > −18.0magexhibit Figure 2 presents the updated versions of Figs. 9a,g from B spiralarms,bars,rings,oredge-ondiscssimilartothestructural Graham&Guzmán(2003):absolute Bmagnitudeandeffective propertiesdefinedbyLiskeretal.(2006a)inthedwarfpopula- radiusR versusmean Bsurfacebrightnesswithintheeffective e tionoftheVirgocluster,correspondingto24percentforatotal radius(cid:10)μ(cid:11) ,withtheAbell496galaxiessuperimposed.Theplot e numberof34dEsinthespectroscopicsampleofAbell496;this contains literature data only for elliptical galaxies and bulges valueislargerthanthepercentagegivenbyLiskeretal.(2007) of lenticulars/spirals (where the bulge/disc decomposition has fortheirdE(di)subclass,butitcanbeexplainedbyabiastoward beenmadeintheoriginalpapers).Allintegratedmeasurements brightgalaxiesin our sample. Anothersubclass pointed outby for spiral and lenticular galaxies have been excluded. Data in Liskeretal.isthatofdEswithbluecentreswhichexhibitrecent computer-readableformatfordE andE galaxiesfromBinggeli orongoingcentralstarformation.Threeobjectsareseenonthe & Jerjen (1998), Caon et al. (1993), D’Onofrio et al. (1994), 90 I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations Faber et al. (1997), Graham & Guzmán (2003), Stiavelli et al. (2001) and homogenization algorithms for these datasets have been kindly provided by Graham. We also included photomet- ricparametersofEanddE/dS0galaxiesfromtheVirgoCluster ACS Survey(Ferrareseet al. 2006);photometricdata ongiant, intermediateellipticalgalaxiesandbulgesofspiralsandlentic- ulars from Bender et al. (1992); photometricparametersof the SérsiccomponentofM32(Graham2002);anddatafor430el- liptical galaxies from the HyperLeda1 database (Paturel et al. 2003), with radial velocities below 10000 km s−1 and brighter thanM =−18.0mag. B Thewell-knownstructuraldichotomybetweendiffusedwarf galaxiesandclassicalellipticalsandbulgesisclearlyvisibleon both plots in Fig. 2. A sequence of classical elliptical galaxies startswiththemostluminousclustergalaxiesatfaintersurface brightnesses and ends with a few compact elliptical galaxies Fig.3. Faber-Jackson relation for ellipticalsand bulges of disc galax- (M 32-like objects) at high surface brightnesses. At the same ies. Abell 496 objects are shown as blue-filled circles. Giant and time,diffusedwarfellipticalgalaxiesformaseparatesequence. intermediate-luminosity ellipticalsareinblue, bulgesof discgalaxies inred,dwarfellipticalsandlenticularsinblack.Theupper endofthe We notice that the counter-argumentsagaint this interpretation arrowrepresentingM32correspondstotheHSTSTISmeasurements ofthestructuraldiagramsexist(e.g.Graham&Guzmán2003). (Josephetal.2001),andthefilledcirclerepresentsthevalueobtained It is remarkable that over a large span of absolute magnitudes fromearlierHSTFOSdata(vanderMareletal.1998),whichwasin (−18 < M < −13 mag) diffuse galaxies show no correlation B agreementwithmorerecentground-basedobservations. betweeneffectivesurfacebrightnessandeffectiveradius:reff al- ways remains between 0.6 and 2.0 kpc (see bottom panel). A number of objects, the brightest representatives of our sample, literatureduetoatemplatemismatch(σ-metallicitydegeneracy, lie in the transition region between the two regimes (“normal” seeChap.1inChilingarian2006). and“diffuse”ellipticals). 