The Virgo stellar over-density Mapping the infall of the Sag(4)

Theconclusionsobtainedfromthesecross-sectionscanbesummarizedinthedi erentspatialperspectivesdis-playedinFigure511,whichclearlyshowsthecoincidenceinpositionoftheVODandthelocationoftheSgrlead-ingtail.NotethatthispartofthestreamdistributesnearlyperpendicularlytotheGalacticdiskinthisdirec-tionofthesky.ThepanelsofFig.5showtwodi erent3-dimensionalprojections(topandbottomrow)ofourSgrmodelcomputedforoblateq=1.4halos(rightcolumn).h=0.8(leftcolumn)andprolateqhTheyvisual-izethesimulatedSgrdwarfanditstidallystrippedtailsinrainbowcolormap,withthecolorencodingthecol-umndensityofsimulationparticles(redbeingthehigh-est,bluethelowest),whichareover-plottedinthepanelsasblackdots.NotethatthecoreofSgrdwarfcorre-spondstothedarkestregionowingtothehighdensityofN-bodyparticlesthatarestillgravitationallyboundtothedwarf’sremnants.ThecoordinatesystemoriginislocatedattheGalacticcenterwiththeredaxis(X)point-ingtowardstheSun(yellowcircle)andtheblueaxis(Z)pointingtowardstheNorthGalacticpole.TheregioninwhichJuri´cetal.(2006)detecttheVODismarkedbythewhitewire-framebox,whereasthelocationofVSSasreportedbyDu auetal.(2006;seeSec.1)isshownbythebluewireframe.Thiscoincidencepointstoaplau-sibleidenti cationofVODwiththeleadingarmoftheSgrtidalstreamor,attheveryleast,tolikelysigni cantpollutionofVODasseenintheSDSSdensitymapsbytheSgrtidaldebris.

Takentogether,theanalysisof gures1,3,4and5clearlysuggeststhattheVirgoover-densitymaycorre-spondtothedetectionoftheSgrstream’sleadingarmfallingontotheMilkyWayplane.Furthermore,theob-servedmorphologyisbestdescribedbymodelsassuminganoblateMilkyWayhalo.Withtheseresultsathand,itappearsfairlyunlikelythattheVODisatidalstreamunrelatedtotheSgrdwarfandyetithasthepredictedlocationandmorphologyoftheSgrleadingarm.

3.2.3.RadialvelocityoftheVirgoover-density

AtpresentthereisnokinematicinformationavailableonthestarsofVirgoover-density.RadialvelocitiesarenecessaryinordertoestablishunambiguouslywhethertheVirgoover-densityisassociatedtotheSgrstream.Theoreticalmodelspredictthat,independentlyofthe

11

plementaryThis gureisasnapshotofasetofmoviesincludedavailableatmaterialhttp://www.astro.princeton.edu/intheelectronicversionofassup-～mjuric/sgr-virgo/

thispaperandalso

The recently discovered Virgo stellar over-density, which expands over \~1000deg^2 perpendicularly to the Galactic disk plane (7< Z <15 kpc, R~7 kpc), is the largest clump of tidal debris ever detected in the outer halo and is likely related with the accre

6Mart´ nez-Delgadoetal.

haloshape,theleadingarmoftheSgrstreamispresentinthisareaoftheskyandmovesonaveragewithnega-tiveradialvelocitiesasitfallsontotheMilkyWaydisk.However,theexactmagnitudeofthemeanradialvelocityisavitaldiscriminatorforthehaloshape,asprolateandoblatehalosleadtofairlydistincttrendsofVetal.2004a;Helmiralongtheleadingarm(Mart´ nez-Delgado2004,Lawetal.2005).WemustremarkthatatpresentthereisnoinformationonthekinematicsoftheSgrleadingarminthisareaofthesky,sincethispartofthestreamwasnotcoveredbythespectroscopicsurveyofM-giantscarriedoutbyMajewskietal.(2004).

