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.astrocladDwarfLGcESO2008
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