4.3.Stellarpopulationproperties 4.2.Kinematicalproperties Our fitting method is not sensitive to the presence or absence The Faber-Jackson relation (Faber & Jackson 1976), reflecting of the Hβ featurein the spectral range:thoughage estimations the connection between the dynamical and stellar masses, is havehigheruncertaintieswhenHβisnotincluded,theyremain shown in Fig. 3. We present a compilation of data for dwarf unbiased(seeAppendixBfordetails). (Geha et al. 2003; van Zee et al. 2004; De Rijcke et al. 2005; To fit the spectra we use the PEGASE.HR synthetic popu- Matkovic´ &Guzman2005),intermediateluminosity,andgiant lationsbuiltusingtheELODIE.3libraryincludingonlystarsin ellipticalgalaxiesandbulgesofbrightlenticulars(Benderetal. the nearest solar neighborhood.These stars are known to have 1992). [Mg/Fe]correlatedwith their[Fe/H]metallicities(see Wheeler Matkovic´ & Guzmán (2005) have analysed a sample of et al. 1989, and references therein). Thus fitting spectra of mostlydwarfandlow-luminosityellipticalgalaxies(M <−18) galaxies with non-solar [Mg/Fe] for metallicities higher than B intheComacluster.InordertohaveaccurateBmagnitudesfor −1.0 dex results in a template mismatch that can bias our es- thesegalaxies,weretrievedthephotometriccatalogueofgalax- timationsofstellarpopulationparameters.Toquantifypossible ies in the direction of Coma2 (Adami et al. 2006) and cross- biases and study the effect in detail we used spectra of early- correlateditwiththeMatkovic´&Guzmáncatalogue.Valuesfor typegalaxiesfromtheSloanDigitalSkySurveyDataRelease6 66matchedobjectshavebeenusedinFig.3. (Adelman-McCarthyetal.2008).SeeAppendixBfordetails. Someobjectsofoursamplelocatedbelowthebulkofgalax- Toobtain[Mg/Fe]abundanceratiosfortheAbell496galax- ies in the luminosity range between M = −17 and −19 mag ies and to check whether our age and metallicity estimations B are mostly lenticular galaxies with large discs, therefore, their arecorrectweusedstellarpopulationmodelsdealingwithLick velocitydispersionstendtobelower(e.g.,ACO496J043306.97- indices of magnesium and iron (Thomas et al. 2003). In or- 131238.8(G-01)). der to compute Lick indices we degraded the spectral resolution to match that needed to compute Lick indices (Worthey Weobserveapopulationofobjectswithhighervelocitydis- et al. 1994; Thomas et al. 2003) by convolving the original persionsthan expectedfortheirluminosities.Mostofthemare spectra with a Gaussian having a width equal to the square located in the inner 80 kpc from the cluster centre and exhibit root of the differences between the squares of the Lick resolu- quite unusual stellar populations. We will discuss their origin andevolutionbelow. tion(σLick),LSF(σinst)andvelocitydispersi(cid:2)onvaluesfoundby We found a number of Abell 496 objects, located sys- thespectralfittingprocedure(σ ):σ = σ2 −σ2 −σ2. g degr Lick FL g tematically below dE galaxies in the literature. Since most Oneshouldalsoapplyavelocitydispersioncorrectionforlarge of the studies (apart from Geha et al. 2003) were based on galaxies,whentheintrinsicbroadeningofthespectrallinesex- spectroscopy with significantly lower spectral resolution than ceeds the Lick resolution. All the objects in our sample have FLAMES/Giraffe, we cannot exclude the possibility of sys- relatively low velocity dispersions, therefore these corrections tematic errors on velocity dispersions for low-σ objects in the werenotnecessary.AsimilarapproachtocomputeLickindices hasbeenusedbyKuntchneretal.(2006).Thespectralrangeof 1 http://leda.univ-lyon1.fr/ FLAMES/Giraffe in the setup we used and the mean heliocen- 2 http://cencosw.oamp.fr/ tric redshift of the cluster z = 0.0330 allow us to compute the I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations 91 followingLickindices:Fe , Mgb,Fe ,Fe ,andFe . 5015 5270 5335 5406 Uncertaintiesonthemeasurementswerecomputedaccordingto Cardieletal.