Environmental effects on the structure of the Dwar

arXiv:astro-ph/9509081v115Sep1995ENVIRONMENTALEFFECTSONTHESTRUCTUREOFTHEDWARFSPHEROIDALGALAXIESM.Bellazzini1,F.FusiPecci2andF.R.Ferraro2.1DipartimentodiAstronomia,Universit´adiBologna,ViaZamboni33,I-40126Bologna,Italy.2OsservatorioAstronomicodiBologna,ViaZamboni33,I-40126Bologna,Italy.e-mail:[e-mail]:39-51-259407ToappearonMonthlyNoticesoftheRoyalAstronomicSociety(Letters)Received:......................................1ABSTRACTThecentralsurfacebrightness(μV(0))ofthedwarfspheroidalsatellitesofourownGalaxyisfoundtocorrelatewiththeirdistancefromtheGalacticCenter(RGC).Thisob-servationalevidencesuggeststhatenvironmentaleffectscouldstronglyinfluencetheirstructure.Thissuggestionisfurthersupportedbyaclear-cutbivariatecorrelationlink-ingμV(0)toalinearcombinationofMV,theabsoluteintegratedmagnitude,andRGC,whichseemstobesharedalsobytheM31spheroidals.PossibleimplicationsfortheFun-damentalPlaneofellipticalgalaxiesandDarkMattercontentofdwarfspheroidalsarealsobrieflydiscussed.1.INTRODUCTIONThesimplefactthatanunequivocaldefinitionofthewholeclassofobserveddwarfspheroidalgalaxies(DSph’s)isstilllackingmaybetakenasanindicationofhowchal-lengingandmysteriousbodiestheyare.Inthisnote,weadoptthedefinitionsreportedbyDaCosta(1992)andGallagher&Wyse(1994,hereafterGW).SothesampleconsideredhereconsistsofninesatellitesofourownGalaxy(i.e.Carina,Draco,Sculptor,Sextans,Fornax,Sagittarius,UrsaMinor,LeoI,LeoII),threesatellitesofM31(i.e.AndI,AndII,AndIII)andtheisolatedgalaxyTucana.AswidelydiscussedintheexaustivereviewsofDaCosta(1992),Pryor(1992),Zinn(1993),Mateo(1993),GW,andFerguson&Binngeli(1994),acompleteunderstandingofthepropertiesofthesestellarsystemsisimportantformanyreasons,likeforinstance:(a)thepossiblerelationwiththeformationprocessoftheir“reference”galaxies(Zinn1993,vandenBergh1994);(b)theirrˆoleintheevolutionarypathwhichleadstoformation2ofellipticalgalaxies(Djorgovski&deCarvalho1990–DdC,GW);(c)theirclaimedhighcontentofDarkMatter(DM)(seePryor1992forareview).HerewepresentanewresultobtainedfromtheanalysisoftheprincipalcorrelationsdetectableamongtheobservedstructuralparametersoftheconsideredsampleofDSph’s,whichaddsempiricalsupporttothehypothesisthatenvironmentalconditions(i.e.,forinstance,thetidalfieldoftheirreferencegalaxy)exertanoticeabledrivinginsettlingtheintrinsicstructureofDSph’s.InSect.2wepresentthedata-setandtheresultsandinSect.3webrieflydiscusstheirimplicationsandsuggestsomeconclusions.2.DATA-SETANDRESULTSThemajorprobleminthestudyofDSph’sisrepresentedbytheintrinsecallylowstatisticalsignificanceofanyglobalanalysisofthesample,eitherbecauseofitspoorness(13objects,atbest),eitherbecauseofthelargeuncertaintiesstillassociatedtothemeasureofreliablestructuralparameters(radii,integratedmagnitudes,etc.)forsodimandsparseobjects.Hence,anycorrelationbasedonthissamplehastobetakenwithparticularcaution.Nevertheless,thesearchforcorrelationslinkingthemainobservablesofgalaxiesisapowerfultoolforthestudyofanyclassofthem,andithasalreadybeenappliedalsotoderiveinterestingevidencesontheDSph’s(DdC,GW,Mateoetal.1993,Caldwelletal.1993,hereafterCASD).2.1Thedata-base.TheobservablesaredrawnmainlyfromthecompilationofGW(theirTable2).ThecentralsurfacebrightnessofCarina,Draco,AndIandAndII,missinginGW,hasbeentakenfromMateo(1993,Table2).TheprojecteddistancesofM31DSphsatellitesarefromvandenBergh(1972).Thecentralvelocitydispersions(σ0[Km/s])andcoreradii3(rc[pc])aredrawnfromMateo(1993,Table2),withtheexceptionofLeoIIwhoseσ0valuehasbeensupersededbyarecentmeasure(Vogtetal.1995).Noticethat,togetastraightforwardparameterwhichscalesdirectelyastheluminousmassdoes,wehavealwaysusedtheabsolutevaluesoftheintegratedabsolutemagnitude(|MV|).2.2Acorrelationbetweencentralstructureanddistancefromtheparentgalaxy.Figure1presentsthecorrelationsoccurringbetweenμV(0)–thecentralsurfacebrightnessinV–andtheparametersLogRGC–thelogarithmofthedistancefromtheGalacticCenter–and|MV|–theintegratedabsolutemagnitude–,forthe8DSph’swhichareneighboursoftheMilkyWay.Atthisstage,theSagittariusDSphhasbeenexcludedfromtheplotstodisplaythebehaviourofanhomogeneoussampleof8objectswithnoevidentsignofstrongtidalpertubationpresentlyinducedbytheGalaxy(butminorsignsoftidaldisturbancehasbeenfoundalsoinsomeotherDSph;Gouldetal.1992;Gerhard1993;Hargreavesetal.1994).Inpanel(1a),thealreadystudiedcorrelationbetween|MV|andμV(0)ispresented.AspreviouslynotedbyGWandCASD,suchacorrelationissignificantlyweakerthanthatfoundforthedwarfellipticals(seeCASD,Fig.8,butalsoDjorgovski&deCarvalho1992).Inpanel(1b),itisshownthatacorrelationatleastassignificantasthepreviousoneisalsostandingbetweenLogRGCandμV(0),inthesensethatDSph’sclosertotheGalaxydisplaydimmercentralregions.Thisisthefirstempiricalfinding,basedexclusivelyonobservedparameters,whichtestifiestheexistenceofapossiblelinkbetweentheintrinsicstructureofGalacticDSph’sandtheirenvironment.NotethatasimilartrendhasbeenshownbyMateoetal.(1991)concerningtheinferredcentralDMdensities,assuminganisotropicvelocitydispersionandawiderdistributionoftheDMwithrespecttostars.4Inordertogivesomeass