HI Recycling Formation of Tidal Dwarf Galaxies

Galactic collisions trigger a number of phenomena, such as transportation inward of gas from distances of up to kiloparsecs from the center of a galaxy to the nuclear region, fuelling a central starburst or nuclear activity. The inverse process, the ejecti

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aGas&GalaxyEvolutionASPConferenceSeries,Vol.**VOLUME**,2000J.E.Hibbard,M.P.RupenandJ.H.vanGorkom,eds.HIRecycling:FormationofTidalDwarfGalaxiesP.–A.DucCNRSandCEA–Saclay,Serviced’astrophysique,OrmedesMerisiers,91191GifsurYvettecedex,FranceE.BrinksDepartamentodeAstronom´ a,Apdo.Postal144,Guanajuato,Gto.36000,MexicoAbstract.Galacticcollisionstriggeranumberofphenomena,suchastrans-portationinwardofgasfromdistancesofuptokiloparsecsfromthecenterofagalaxytothenuclearregion,fuellingacentralstarburstornuclearac-tivity.Theinverseprocess,theejectionofmaterialintotheintergalacticmediumbytidalforces,isanotherimportantaspectandcanbestudiedespeciallywellthroughdetailedHiobservationsofinteractingsystemswhichhaveshownthatalargefractionofthegaseouscomponentofcol-lidinggalaxiescanbeexpelled.Partofthistidaldebrismightfallback,bedispersedthroughouttheintergalacticmediumorrecondensetoformanewgenerationofgalaxies:theso–calledtidaldwarfgalaxies.Thelat-terarenearbyexamplesofgalaxiesinformation.Thepropertiesoftheserecycledobjectsarereviewedhereanddi erentwaystoidentifythemarereviewed.1.RecyclingtheHIgasTheVLAandothersynthesisarrayshave,fortwentyyearsandmore,revealed

thespectaculardistributionoftheatomichydrogen(Hi)ininteractingsystems.Longtidaltailsthatareevenmoreprominentthantheiropticalcounterparts,bridges,andring–likestructuresareamongthemanyweirdandwonderfulfea-tureswhichshowupinhighresolutionmaps(seeSchiminovichetal.,thisvol-ume).BecauseingeneraltheHiindiskgalaxiesextendswellbeyondtheopticalR25radius,thisatomichydrogenreactsverye cientlytoanyexternalpertur-bationasevidencedbythefactthatmostoftheneutralgasisfoundoutsidethecollidingdisks.InsystemslikeArp105andNGC7252(seeFig.1),upto90%ofthegasvisibleat21cmissituatedintheintergalacticmedium(IGM)andlittle,ifanygasremainswithintheparentspirals.Inadditiontogasbeingactivelyremoved,alargepartoftheoriginalHihaslikelybeenfunnelledinwardwhereitwastransformedintoanotherphase(seereviewbyStruck,1999).Thefateofthetidallyexpelledgaswilllargelydependonitsdensityandlocationwithrespecttotheinteractinggalaxies.Whereasthecloudsclosesttotheinteracting(ormerging)pairwillfallbackattimescalesofafewMyr

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Galactic collisions trigger a number of phenomena, such as transportation inward of gas from distances of up to kiloparsecs from the center of a galaxy to the nuclear region, fuelling a central starburst or nuclear activity. The inverse process, the ejecti

2Duc&

Brinks

Hidistributionsforasampleofinteractingsystemswhichhaveformedsometidaldwarfgalaxies.TheHicontoursaresuperimposedonopticalV–bandimages;referencesfortheHiVLAdata:1)Ducetal.(1997);2)Ducetal.(2000);3)Malphrusetal.(1997),andDuc&Mirabel(1998);4)Dickey(1997);5)Hibbardetal.(1994)Figure1.

(Hibbard&Mihos,1995),themostdistantoneswillbecomegravitationallyunbound.Theymightslowlydi useandenrichtheIGMwithheavyelementsorthestarsmightevenformthebasisofthefaintbackgroundlightintheintraclustermedium(ICM)iftheinteractingsystembelongstoacluster.Undercertainconditions,however,tidaldebrismayberecycled.Ifself–gravityissu cientlylarge,expelledcloudswillcondenseandcollapseagaintobuildnewstar–formingobjects.O springasmassiveasmagellanicdwarfirregulargalaxieshasbeenobservedaroundseveralinteractingsystems(seeforexamplesFig.1).Generallysituatedatthetipof50–100kpclongtidaltails,theyarereferredtoasTidalDwarfGalaxies(TDGs).ThetotalgasfractionthatcouldendupinaTDGisyetlargelyunknownanditishopedthatnumericalsimulationsmightonedaygetahandleonthis.Observationally,HicloudstowardsTDGsasmassiveas5109M⊙havebeenmeasured.

Galactic collisions trigger a number of phenomena, such as transportation inward of gas from distances of up to kiloparsecs from the center of a galaxy to the nuclear region, fuelling a central starburst or nuclear activity. The inverse process, the ejecti

FormationofTidalDwarfGalaxies

2.Formingtidaldwarfgalaxies3

NotonlyHicloudsareexpelledduringtidalinteractions,butanymaterialthatwasoriginally(rotating)inadisk,inparticularthestars.ThereforeTDGsaremixedbags,composedofyoungstarsformedinsitufromcollapsinggascloudsandanolderpopulationpulledoutfromtheirparentgalaxies.Thelattercomponentmaybeunimportantinsystemsinvolvinggas–richearlytypegalaxies(i.e.NGC5291,Duc&Mirabel1998;seealsoFig.1).Instabilitiesinthegaseous—Hi—componentseemtobethedrivingfactorintheformationofTDGs.Starformationoccursatrateswhichmightreach0.1M⊙yr 1.GiventhatTDGscontainhugeHireservoirs,typically109M⊙,itistobeexpectedthattherelativeimportanceofanolderstellarpopulationwilldecreasewithtimeasSFproceeds.AppreciablequantitiesofmoleculargashavealsorecentlybeendetectedinTDGsbyBraineetal.(2000).TheysuggestthatthisgashasbeenformedinsituoutofthecollapsedHi.

3.Identifyingtidaldwarfgalaxies

Tidalobjectswillsurviveprovidedthattheyhaveapotentialwellthatisdeepenoughtosustainthemselvesagainstinternalorexternaldisruption.WhenidentifyingTDGs,itisthereforeimportanttocheckthattheyarenotsimplytheagglomerateddebrisofacollisionbutareself–gravitatingentities(Ducetal.2000).Oneshouldhencetrytodistinguishthosetidalfeaturesthatarekinematicallydecoupledfromtheirhosttails,thekinematicsofwhichisgovernedbystreamingmotions.Becauseofthedi cultyofobtaininghighsensitivity,highresolutionHidata,andproblemsrelatedtoprojectione ectsalongthelineofsight,thisisadi culttaskthatrequiresacarefulexaminationoftheavailabledatacubes,suchasthoseprovidedbysynthesisradioobservations(fortheneutralgascomponent), …… 此处隐藏：6833字，全部文档内容请下载后查看。喜欢就下载吧 ……