(1998). There are three sources of systematic errors that can affect the measurementsofLick indices:(1)data reductionissues re- sultinginadditiveerrors,suchasproblemswiththediffuselight or sky subtraction; (2) difference in the spectral resolution between given observations and the Lick/IDS system; (3) uncer- taintiesinthedeterminationoftheradialvelocities. The diffuse light subtraction for FLAMES/Giraffe is done with a highaccuracythanksto the rathersparse packingofthe MEDUSA fiber traces on the CCD plane, therefore, it cannot resultinsignificanterrors.Weobservedtheskysimultaneously with the galaxies in different parts of the field of view, so it is possible to assess the quality of sky subtraction by comparing individual sky spectra obtained in different fibers. No systematic difference is observed, we therefore conclude that the sky subtractionprocedureisreliable. The radial velocities and velocity dispersions of the Abell496galaxiesaremeasuredwithhighaccuracy,duetothe factthattheFLAMES/GiraffeLSFhasalmostinfiniteresolution intermsofsquaredifferences,comparedtotheLick/IDSsystem. Fig.4. Mgb – σ (top) and (cid:10)Fe(cid:11)−σ (bottom) relations. Only points We also notice that in the course of this study to analyze 0 0 Lick indices for both FLAMES/Giraffe and SDSS spectra (see havingΔ(Mgb)<0.5Åareshown.SDSSgalaxieswithagesolderthan 3Gyrareshownaslight-bluedots.Theblack-solidlinerepresentsthe AppendixB),weusethemodelsbyThomasetal.(2003),rather correlationbetweenMgbandσ foundbySmithetal.(2006). 0 thanthemodelsbyWortheyetal.(1994).Therefore,converting the computed values of Lick indices into the Lick/IDS system throughtheobservationsoftheLick/IDSstarsisnotnecessary. Moreoverit was notpossible to performthis empiricalconver- the aperture effect may not be very important for giant galax- sionbecauseLickstarshavenotbeenobserved. ies(asmentionedinGallazzietal.2006)becausetheσgradient The measurements of the Lick indices are presented in willcompensatethemetallicitygradient;itmaybeimportantfor AppendixA(TableA.3). dwarfsandintermediateluminosityobjects. Therehave beena numberof recentstudies (Gallazzietal. Whenmanyclustersofgalaxiesatdifferentredshiftsareob- 2006; Smith et al. 2006; Yamada et al. 2006; Carretero et al. servedwiththesameinstrument(e.g.,Gallazzietal.2006;Smith 2007) addressing correlations between galaxy absorption line etal.2006)theapertureeffectwillincreasethespreadofabsorp- strengths and velocity dispersions. Most of them are based tionlinestrengthmeasurementsforagivenvelocitydispersion. on observations obtained with multi-object spectrographs with InFig. 4,we presentthemeasurementsofMgband(cid:10)Fe(cid:11) = fixed spatial aperture sizes (as in the present study). Although 0.72Fe +0.28Fe versusvelocitydispersion.Wealsoput 5270 5335 thisobservationaltechniqueallowsfortheacquisitionofspectra measurements for ∼700 early-type galaxies older than 3 Gyr ofmanyobjectsduringasingleexposuretime,ithassomedis- with redshifts z < 0.033 observed in the SDSS on the same advantageswhenstudyingstellarpopulations.Early-typegalax- plots. We have used values of Lick indices provided by the ies(giantsanddwarfs)andbulgesofspiralsareknowntohave SDSS. The solid line on the Mgb-σ diagram corresponds to rather strongmetallicity gradients(see, e.g., Sánchez-Blázquez thebest-fittingrelationfromtheNationalOpticalAstronomical et al. 2006c) and often evolutionary decoupled nuclei (e.g., Observatory(NOAO)FundamentalPlaneSurvey(NFPS,Smith Sil’chenko2006;Chilingarianetal.2007b,2008b;Peletieretal. et al. 2006). Measurements for a sample of early-type galax- 2007).Youngand/ormetal-richstellarpopulationsinnucleiwill iesfromSánchez-Blázquezetal.