Table Of ContentBirth, life and survival of Tidal Dwarf Galaxies
Pierre-AlainDuc
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Abstract Advances on the formation and survival of the so–called Tidal Dwarf
] Galaxies (TDGs) are reviewed. The understandingon how objects of the mass of
O
dwarfgalaxiesmayformindebrisofgalacticcollisionshasrecentlybenefitedfrom
C thecouplingofmulti-wavelengthobservationswithnumericalsimulationsofgalaxy
. mergers. Nonetheless, no consensualscenario has yet emergedand as a matter of
h
p fact the very definition of TDGs remains elusive. Their real cosmological impor-
- tance is also a matter of debate, their presence in our Local Group of galaxies as
o
well.Identifyingold,evolved,TDGsamongthepopulationofregulardwarfgalax-
r
t ies and satellites may notbe straightforward.Howevera numberof specific prop-
s
a erties (location, dark matter and metal content) that objects of tidal origin should
[ haveareremindedhere.ExamplesofnewlydiscoveredgenuineoldTDGsarounda
2 nearbyellipticalgalaxyarefinallypresented.
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8 1 Introducing Tidal DwarfGalaxies
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ThereisyetnoconsensualdefinitionofaTidalDwarfGalaxy(TDG).Let’showever
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sticktotheacronymtodefineit:
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1 •Tidalreferstoanobjectmadeofmaterialthatwastidallyexpelledfromgalaxies.
: Thedefinitionmaybeabitenlargedandincludesobjectsborningeneralindebrisof
v
galaxy-galaxycollisions:tidal tails of mergers,butalso collisionalringsand even
i
X gas stripped in the intergalacticmedium by other processes than tidal forces. The
r keyelementisthatthebuildingmaterialofTDGsusedtobelongtoalargerparent
a
galaxyandwasthuspre-enriched.Inpracticelate–type,rotatingcollidinggalaxies
generatemoredebristhanearly–type,dynamicallyhotgalaxies.Asaconsequence,
TDGsshouldmostlybeproducedbywetmergersinvolvingspiralgalaxies.
•Dwarfmeansthattheobjectbornintidaltailsshouldhavethesizeandmassofa
Pierre-AlainDuc
LaboratoireAIMParisSaclaye-mail:[email protected]
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dwarfgalaxy–althoughthisisaloosecriteriongiventhelargerangeofsizes/masses
exhibitedbydwarfgalaxies(fromtheultrafaintonesaroundtheMilkyWaytothe
Magellanic type objects). This specification allows to disentangle TDGs from the
SuperStarClustersandothercompactstellarobjectsthatareformedaswellincol-
lidingsystems.
•Galaxyimpliesthatthesystemiskinematicallydecoupledfromitsparentgalaxy
andgravitationallybound.Thisincreasesitsabilitytosurviveexternalgravitational
stirring or internal destructive processes such as stellar feedback. In other words,
TDGsare nottransientobjectsbutcorrespondto genuinecondensationsof matter
thathavecollapsedin-situwithincollisionaldebris.
HowdoesthisdefinitionofaTDGtranslateintoobservationalproperties?
•Beingrecycledobjects,TDGshaveinheritedfromtheirparentsthemetalcontent
oftheirinterstellarmedium.Thustheirmetallicitytellsaboutthepastchemicalen-
richmentoftheirparents,andisthusnotcorrelatedwiththeiractualmass,contrary
to conventionalgalaxies. Made out of pre-enriched material, they should have an
excess of heavy elements, providedthat their parents were themselvesmetal rich.
Thisimpliesaswellthattheirdustcontentandmoleculargascontent,astracedby
CO,ishigherthaninregularstar–formingdwarfgalaxies.
• Made out of material expelled from the dark–matter poor disks of their parent
galaxies,TDGshaveaccretedlittleoftheirdark–mattercontent.Asaconsequence,
theirluminousmass(starsandgas)shouldbeclosetotheirdynamical,total,mass,
contrarytoconventionaldark–matterdominatedgalaxies(butseeSection3).
Examplesof observedTidalDwarfGalaxies are shownin Fig. 1. On these im-
agesofcollidingsystems,theTDGsappearasredstainsonblueribbons,i.e.star–
forming objects within gas–rich tails. The most massive of them are usually lo-
catedneartheirtip.SeveralpapershaveexploitedtherichUltraviolet/GALEXIn-
frared/Spitzerdatabasesoninteractinggalaxiesandinvestigatedindetailshowstar–
formationproceedsincollisionaldebris(e.g.[27,25,2]).
