Properties of Dark Matter Haloes

arXiv:astro-ph/0206126v18Jun2002PropertiesofDarkMatterHaloesF.CombesObservatoiredeParis,LERMA,61Av.del’Observatoire,F-75014,Paris,FranceAbstractAnoverviewispresentedofthemainpropertiesofdarkmatterhaloes,asweknowthemfromobservations,essentiallyfromrotationcurvesaroundspiralanddwarfgalaxies.Detailedrotationcurvesarenowknownformorethanathousandgalaxies,revealingthattheyarenotsoflatintheouterparts,butrisingforlate-types,andfallingforearly-types.Awellestablishedresultnowisthatmostbrightgalaxiesarenotdominatedbydarkmatterinsidetheiropticaldisks.OnlyfordwarfsandLSB(LowSurfaceBrightnessgalaxies)darkmatterplaysadominantroleinthevisibleregions.The3D-shapeofhaloesareinvestigatedthroughseveralmethods,thatwillbediscussed:polarrings,flaringofHIplanes,X-rayisophotes.Itisnotyetpossiblewithrotationcurvestoknowhowfarhaloesextend,buttentativeshavebeenmade.Itwillbeshownthatthedarkmatterappearstobecoupledtothegasinspiralsanddwarfs,suggestingthatdarkbaryonscouldplaythemajorroleinrotationcurves.Theoriesproposingtoreplacethenon-baryonicdarkmatterbyadifferentdynamicalorgravitylaw,suchasMOND,havetotakeintoaccountthedarkbaryons,especiallysincetheirspatialdistributionislikelytobequitedifferentfromthevisiblematter.Keywords:darkmatter,galaxies,rotationcurves,flaring,flattening1RotationCurvesAtgalacticscales,thebesttoolstoprobethedarkmattercontentoftheuniversearecombinedHIandHαorCOrotationcurvesofspiralgalaxies(seethereviewbySofue&Rubin2001).Theopticalrotationcurvesprovidehighspatialresolutioninthevisibledisk,andinparticularinthecenter,totracecentralmassconcentrations,whileonlytheHIgasextendfarenoughinradiustotracetheouterparts,wheredarkmatterisdominating.Alotofprogresshasbeenmaderecentlyinourknowledgeofdarkmattercontentofgalaxies,becauseoflargesamplesobservedin2DFabry-PerotHαspectroscopy,andalsoI-bandornear-infraredphotometry(Mathewsonetal1992,Schommeretal1993,Eskridgeetal2000).B-bandimagesofgalaxiessufferfromextinction,inPreprintsubmittedtoElsevierScience1February2008Fig.1.ExamplesofHαrotationcurves(dots)andtheirfitswithI-bandimages(fulllines);thecorrespondingM/Lratiosisindicatedaboveeachpanel,fromBuchhorn(1993).particularinthecenterofgalaxies,leadingtounderestimatingthestellardiskcontributiontothemass,andmagnifyingthecontributionofanhypotheticdarkcomponent.Alsothemass-to-lightratiosarevaryingmorestronglywithstellarpopulationsintheblue.ThisisillustratedbythelargerscatteroftheTully-Fischerrelationintheblue(e.g.Verheijen2001).1.1RotationcurvesofnormalspiralsThenewfeatureresultingfromtheserecentsurveysisthatformostspiralgalaxies,thedarkmatterisnotdominantwithintheopticaldisk.Indeed,the500rotationcurvesobservedbyMathewsonetal.(1992)havebeenrepro-ducedremarquablywellbyBuchhornwithmass-to-lightratiosconstantwithradius(e.g.Freeman1993,andfig1)andwithvaluescompatiblewithwhatisknownfromstellarpopulations.Thefactthatthebaryonicmatterisactu-allydominantisreflectedbytheverygoodfitofalloscillationsor”wiggles”intheobservedrotationcurves,correspondingtospiralarmsinthedisk.Anon-baryoniccomponentwouldnotfollowthespiralinstabilitiesinthedisk,andwouldhavedilutedtheseoscillationsintherotationcurves.Thispointisrelatedtothemaximumdiskhypothesis:thelattertriestofitrotationcurvesinattributingthemaximummasstothedisk,compatibletothecentralpartofthecurve.Then,keepingtheM/Lratioconstantwithradius,therotationcurvehappenstobereproducedquitewellovertheopticaldisk,withoutdarkmatter.Ofcourse,itisstillpossibletoreduceM/Lofthestellarcomponent,andalsofittherotationcurvewiththeadditionofadarkmattercomponent.Butthepeculiarstreamingmotionsfeaturesarethenlesswellreproduced(Sackett1997,Palunas&Williams2000).Also,thefactthatmostgalaxydiskspossessbars,andthesebarsarerotatingrapidly(theircororationislocatedthroughresonancesinthemiddleofthedisk),pleadesinfavorofadiskdominatedbythevisiblematter,withaneg-2Fig.2.(a)Theuniversalrotationcurveofspiraldisksatdifferentluminosities(MI);radiiarenormalisedtoR200,themeanradiuscontainingameanhalooverdensityof200.(b)Theslopeoftherotationcurveintheregion(0.6-1)RoptversustherotationvelocityVoptatRopt.FromPersicetal.1996).ligiblecontributionofsphericaldarkmatter;dynamicalfrictionagainstthedarkmattercomponentwouldslowdownthebarsinafewdynamicaltimes(Debattista&Sellwood1998).Thebottomlineisthereforethatdarkmatterisonlyneededatlargeradii,intheHI-21cmextensions.Thefactthattherotationcurveisflatintheouterparts,whileitisnolongerattributabletothestellardiskorbulge,hasbeencalledtheconspiracy.Whydoesthevelocityduetothesphericalnon-baryonicdarkmattercoincideexactlytothatofthestellarcomponent?Infact,rotationcurvesarenotallflat,dependingontheirmorphologicaltypes(Casertano&vanGorkom1991):early-typegalaxieshaverotationcurvesthatbegintofalldown,whilelate-typeanddwarfshavenotyetreachedtheirmaximumvelocityatthelastobservedradius.Whencompiling1100rotationcurves,andnormalisingthemtotheirexpo-nentialradialscales,Persicetal.(1996)foundthatspiraldiscs(oncethecontributionofthebulgeisremovedingalaxies),mayhaveanuniversalro-tationcurve,onlydeterminedfromtheirtotalluminosity(cffig2).Athigh