(2006a−c),kindlyprovidedby not strongly affect the aperture measurements for galaxies lo- Sánchez-Blázquezinacomputerreadableform,areshowninor- cated 100 Mpc or further, because their spatial sizes are small ange.ThereisrelativelygoodagreementbetweenNFPS,SDSS, and contributions to the total fluxes within a 1.5 arcsec-wide Sánchez-Blázquezetal.(2006a),andthecorrelationwefindfor aperturearenegligible,metallicitygradientsmaybiastheresults Abell496galaxies.Ourobjectstendtobeslightlyricherinmag- quitestrongly. nesium,in contrasttotheSDSSones.Inourcase,wehaveob- If members of a given cluster of galaxies are analyzed, the servedmostlygalaxiesneartheAbell496clustercore,whilein scale ofthe gradientwill dependontheir effectiveradii,which thecaseofNFPS,galaxieshavebeenobservedintheperipheral are rather tightly connected to luminosities. At the same time partsof the clustersaswell, and forSDSS therewas a number since luminosities of early-type galaxies correlate with central ofgroupandfieldgalaxies(sincewedidnotapplyanyenviron- velocitydispersions(Faber&Jackson1976),forlow-σobjects mentalselectioncriteria).Smithetal.(2006;seealsoSil’chenko the same aperture size in average will contain a larger frac- 2006,foranapplicationtolenticulargalaxies)demonstratedthat tion of the galaxy,leading to an underestimationof the central objectsinthecoresofclusterstendtobeabovetheaverageline, or effective metallicity. This may cause a change in the slope andconversely,galaxiespopulatinglessdenseenvironmentsare of the Mgb-σrelation, especially for fainter objects(makingit lessmetal-rich. steeper). On the other hand, the velocity dispersion also varies We notice four high-σ outliers from the Abell 496 sam- with radius; however, for low-luminosity and dwarf galaxies, ple significantly above the sequence of Smith et al. (2006) σ-profiles turn to be almost flat (Simien & Prugniel 2002; in the Mgb-σ plot. Those objects (ACO496J043333.17- Geha et al. 2002, 2003; van Zee et al. 2004). Therefore,while 131712.6 (G-17); ACO496J043337.35-131520.2 (G-20); 92 I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations Fig.5.(cid:10)Fe(cid:11)vs.Mgbwithsizesofsymbolsindicatingthevaluesofcen- tral velocity dispersions. Models from Thomas et al. (2003) for dif- ferentvaluesof[α/Fe]enrichmentareoverplotted.Onlypointshaving Δ(Mgb)<0.5Åareshown. ACO496J043338.22-131500.7 (G-21); ACO496J043341.69- 131551.8 (G-24)) are all located in the innermost part of Abell 496 and have probably experienced strong tidal harassment(seediscussion). The (cid:10)Fe(cid:11) versusMgb relation is shown in Fig. 5. The sizes of symbols reflect values of the velocity dispersion. The data forearly-typegalaxiesfromSánchez-Blázquezetal.(2006a−c) areoverplotted.ThemodelsofThomasetal.(2003)for−0.3 < [Mg/Fe] < +0.5 dex are shown as crosses; each sequence in- cludes models for an indicated metallicity and ages from 3 to 15 Gyr (bottom-left to top-right). Nearly all objects with low velocity dispersions exhibit solar [Mg/Fe] ratios (the corresponding models are shown in cyan). High-σ objects have [Mg/Fe]>0.2dexand,ingeneral,tendtohavehighermetallic- ityvalues. Fig.6.Relationsbetweencentralvelocitydispersionandstellarpopu- In Appendix B, we demonstrate that non-solar[Mg/Fe] ra- lationparameters:SSP-equivalentage(top),metallicity(middle),anda tios bias neither age, nor metallicity estimations obtained by combinationoftheseparametersfreeoftheeffectoftheage-metallicity spectral fitting in the wavelength range of FLAMES/Giraffe. degeneracy(bottom).Thepresenceofembeddedstructuresinthegalax- Indeed, the measurements do suffer from the well-known age- iesisindicated.Valuesforearly-typegalaxiesfromSánchez-Blázquez metallicity degeneracy (see, e.g., Worthey 1994) expressed as etal.