Towards a Phylogenetic Analysis of Galaxy Evolution a Case Study with the Dwarf Galaxies o

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arXiv:astro-ph/0605221v1 9 May 2006Astronomy&Astrophysicsmanuscriptno.astrocladDwarfLGc󰀂ESO2008

February5,2008

TowardsaPhylogeneticAnalysisofGalaxyEvolution:

aCaseStudywiththeDwarfGalaxiesoftheLocalGroup

D.Fraix-Burnet1,P.Choler2,andE.J.P.Douzery3

1Laboratoired’AstrophysiquedeGrenoble,BP53,F-38041Grenoblecedex9,France

2Laboratoired’´EcologieAlpine,BP53,F-38041Grenoblecedex9,France

3LaboratoiredePalontologie,PhylognieetPalobiologie,InstitutdesSciencesdel’´EvolutiondeMontpellier,F-34095Montpelliercedex5,

France.

Received:February28,2006;accepted:April25,2006

ABSTRACT

Context.TheHubbletuningforkdiagramhasalwaysbeenthepreferredschemeforclassificationofgalaxies.Itisbasedonmorphologyonly.

Attheopposite,biologistshavelongtakenintoaccountthegenealogicalrelatednessoflivingentitiesforclassificationpurposes.

Aims.Assumingbranchingevolutionofgalaxiesasa’descentwithmodification’,weshowherethattheconceptsandtoolsofphylogenetic

systematicswidelyusedinbiologycanbeheuristicallytransposedtothecaseofgalaxies.

Methods.Thisapproachthatwecall“astrocladistics”isappliedtoDwarfGalaxiesoftheLocalGroupandprovidesthefirstevolutionarytree

forrealgalaxies.

Results.Thetreesthatwepresentherearesufficientlysolidtosupporttheexistenceofahierarchicalorganizationinthediversityofdwarf

galaxiesoftheLocalGroup.Thisalsoshowsthatthesegalaxiesallderivefromacommonancestralkindofobjects.Wefindthatsomekindsof

dIrrsareprogenitorsofbothdSphsandotherkindsofdIrrs.Wealsoidentifythreeevolutionarygroups,eachonehavingitsowncharacteristics

andownevolution.

Conclusions.Thepresentworkopensanewwaytoanalyzegalaxyevolutionandapathtowardsanewsystematicsofgalaxies.Workonother

galaxiesintheUniverseisinprogress.

Keywords.Galaxies:fundamentalparameters–Galaxies:evolution–Galaxies:formation

1.Introduction

SinceHubble(1922,1926),classificationofgalaxiesrelieslargelyonmorphology:spirals(flattenedgalaxieswithspiralarms),

barredspirals,ellipticals(galaxieswithnoobviouspattern)andirregulars(Kormendy&Bender1996;Roberts&Haynes1994).

Subdivisionshavebeencreatedsincethen(deVaucouleurs1959;Sandage1961)inanattempttotransformmorphologyinto

amorequantitativeparameter.Theuseofalimitednumberofadditionalcharacters—radioandX-rayproperties,environment,

nucleusactivity,gascontent,starformation,coloursandothers—hasledtotherecognitionofnumerousclasseswhichareessen-

tiallycataloguesofobjects.Thistraditionalapproachofclassificationmightlookinadequatetodescribethenowacknowledged

complexityofgalaxies.Multivariateanalysisofthesecharactershasbeenonceproposedtotackletheproblemofsynthesizingthe

classificatoryinformationbroughtbyindependentcharacters(Whitmore1984;Watanabeetal.1985).However,theseapproaches

remainbasedonoverallsimilaritiesestimations.

Indeed,galaxiesarecomplexsystemsinwhichseveralphysicalandchemicalprocessesgoverntheevolutionoftheirbasic

constituents:stars,gas,dust,moleculesandprobablyblackholes(e.g.Vilchezetal.2001).Surprisingly,besidesHubble’sprimer

hypothesesontheevolutionofgalaxiesleadingtothefamousHubbleortuningforkdiagram(Hubble1936),therehasbeenno

furtherattempttogroundclassificationofgalaxiesonhistorical/evolutionaryrelationships.Nowadays,thephysicalandchemical

processesareindividuallyoftenroughlyunderstoodandsometimeswellmodelled(e.g.Sauvageetal.2002),buttheyarenot

collectivelyintegrated,andgraspingthecausesofgalaxydiversityintheirentirecomplexityremainsadifficulttask.2D.Fraix-Burnetetal.:TowardsaPhylogeneticAnalysisofGalaxyEvolution

Table1.Listofcharactersusedinthecladisticsanalysis.Theconstraintontheevolutionofeachcharacterisindicatedby“o”

for“ordered”and“u”for“unordered”.

1Ellipticityu

2Coreradiuso

3VLuminosity(integrated)o

4B-V(integrated)o

5U-B(integrated)o

6Totalmasso

7Centralmassdensityu

8HImasso

9Mass/luminosityratioo

10HImass/totalmassratioo

11HImass/Bluminosityratioo

12Dustmasso

13HIfluxo

14COfluxu

15Hαfluxo

16Fe/Hratioo

17O/Hratiou

18N/Oratioo

19StarFormationRateu

20Rotationalvelocityu

21VelocitydispersionofISMu

22Rotationalvelocity/Velocitydispersiono

23Maximumrotationalvelocityu

24Centralvelocitydispersionu

1Thisisnottobeconfusedwiththehierarchicalorganisationofgalaxydiversitydealtwithinthis

paper.D.Fraix-Burnetetal.:TowardsaPhylogeneticAnalysisofGalaxyEvolution3

Fig.1.Treeof14galaxiesobtainedbyamaximumparsimonyanalysisof24charactersasdescribedinthetext.Ithas167steps,

ConsistencyIndex=0.68,RetentionIndex=0.75,RescaledConsistencyIndex=0.51.Numbersateachnodearebootstrap(above)

anddecay(below)values.Bootstrappercentagesareobtainedafter1,000resamplingofcharacters.Decayindexescorrespondto

theminimumnumberofcharacter-statechangestobeaddedtotreelengthtobreakthecorrespondingnode.

Fig.2.Thistreewiththe36DwarfgalaxiesoftheLocalGroup,isthestrictconsensusof1041mostparsimonioustreeshaving