TherehavebeenradialvelocitymeasurementsofRRLyraestarsintheVirgoconstellationthathaveshownthepresenceofaspatialover-density(calledthe12h.4clump,seeSec.1)locatedatD 20kpc.Du auetal.(2006) ndameanradialvelocityinthegalacticrestframeof(Vr)gsrlowvelocitydispersion 99.8(km/sσ=17.3(seekmTables 1),1)unexpectedwithafairlyforthesmoothpopulationofRRLyraeintheMilkyWay,thatsuggestsapossibletidalstreamdetection.Accord-ingtothetheoreticalmodelsoftheSgrstream,theposi-tivevalueof(Vr)leadingarm.gsrexcludesamembershipofthesestarstotheHowever,itisunclearwhetherthe12h.4clumpisnotassociatedtotheSgrstreamatall,sincetheoblatehalomodels(qh=0.8inthiswork)alsopredictthepresenceofthetrailingarmstarsinthatvol-umeoftheMilkyWay(seeFig.5,wheretheblueboxrepresentsthe12h.4clumpposition)withanaveragedradialvelocityof(Vr)gsr=90km/s,fairlysimilartothatreportedbyDu auetal.(2006)((Vtheirsampleselection).r)gsr=+83or+100km/s,dependingofTherefore,ifthe12h.4clumpprovestobelongtotheSgrtrailingarmitwouldfavorthemodelswithqh<1.

4.DISCUSSION

TheoreticalmodelsoftheSgrstreamarenotabletoreproducealltheobservationalconstraints.Allmodelsagreethatthegeometryofthetidalstream(i.eits3Dspatialdistribution)clearlyrulesouthalomodelswithaprolateshape(Mart´ nez-Delgadoetal.2004a,Johnstonetal.2005,Lawetal.2005)andthatMilkyWayhaloswithqh<1arepreferredata3σlevel.Paradoxically,themodelsthatbestexplainthestreamstructurefailtoreproducetheradialvelocitytrendalongtheleadingarmunlessprolatehalomodels(qthepresenth>1)areinvoked(Helmi2004).Apparently,statusoftheSgrstreammodelingisthatofa shbitingitstail.

Itisnotclearhowtoreconcileobservationsandthe-ory.Possibleexplanationsforthismismatchare:(i)theMilkyWaymodelsareover-simplistic(althoughsimilarmodelssuccessfullyreproduceothertidalstreamssuchasMonoceros,Pe narrubiaetal2005,andtheMagellanicstream,Ruzicka,Palous&Theis2006,bothstreamsbe-ingbestreproducedbymodelswithoblatehalos)or(ii)thepresenceofpossiblesystematicerrorsonthedataoftheSgrtidalstreamusedinthecomputationofthesesimulations(assuggestedbyChouetal.2006).

Inthiscontribution,wehaveinvestigatedtheoriginoftheVirgoover-densitydiscoveredfromtheanalysisoftheSDSSdata.TheVODisthelargestclumpoftidaldebriseverdetectedintheouterhalooftheMilkyWay.We ndthattheposition,apparentangularsize,stellardensityandthreedimensionalgeometrymatchthethe-

oreticalpredictionsfromthepresentSgrstreammodels.

Accordingtothesemodels,theVirgoover-densitycor-respondstoSgrleadingarmfallingontotheMilkyWaydisk.InagreementwithotherportionsoftheSgrstream,thespatialdistributionoftheVODisbestreproducedbymodelswithoblatehalosqthataunambiguousassociationh<1.However,wemustre-markbetweentheVODandtheSgrstreamcannotbeestablishedinabsenceofstellarkinematics.

IftheVirgoover-densityprovestobelongtotheSgrstreamitmightrepresentthekeypiecetosolvethepara-doxdescribedabove,aswehaveforthe rsttimetheopportuni …… 此处隐藏：5966字，全部文档内容请下载后查看。喜欢就下载吧 ……