(2006a,b)andmeasurementsfor848SDSSgalaxiesareoverplot- Δt/ΔZ ≈ 3/2or Δ(log t)/ΔZ ≈ 2/3.In AppendixA,we pro- ted.ColoursarethesameasinFig.4. 10 videmapsofχ2 intheage-metallicityspaceforeveryobjectin oursample.Theelongatedshapeoftheχ2minima,corresponds exactlytotheexpecteddegeneracy. In Fig. 6, we compare the age t and metallicity estimates tend to beolderin dense environments,since we find veryfew versus velocity dispersion for Abell 496 galaxies and for the objectsthereyoungerthan10Gyrhavingσ>60kms−1. samplesofSDSSobjectsmentionedaboveandearly-typegalax- Metallicityversusσ(middlepanelofFig.6)exhibitsamuch ies from Sánchez-Blázquez et al. (2006a). We used the values strongercorrelationthanagevs.σ. TheAbell496galaxiesap- fromSánchez-BlázquezobtainedbyinvertingthegridoftheHβ pear to be almost on the low-σ extension of the sequence of and (cid:10)MgFe(cid:11) Lick indices. The top panel presents t versus σ. the SDSS galaxies, although there are several faint Abell 496 Abell 496 galaxies are systematically older than objects from dwarfswithratherhighmetallicities.ThebottompanelinFig.6 theSDSSsampleandthespreadofagesisquitelarge.However, shows a combinationof age and metallicity giving an edge-on wenote(a)theabsenceofyounggalaxieswithhighvelocitydis- view of the age-metallicity degeneracy: 0.4log t + 0.6Z ver- 10 persions;(b)thefactthatsmallgalaxies(withlowvelocitydis- sus velocity dispersion. The correlation becomes much tighter persions)tendtobeyoungerthanlargeones;and(c)thespread than Z−σ, and loci of Abell 496 and SDSS galaxies follow of age estimates is larger for low-mass objects. Two explana- the same correlation. The spread of points (standard devia- tionsforthe offsetbetweenthe Abell496galaxiesandthetwo tion) for a given value of the velocity dispersion is less than othersamplesonthetoppanelaretheenvironmentaleffectsand 0.1 dex. A number of outliers in the Abell 496 sample are the sampleselection.We haveselectedtheSDSSgalaxieswith seen: two galaxies in the high-σ area, located in the very cen- a minimal signal-to-noise ratio, so dwarf galaxies have been tre of the cluster (A496cE and A496g1 using the terminol- nearly automatically excluded. On the other hand, we confirm ogy from Chilingarian et al. 2007c); and three dwarf galaxies fromoursample(whichincludesthegalaxiesin the centralre- (ACO496J043324.61-131111.9 (G-08); ACO496J043339.07- gion of a massive cluster) that intermediate-luminosity objects 131319.7 (G-22); and ACO496J043355.55-131024.9 (G-37)). I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations 93 This quantity, 0.4log t + 0.6Z, is considered an indicator of 10 theaveragestrengthofabsorptionlinesinthespectrum. There are seven galaxies in our sample exhibit- ing relatively young stellar populations (t < 3 Gyr): ACO496J043321.37-130416.6 (G-07); ACO496J043325.54- 130408.0 (G-12); ACO496J043334.54-131137.1 (G-19); ACO496J043350.17-125945.4 (G-34); ACO496J043351.54- 131135.5 (G-35); ACO496J043356.18-125913.1 (G-38); and ACO496J043415.37-130823.5 (G-46). Three of them, ACO496J043321.37-130416.6 (G-07); ACO496J043356.18- 125913.1 (G-38); and ACO496J043415.37-130823.5 (G-46) have narrow [OIII] emission lines in the spectra (σ between 20 and 40 km s−1) and there is no evidence for the [NI](λ=5199Å)lineinatleasttwoofthem,suggestingongo- ingstarformationratherthanshockedgas.Thefirsttwo,aswell asACO496J043351.54-131135.5(G-35),containbluespatially unresolvedcentralregionsclearly visible on colourmaps.This canbeconsideredasanargumentforthepresenceofyoungstars and/orstarformationonlyinthecoresofthegalaxies.Allseven galaxiesarelocated quitefar fromthe cluster centre(projected distances are 150−200 kpc for ACO496J043334.54-131137.1 (G-19)andACO496J043351.54-131135.5(G-35)and>450kpc fortheotherfive). A significant fraction of galaxies (13 of 46) exhibit red spatially unresolved cores in the colour maps. Stellar populations of these objects determined by spectral fit- Fig.7.Ageand[Mg/Fe]abundanceratiosofgalaxiesasafunctionof ting are rather old and metal rich, none of the spectra projecteddistancetotheclustercentre.Onlygalaxieshavinguncertain- shows emission lines. In some cases (ACO496J043320.35- tieson[Mg/Fe]determinationsbetterthan0.1dexareshowninthebot- 130314.9 (G-06); ACO496J043331.48-131654.6 (G-15); tompanel.Theremaininggalaxiesareshowninblueinthetoppanel. ACO496J043333.53-131852.6 (G-18); ACO496J043342.10- 131653.7 (G-25); ACO496J043346.71-131756.2 (G-31); and ACO496J043401.57-131359.7 (G-40)) the red cores represent the central parts of extended bar-like structures, clearly visible IntheirstudyofVirgoearly-typedwarfgalaxies,Liskeretal. on colour maps, which are redder than the outer parts of (2007,andreferencestherein)donotgivethestellarpopulation their host galaxies. However, in most of the other red-core properties of the three subclasses, which are the result of in- galaxies only the cores show peculiar colours, while the fallinggalaxytransformation.Ifweputtogethertheeightdwarfs discs/spheroids look more or less uniform on the maps. In the correspondingtotheirdE(di)subclassandthefiveotherdwarfs, caseofACO496J043306.97-131238.8(G-01),anedge-ondisky which exhibit very young central stellar populations, this sub- galaxy, dust absorption is responsible for the redder colour of sample presents the same spatial and velocity distributions as thecentralregion. theirleastunrelaxeddwarfgalaxypopulation.Thevelocityhis- Tryingtorelatethepresenceofembeddedstructures(discs, togram is more spread for the 13 selected dwarfs compared to spiralarms,bars)tothestellarpopulationparameters,westress that of the 24 remaining early-type dwarfs, including the two thatthemajorityofthegalaxiesexhibitingembeddedstructures galaxies for which we did not succeed to fit the spectra due have luminosity-weightedages of between 4 and 10 Gyr (only to low signal-to-noise ratios. If we also take into account their ACO496J043346.71-131756.2(G-31)andACO496J043403.19- corresponding spatial distributions, we can conclude that the 131310.6 (G-41), with bars and faint spiral arms, are about subclass isprobablynotfullyrelaxed.A cleardifferenceexists 14Gyrold,buttheirabsolutebluemagnitudeisbelow−18mag; in the stellar populationages, since among the 22 dwarfs with ACO496J043321.37-130416.6 (G-07) is a young star-forming noembeddedstructures,onlysixgalaxieshaveagesbetween3 galaxy)andallofthemhavefeaturesvisibleoncolourmaps.No and9Gyr. objectswithembeddedstructuresarefoundintheveryinnerpart Thebrighterpartofoursamplecanbecomparedtoexisting of the cluster (d < 60 kpc) where the luminosity-weighted studies of stellar populations of early-type galaxies. Poggianti proj ages of the five galaxies in our sample are above 12 Gyr. In etal.