At this stage, it is worthwhile noting that the vast majority of the Tidal Dwarf
Galaxies so far securely identified are young objects, formed in mergers that oc-
curredlessthanoneGyrago.Theystillexhibittheumbilicalcordlinkingthemto
their parents... i.e. the tails and bridges in which they were formed have not had
thetimetoevaporate.Onceevolved,TDGsshouldbecomeundistinguishablefrom
regularsatellitegalaxiesonopticalimages.
2 Birth ofTidal DwarfGalaxies:models,simulations
Observationsgive some clues on the formationmechanismof tidal dwarfs. In the
young TDGs observed so far, the atomic hydrogen makes the bulk of their mass.
Thereforegasshouldplayakeyrole.Onthetheoreticalside,severalscenarioshave
beenproposed,supportedbyvarioustypesofnumericalsimulations:
Birth,lifeandsurvivalofTidalDwarfGalaxies 3
Fig. 1 Sample of colliding systems exhibiting TDG candidates. The distribution of the gas is
showninblueandthestar-formingregionsinred.
•(1)Localgravitationalinstabilitiesinthestellarcomponent.Simulationsofmerg-
erswhichonlyincludethestellarcomponentareapparentlyabletoproducealong
tidaltailsgravitationalboundstellarobjects,somereachingthemassofdwarfgalax-
ies[1].HoweverithasbeenclaimedthattheymightinfactbeartifactsoftheN-body
simulations[30]
• (2) Local gravitational instabilities in the gaseous component. In simulations
whichincludethegascomponent,realmassivegascondensationsmaylocallygrow
inthetailsandformobjectssimilartoTDGs[30]
•(3)EjectionofJeans-unstablegasclouds.Duetotheincreasedvelocitydispersion
inducedbygalaxy-galaxyinteractions,theJeansmassoftheindividualcloudcom-
plexesincreasesintheouterdisksoftheparentgalaxies.Theyarethenpulledout
by tidal forces,becomeunstable and collapse when reachinglarge galacto-centric
distances[11].
•(4)A top–downkinematicalscenario.Theglobaltidalfield ofgalaxieswithex-
tendeddarkmatterhaloscanefficientlycarryawayfromtheirdiskalargefraction
of the gas, while maintaining its surface density to a high value [9]. In fact tidal
forcescontributetostretchthegasonlyatlowgalacto-centricdistances,i.e.atthe
baseofthetail.Asaresult,gasaccumulatesnearthetipoftidaltail,andthencol-
lapsesandfragments,throughaprocessapparentlyoppositetothebottom–upone
favoredwiththeColdDarkMattermodelforthebuildingupofclassicalgalaxies.
•(5)Thefullycompressivemodeoftidalforces.Atlocationswheretidalforcesare
compressiveratherthandestructive,star/clusterformationmaybetriggeredand/or
4 Pierre-AlainDuc
Fig.2 Formationoftidaldwarfgalaxiesinhighresolutionnumericalsimulationofamajormerger
[6].Twosnapshotsareshown,resp.afterthefirstencounterandthemerger(Bellesetal.,inprep)
already formed stellar objects, such as TDGs, may be protected from disruption
[24].
• (6) Merger between Super-Star-Clusters. SSCs with a range of masses may be
formedin mergers.Some of them mightmergeto reach the mass of dwarf galax-
ies [12]. TheTDGs bornthatway would thenresemble the Ultra CompactDwarf
Galaxies(UCDs)identifiedinnearbygroupsandclustersofgalaxies.
The variety of proposed scenarios tells how much having ad-hoc initial condi-
tionsandallnecessaryingredientsinthesimulationsisimportant:ifthedarkmatter
halo is truncatedin the numericalsimulations(to lower their computationalcost),
scenario(4)willnotwork;scenarios(2)-(5)requirepropertreatmentofthegaseous
component,includingfeedback.Scenario(6)needshighresolution,soastoresolve
SuperStarClusters.Fig.2presentsoneofsuchsimulationsfulfillingmostofthese
criteria.Thenumericalmodelusedatotalof36millionparticles,including12mil-
lion“sticky”particlesforthegascomponent,andminimalgridcellsizeof32pc[6].