(2001)presentedtheluminosity-weightedagesandmetal- Fig. 7, the ages and α-enhancements of galaxies at different licities for several dozens of Coma cluster early-type galaxies. projected distances from the Abell 496 centre are shown. In a From their Fig. 2, it is clear that there are two populations of transition area (d between 60 and 230 kpc) the age of the galaxies,oldandyoung.TheageoftheoldonesforM >−19.0 proj B youngest stellar populations decreases when the projected dis- anticorrelateswiththeluminosity.Inoursample,however,wedo tance increases, as it can be seen in Fig. 7. Half of the galax- nothaveanybrightgalaxywithayoungstellarpopulation.This ieswithembeddedstructuresarefoundinthisareabutsomeof isprobablyduetotheselectionofobjectsinoursample:(1)we themcouldhaveadistancetotheclustercentrelargerthantheir tried to avoid galaxies with redshifts already in the literature; projected distance. In the bottom panel of Fig. 7, the ratio of (2) young galaxies have higher surface brightnesses compared α-elementsoverironinthetransitionareaisnotaslargeasfor tooldonesforagivensize,therefore,theyhaveabetterchance theinnermostpartoftheclusterandspreadsoverthesamerange to have already been observed in redshift surveys with limited asforgalaxiesatlargerprojecteddistances. aperturemagnitudes,andthustobeexcludedfromoursample. 94 I.V.Chilingarianetal.:Abell496:Kinematicsandstellarpopulations Sánchezetal.(2007)haveobtainedinternalkinematicsand stellar population parameters for galaxies in the core of the Abell2218clusterusing3D-spectroscopyoftheclustercentre. ThebehaviorofageestimatesissimilartothatoftheAbell496 galaxies:forlow-massobjects,agestendtobeyoungerandmore spreadoutthanforlargegalaxies. 5. Discussion 5.1.OntheoriginofdE/dS0galaxies We discuss here what we can learn from the stellar population properties of dE/dS0s in Abell 496 regarding their origin and evolution. First, what is the explanation for the spread in the stellaragesandmetallicitiesofourdwarfgalaxysample?Isthis spreadrelatedtosomedifferencesinthetimeneededforgalax- iestobeaccretedintotheclusterandtoreachtheclustercore? Low-massgalaxiesare more sensitivethan massive galaxiesto bothinternalandexternalprocessesaffectingtheirgascontents andstarformationrates.Therefore,the“insitu”formationsce- Fig.8.1-σconfidencelevelsofageandmetallicitydeterminationsfor nario,whichisabletoreproducethevarioustimescalesforstar the Abell 496 galaxies. Objects in the three velocity dispersion bins formation,could work as well as an accretion scenario. Which are plotted using different colours: red for σ < 30 km s−1, blue for scenario can explain that star formation occurs in the nuclear 30<σ<60kms−1,andblackforσ>60kms−1. regionsforanumberofdwarfs?Starformationisdriveninpar- ticularbythegascontent,andifthegasisrapidlyremovedfrom its host galaxy, star formation can be stopped in a very short between the dynamical mass and metallicity can be explained time.Ontheotherhand,aninfallofgasinthegalaxycentrewill by the lower efficiency of star formation in galaxies of lower inducestarformationinthenuclearregion.Thescenarioshould massesduetomoreefficientsupernovafeedback.However,un- also reproducethe observed[Mg/Fe]ratio andits relationwith derthisscenario,onewouldnotobserveanyolddwarfellipticals thevelocitydispersion. becauseyoungerstars,formedinthesecondaryepisodesofstar Three possible scenarii for the gasremovalusually consid- formation, would dominate the light even though their masses eredfordEgalaxiesare:(1)supernova-drivenwindsattheearly mightbe low comparedto the massesof the first generationof stages of galaxy evolution (Dekel & Silk 1986); (2) ram pres- stars. Therefore,one still needs a mechanism to sweep out the