Theproductionofstarclusterswithmassesdownto105M⊙wasdirectlyresolvedin
thesesimulations.Themassspectrumofobjectsproducedduringthemergerseems
tobebimodal.Twodistinctfamiliesarise:(a)compactSSCs,withmasseslessthan
108M⊙whichseempressuresupportedandmaybetheprogenitorsofglobularclus-
ters;(b)extendedobjectswith massesabove108 M⊙ whichareusuallysupported
byrotation.ThelatterhavethepropertiesofobservedTidalDwarfGalaxies.Thus
TDGsarenotsimplythehighmassendofSSCs,aconclusionthatwasalsoreached
fromtheanalysisofHSTimages[16].Furthermore,analyzingsnapshotsofthesim-
ulationforaperiodofoneGyr,wefoundnoevidencethatthelatterevolveintothe
former,viamerging.TheTDGprogenitorsarevisiblesoonafterthefirstencounter,
intheoutskirtsofthecollidinggalaxies,atatimewhenthetidaltailshavenotyet
completely unfolded.They quickly collect all their building material. After about
Birth,lifeandsurvivalofTidalDwarfGalaxies 5
100Myr,theirmassisstabilized.Rotationalsupportappearsaswellveryearlyon.
Only a few massive objects are formed later on within the tidal tails. Note how-
everthatstar-formationandgasfeedbackarenotproperlyhandledwiththe sticky
particles used in these simulations. Investigationsmay now be carried using fully
hydrodynamicalsimulations[26].
3 Life ofTidal DwarfGalaxies:dynamics
Fig.3 PrototypicalmergerNGC7252.ThemomentmapsoftheHItowardstheTDGcandidate
(top),aswellasitsposition–velocitydiagramsareconsistentwithrotation(Bellesetal.inprep.)
Numerical models predict that TDGs, or at least the more massive of them,
shouldbe supportedby rotation.Optical slit spectroscopyof numerousTDG can-
didateshasbeencarriedout[29,18],showingstrongvelocitygradients,consistent
with rotation, but also with artificial slit effects [28]. Integral field spectroscopy
data [4], as well as high resolution HI and CO datacubes[15, 5] are available for
severalTDGsandconfirmthatTDGsrotate,thoughatvelocitiesmuchlowerthan
measured with slit spectroscopy. For the TDGs located in the collisional ring of
NGC 5291, the data quality was sufficient to allow a determination of their total
mass,andcomparingitwiththeluminousmass(HI,H ,stars),oftheirdarkmatter
2
content.Aspredictedbynumericalsimulationsandearlyestimatesofthedynami-
calmassbasedonthemillimeterCOlinewidth[7],theinferredM/Lismuchlower
than in regular, dark matter dominated dwarf galaxies. However an unanticipated
mismatch by a factor of 2–3 between the dynamicaland luminousmass has been
noticed[5].AsimilardiscrepancywasfoundfortheTDGcandidateVCC2062,in
theVirgocluster[10],andmorerecentlyinoneofthe TDGshostedbythe proto-
typicaladvancedmergerNGC7252.Theresultsofthislatterstudyareillustratedin
Fig.3.
6 Pierre-AlainDuc
Various hypotheses have been put forward to account for the missing mass in
TDGs. Cosmological dark matter accreted from disrupted satellite galaxies [23]
mightbepresentinthediskofspiralsandthusinTDGs.Theoryofmodifiedgrav-
ity,suchasMOND,predictsrotationcurvesforTDGssimilartotheobservedones
[13,20].Analternativeideaisthatspiralsdiskscontaindarkbaryons,forinstance
intheformofverycoldmoleculargasnotaccountedforbyCOobservations[22].
ThepresenceofdarkgascloudsintheMilkyWayhadbeeninferredusinggamma
rays[14].Inthefar-infrareddomain,Planckisalsomakingacensusofthemolecu-
larcomponentnotmappedbystandardtracers.Whetheritmayexplainentirelythe
missingmassobservedinTDGsisstillanopenquestion.
4 Death ofTidal DwarfGalaxies:lifeexpectancy, census
IfTidalDwarfGalaxiestellsomethingaboutdarkmatterandthusaboutcosmology
are they cosmologicalimportantobjects? To answer this question,one should de-
terminehowmanyofthemareproducedpermerger,andthenhowmanymanageto
survive.Bothsimulationsandobservationsmaygivecluesontheseissues.