surestrippingbytheintra-clustermedium(e.g.,Marcolinietal. remaininggasfromthelow-massgalaxiesearlierorlaterduring 2003); and(3) tidalharassmentdueto distantandrepeateden- theirlifetime. counters with other cluster members (Moore et al. 1998). The In the case of Abell 496, [Mg/Fe] ≈ 0 dex for nearly all first oneis oftenreferredto as“internal”andlattertwo as“ex- low-massobjects(σ <60kms−1),meaningthatthestarforma- 0 ternal”agentsofdEgalaxyformationandevolution. tionepochdurationswereatleast1−2Gyr,theminimalrequired Theideaofsupernova-drivenwindsisbasedontheassump- timetocompletetheironenrichment(Matteucci1994).Hence, tion that the gravitational field of dwarf galaxies is not suffi- we cannot consider the scenario of gas removal by supernova- cientlystrongtokeeptheinterstellarmediumfrombeingswept drivenwindsastheonlyexplanationfortheobservedproperties out by SN II explosions during the first intense star formation ofdEgalaxies,althoughitallowsustoreproducetheobserved episode. mass-metallicitycorrelation. Several different models for galactic winds exist (see Ram-pressurestrippingoflate-typedwarfgalaxies(dIrr)or De Rijcke et al. 2005, for a detailed review). Simple mod- dwarfspiralsappearstobeanacceptablewaytoremovethegas els discussed in Yoshii & Arimoto (1987) lead to abrupt gas from dE progenitors.If we assume that late-typegalaxieshave loss and interruptionof the star formationepisodeaftera short formed outside the central region of the cluster, and later fell time (107 years). Consequently, later explosions of SNIa on a ontoit,sufficienttimeisleftforironenrichment,sincethetyp- timescaleofgigayearswillnotcontributetotheironenrichment icalinfalltimeisafewGyr.Ifram-pressurestrippingplaysthe of the stellar population (Matteucci 1994). Thus, a short star leading role in gas removal, one would expect a large spread formationepisodewillleadtoanoverabundanceofα-elements of luminosity-weighted ages for low-mass objects that can be overiron([α/Fe] > 0).Thisphenomenonisobservedinglobu- completely strippedduringtheir first passage throughthe clus- larclustersand,usually,ingiantearly-typegalaxies(Kuntschner ter centre. This stripping can occur at any moment during the etal.2006;Sil’chenko2006).IfgasisremovedfromdEgalaxies galaxy lifetime. Due to our small sample we cannotgive a de- bysupernova-drivenwinds,we wouldexpectto see[Mg/Fe] > cisiveanswertowhetherthespreadofageestimationsinFig.6 0 dex, and it should anticorrelate with the dynamical mass of is the result of ram-pressure stripping of late-type progenitors, galaxies (or σ). This does not agree with what we see in our or due to the low quality of measurements (including the age- data. metallicitydegeneracyeffects). Another study by Chiosi & Carraro (2002), contrary to Smith etal. (2007) pointedoutthatthereis a conspiracyin Yoshii&Arimoto(1987),predictsverylongandoscillatingstar the sense thatthe distributionof galaxiesin the Z −logt plane formationhistoriesindwarfgalaxies:supernovaexplosionsdis- at fixed velocity dispersion is aligned with the age-metallicity perse the gas, stopping star formation, but later the gas cools degeneracy. In Fig. 8, we plotted the relationship between age down,fallsbackinandanotherstar-formationepisodebegins.In andmetallicityfordifferentintervalsofsigmawiththeSmithet thiscase,the[Mg/Fe]ratiosdecreaseto∼0dex,thecorrelation al. sample of faint red galaxies in three clusters located inside

Kinematics and stellar populations of low-luminosity early-type galaxies in the Abell 496 cluster

Similar Kinematics and stellar populations of low-luminosity early-type galaxies in the Abell 496 cluster