Fig.4 TDGformationathighredshift,probedbysimulationsofclumpydiskgalaxieswithahigh
gasfraction(Bournaudetal.,2010,submitted)
Fromthesimpleargumentthat,athighredshift,tidalcollisionsshouldhavebeen
numerous,someresearchersreachedtheradicalconclusionthatmostdwarfsinthe
Universeshouldhaveatidalorigin[21].Sincethisisratherunlikely,theColdDark
Matter paradigm, and the hierarchical mass assembly it implies have been ques-
tioned[17].
Analyzingalargesetofnumericalsimulations,weconcludedthattheformation
ofTDGswasinfactnotaveryefficientprocessingalaxycollisions:specificcon-
ditionsshouldbemet,suchaslowimpactvelocities,upto250kms−1,leadingto
mergers, prograde encounters, mass ratios up to 4:1– excluding minor mergers –,
and above all initially extended gas in the parent galaxies [3]. Furthermore, only
TDGslocatednearthetipofthetidaltailsareabletosurvivemorethan1Gyr.The
productionrateisthenabout1TDGperfavorablemerger.Evenifthemergingrate
increases with redshift,it would then be unlikelythat TDGs contributemore than
a few percentto the populationof dwarf galaxies.However,the initial parameters
Birth,lifeandsurvivalofTidalDwarfGalaxies 7
of our simulations[3] are valid fornearbymergers.Simulationsof mergerstuned
forthedistantUniversewhichinparticularassumethatthediskoftheparentgalax-
ieshadahighergasfraction(upto50%)andwasmoreturbulent,arepresentedin
Fig. 4. Theydidnotgeneratetheverylongtidaltailsobservedinnearbymergers.
Howeveralargenumberofclumpsofmatter,withtypicalmassesof108−109M⊙,
initiallyformedinthedisksmaybekickedoutbythecollision.Suchobjects,once
independent,haveallthepropertiesexpectedforTDGs.
If such process is as efficient as these simulations show, the Local Universe
should be full of such second generation, dark matter poor, dwarf galaxies. Is it
reallythe case? The distributionof theLocalGroupdwarfspheroidalsonspecific
planes/circlesmaysuggestthattheyareoldTDGs[19].Giventheirlocation,even
theMagellanicCloudswerespeculatedtohavebeensynthesizedinanoldmerger
thatbuiltthepresent-dayAndromedagalaxy[31].Howevertovalidatesuchanhy-
pothesis, one needs to check whether it is consistent with all the other properties
expected for TDGs: lack of dark matter, specific star formation and chemical en-
richmenthistories.Andwhatissofarknownaboutthepropertiesoftheneighbors
oftheMilkyWaydoesnotreallysupportthetidalhypothesis.
OldTDGsremaintobefound,firstlookingatenvironmentswheretheyareex-
pectedtohaveformedefficiently,i.e.wheremajormergershavelikelyoccurred.If
Early-typegalaxiesresultfrommajormergers,someoftheirsatellitesmightbeof
tidal origin. Fig. 5 presents deep optical images of a nearby elliptical galaxy that
revealedthepresenceofthreegas–richTDGcandidates,whichhavelikelyformed
2-3Gyrago.
Such systematic census of old TDGs, coupledwith new numericalsimulations
ofmergers,withapropertreatmentofthegasandstar-formation,shouldbepursued
todeterminetherealnumericalimportanceoftidaldwarfs.
Fig.5 DiscoveryofoldTDGs
around anellipticalgalaxy
withtheCFHTMegaCam
camera.Theimagewasob-
tainedaspartoftheATLAS3D
survey (Cappellari et al.,
2011).Thethreecandidates
whichhavemorphologiesof
dEsbutaregas–rich–seethe
bottomg–bandimageswith
contoursoftheHIemission
fromtheWSRT(Serraetal.,
inprep.)superimposed–lie
alonga160kpclongtidal
tail,visibleonthetopimage,
wherelowsurfacebrightness
featureshavebeenenhanced.
Themergerhasanestimated
ageof2-5Gyr(Ducetal.,
2011,submitted)
8 Pierre-AlainDuc
Acknowledgements Iwishtothankallmycollaborators,observersandsimulationexperts,inpar-
ticularFre´de´ricBournaud,Pierre-EmmanuelBelles,Me´de´ricBoquien,EliasBrinks,UteLisenfeld,
JonathanBraine,PeterWeilbacher,EtienneFerriereandDanielMiralles.TheATLAS3Dteam,in
particular PaoloSerra, isthanked for providing thedatashown inFig.5.Manythanks toPolis
Papaderosfortheinitiativeandorganizationofthissymposium.
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